Cleanup: ID management: remove unused old `BKE_libblock_copy_for_localize` function.

3 files changed, 2217 insertions, 10 deletions

diff --git a/source/blender/blenkernel/BKE_lib_id.h b/source/blender/blenkernel/BKE_lib_id.h
index 5de669cb620..bb875f8d1c9 100644
--- a/source/blender/blenkernel/BKE_lib_id.h
+++ b/source/blender/blenkernel/BKE_lib_id.h
@@ -152,8 +152,6 @@ void BKE_libblock_copy_ex(struct Main *bmain,
                           const int orig_flag);
 void *BKE_libblock_copy(struct Main *bmain, const struct ID *id) ATTR_WARN_UNUSED_RESULT
     ATTR_NONNULL();
-/* Special version: used by data-block localization. */
-void *BKE_libblock_copy_for_localize(const struct ID *id);
 
 void BKE_libblock_rename(struct Main *bmain, struct ID *id, const char *name) ATTR_NONNULL();
 void BLI_libblock_ensure_unique_name(struct Main *bmain, const char *name) ATTR_NONNULL();
diff --git a/source/blender/blenkernel/intern/lib_id.c b/source/blender/blenkernel/intern/lib_id.c
index 5e1027c62af..0f880d16358 100644
--- a/source/blender/blenkernel/intern/lib_id.c
+++ b/source/blender/blenkernel/intern/lib_id.c
@@ -1321,14 +1321,6 @@ void *BKE_libblock_copy(Main *bmain, const ID *id)
   return idn;
 }
 
-/* XXX TODO: get rid of this useless wrapper at some point... */
-void *BKE_libblock_copy_for_localize(const ID *id)
-{
-  ID *idn;
-  BKE_libblock_copy_ex(NULL, id, &idn, LIB_ID_COPY_LOCALIZE | LIB_ID_COPY_NO_ANIMDATA);
-  return idn;
-}
-
 /* ***************** ID ************************ */
 ID *BKE_libblock_find_name(struct Main *bmain, const short type, const char *name)
 {
diff --git a/source/blender/blenkernel/intern/mesh_normals.cc.orig b/source/blender/blenkernel/intern/mesh_normals.cc.orig
new file mode 100644
index 00000000000..18d384e8589
--- /dev/null
+++ b/source/blender/blenkernel/intern/mesh_normals.cc.orig
@@ -0,0 +1,2217 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2001-2002 by NaN Holding BV.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup bke
+ *
+ * Mesh normal calculation functions.
+ *
+ * \see bmesh_mesh_normals.c for the equivalent #BMesh functionality.
+ */
+
+#include <climits>
+
+#include "CLG_log.h"
+
+#include "MEM_guardedalloc.h"
+
+#include "DNA_mesh_types.h"
+#include "DNA_meshdata_types.h"
+
+#include "BLI_alloca.h"
+#include "BLI_bitmap.h"
+
+#include "BLI_linklist.h"
+#include "BLI_linklist_stack.h"
+#include "BLI_math.h"
+#include "BLI_memarena.h"
+#include "BLI_stack.h"
+#include "BLI_task.h"
+#include "BLI_utildefines.h"
+
+#include "BKE_customdata.h"
+#include "BKE_editmesh_cache.h"
+#include "BKE_global.h"
+#include "BKE_mesh.h"
+
+<<<<<<< Updated upstream
+#include "atomic_ops.h"
+
+// #define DEBUG_TIME
+=======
+#define DEBUG_TIME
+>>>>>>> Stashed changes
+
+#ifdef DEBUG_TIME
+#  include "PIL_time.h"
+#  include "PIL_time_utildefines.h"
+#endif
+
+static CLG_LogRef LOG = {"bke.mesh_normals"};
+
+/* -------------------------------------------------------------------- */
+/** \name Private Utility Functions
+ * \{ */
+
+/**
+ * A thread-safe version of #add_v3_v3 that uses a spin-lock.
+ *
+ * \note Avoid using this when the chance of contention is high.
+ */
+static void add_v3_v3_atomic(float r[3], const float a[3])
+{
+#define FLT_EQ_NONAN(_fa, _fb) (*((const uint32_t *)&_fa) == *((const uint32_t *)&_fb))
+
+  float virtual_lock = r[0];
+  while (true) {
+    /* This loops until following conditions are met:
+     * - `r[0]` has same value as virtual_lock (i.e. it did not change since last try).
+     * - `r[0]` was not `FLT_MAX`, i.e. it was not locked by another thread. */
+    const float test_lock = atomic_cas_float(&r[0], virtual_lock, FLT_MAX);
+    if (_ATOMIC_LIKELY(FLT_EQ_NONAN(test_lock, virtual_lock) && (test_lock != FLT_MAX))) {
+      break;
+    }
+    virtual_lock = test_lock;
+  }
+  virtual_lock += a[0];
+  r[1] += a[1];
+  r[2] += a[2];
+
+  /* Second atomic operation to 'release'
+   * our lock on that vector and set its first scalar value. */
+  /* Note that we do not need to loop here, since we 'locked' `r[0]`,
+   * nobody should have changed it in the mean time. */
+  virtual_lock = atomic_cas_float(&r[0], FLT_MAX, virtual_lock);
+  BLI_assert(virtual_lock == FLT_MAX);
+
+#undef FLT_EQ_NONAN
+}
+
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Mesh Normal Calculation
+ * \{ */
+
+void BKE_mesh_normals_tag_dirty(Mesh *mesh)
+{
+  mesh->runtime.cd_dirty_vert |= CD_MASK_NORMAL;
+  mesh->runtime.cd_dirty_poly |= CD_MASK_NORMAL;
+}
+
+/**
+ * Call when there are no polygons.
+ */
+static void mesh_calc_normals_vert_fallback(MVert *mverts, int numVerts)
+{
+  for (int i = 0; i < numVerts; i++) {
+    MVert *mv = &mverts[i];
+    float no[3];
+
+    normalize_v3_v3(no, mv->co);
+    normal_float_to_short_v3(mv->no, no);
+  }
+}
+
+/* TODO(Sybren): we can probably rename this to BKE_mesh_calc_normals_mapping(),
+ * and remove the function of the same name below, as that one doesn't seem to be
+ * called anywhere. */
+void BKE_mesh_calc_normals_mapping_simple(struct Mesh *mesh)
+{
+  const bool only_face_normals = CustomData_is_referenced_layer(&mesh->vdata, CD_MVERT);
+
+  BKE_mesh_calc_normals_mapping_ex(mesh->mvert,
+                                   mesh->totvert,
+                                   mesh->mloop,
+                                   mesh->mpoly,
+                                   mesh->totloop,
+                                   mesh->totpoly,
+                                   nullptr,
+                                   mesh->mface,
+                                   mesh->totface,
+                                   nullptr,
+                                   nullptr,
+                                   only_face_normals);
+}
+
+/* Calculate vertex and face normals, face normals are returned in *r_faceNors if non-nullptr
+ * and vertex normals are stored in actual mverts.
+ */
+void BKE_mesh_calc_normals_mapping(MVert *mverts,
+                                   int numVerts,
+                                   const MLoop *mloop,
+                                   const MPoly *mpolys,
+                                   int numLoops,
+                                   int numPolys,
+                                   float (*r_polyNors)[3],
+                                   const MFace *mfaces,
+                                   int numFaces,
+                                   const int *origIndexFace,
+                                   float (*r_faceNors)[3])
+{
+  BKE_mesh_calc_normals_mapping_ex(mverts,
+                                   numVerts,
+                                   mloop,
+                                   mpolys,
+                                   numLoops,
+                                   numPolys,
+                                   r_polyNors,
+                                   mfaces,
+                                   numFaces,
+                                   origIndexFace,
+                                   r_faceNors,
+                                   false);
+}
+/* extended version of 'BKE_mesh_calc_normals_poly' with option not to calc vertex normals */
+void BKE_mesh_calc_normals_mapping_ex(MVert *mverts,
+                                      int numVerts,
+                                      const MLoop *mloop,
+                                      const MPoly *mpolys,
+                                      int numLoops,
+                                      int numPolys,
+                                      float (*r_polyNors)[3],
+                                      const MFace *mfaces,
+                                      int numFaces,
+                                      const int *origIndexFace,
+                                      float (*r_faceNors)[3],
+                                      const bool only_face_normals)
+{
+  float(*pnors)[3] = r_polyNors, (*fnors)[3] = r_faceNors;
+
+  if (numPolys == 0) {
+    if (only_face_normals == false) {
+      mesh_calc_normals_vert_fallback(mverts, numVerts);
+    }
+    return;
+  }
+
+  /* if we are not calculating verts and no verts were passes then we have nothing to do */
+  if ((only_face_normals == true) && (r_polyNors == nullptr) && (r_faceNors == nullptr)) {
+    CLOG_WARN(&LOG, "called with nothing to do");
+    return;
+  }
+
+  if (!pnors) {
+    pnors = (float(*)[3])MEM_calloc_arrayN((size_t)numPolys, sizeof(float[3]), __func__);
+  }
+  /* NO NEED TO ALLOC YET */
+  /* if (!fnors) fnors = MEM_calloc_arrayN(numFaces, sizeof(float[3]), "face nors mesh.c"); */
+
+  if (only_face_normals == false) {
+    /* vertex normals are optional, they require some extra calculations,
+     * so make them optional */
+    BKE_mesh_calc_normals_poly(
+        mverts, nullptr, numVerts, mloop, mpolys, numLoops, numPolys, pnors, false);
+  }
+  else {
+    /* only calc poly normals */
+    const MPoly *mp = mpolys;
+    for (int i = 0; i < numPolys; i++, mp++) {
+      BKE_mesh_calc_poly_normal(mp, mloop + mp->loopstart, mverts, pnors[i]);
+    }
+  }
+
+  if (origIndexFace &&
+      /* fnors == r_faceNors */ /* NO NEED TO ALLOC YET */
+          fnors != nullptr &&
+      numFaces) {
+    const MFace *mf = mfaces;
+    for (int i = 0; i < numFaces; i++, mf++, origIndexFace++) {
+      if (*origIndexFace < numPolys) {
+        copy_v3_v3(fnors[i], pnors[*origIndexFace]);
+      }
+      else {
+        /* eek, we're not corresponding to polys */
+        CLOG_ERROR(&LOG, "tessellation face indices are incorrect.  normals may look bad.");
+      }
+    }
+  }
+
+  if (pnors != r_polyNors) {
+    MEM_freeN(pnors);
+  }
+  /* if (fnors != r_faceNors) MEM_freeN(fnors); */ /* NO NEED TO ALLOC YET */
+
+  fnors = pnors = nullptr;
+}
+
+struct MeshCalcNormalsData {
+  const MPoly *mpolys;
+  const MLoop *mloop;
+  MVert *mverts;
+  float (*pnors)[3];
+  float (*vnors)[3];
+};
+
+static void mesh_calc_normals_poly_cb(void *__restrict userdata,
+                                      const int pidx,
+                                      const TaskParallelTLS *__restrict UNUSED(tls))
+{
+  MeshCalcNormalsData *data = (MeshCalcNormalsData *)userdata;
+  const MPoly *mp = &data->mpolys[pidx];
+
+  BKE_mesh_calc_poly_normal(mp, data->mloop + mp->loopstart, data->mverts, data->pnors[pidx]);
+}
+
+static void mesh_calc_normals_poly_and_accum_cb(void *__restrict userdata,
+                                                const int pidx,
+                                                const TaskParallelTLS *__restrict UNUSED(tls))
+{
+  const MeshCalcNormalsData *data = (MeshCalcNormalsData *)userdata;
+  const MPoly *mp = &data->mpolys[pidx];
+  const MLoop *ml = &data->mloop[mp->loopstart];
+  const MVert *mverts = data->mverts;
+  float(*vnors)[3] = data->vnors;
+
+  float pnor_temp[3];
+  float *pnor = data->pnors ? data->pnors[pidx] : pnor_temp;
+
+  const int i_end = mp->totloop - 1;
+
+  /* Polygon Normal and edge-vector */
+  /* inline version of #BKE_mesh_calc_poly_normal, also does edge-vectors */
+  {
+    zero_v3(pnor);
+    /* Newell's Method */
+    const float *v_curr = mverts[ml[i_end].v].co;
+    for (int i_next = 0; i_next <= i_end; i_next++) {
+      const float *v_next = mverts[ml[i_next].v].co;
+      add_newell_cross_v3_v3v3(pnor, v_curr, v_next);
+      v_curr = v_next;
+    }
+    if (UNLIKELY(normalize_v3(pnor) == 0.0f)) {
+      pnor[2] = 1.0f; /* other axes set to 0.0 */
+    }
+  }
+
+  /* Accumulate angle weighted face normal into the vertex normal. */
+  /* inline version of #accumulate_vertex_normals_poly_v3. */
+  {
+    float edvec_prev[3], edvec_next[3], edvec_end[3];
+    const float *v_curr = mverts[ml[i_end].v].co;
+    sub_v3_v3v3(edvec_prev, mverts[ml[i_end - 1].v].co, v_curr);
+    normalize_v3(edvec_prev);
+    copy_v3_v3(edvec_end, edvec_prev);
+
+    for (int i_next = 0, i_curr = i_end; i_next <= i_end; i_curr = i_next++) {
+      const float *v_next = mverts[ml[i_next].v].co;
+
+      /* Skip an extra normalization by reusing the first calculated edge. */
+      if (i_next != i_end) {
+        sub_v3_v3v3(edvec_next, v_curr, v_next);
+        normalize_v3(edvec_next);
+      }
+      else {
+        copy_v3_v3(edvec_next, edvec_end);
+      }
+
+      /* Calculate angle between the two poly edges incident on this vertex. */
+      const float fac = saacos(-dot_v3v3(edvec_prev, edvec_next));
+      const float vnor_add[3] = {pnor[0] * fac, pnor[1] * fac, pnor[2] * fac};
+
+      add_v3_v3_atomic(vnors[ml[i_curr].v], vnor_add);
+      v_curr = v_next;
+      copy_v3_v3(edvec_prev, edvec_next);
+    }
+  }
+}
+
+static void mesh_calc_normals_poly_finalize_cb(void *__restrict userdata,
+                                               const int vidx,
+                                               const TaskParallelTLS *__restrict UNUSED(tls))
+{
+  MeshCalcNormalsData *data = (MeshCalcNormalsData *)userdata;
+
+  MVert *mv = &data->mverts[vidx];
+  float *no = data->vnors[vidx];
+
+  if (UNLIKELY(normalize_v3(no) == 0.0f)) {
+    /* following Mesh convention; we use vertex coordinate itself for normal in this case */
+    normalize_v3_v3(no, mv->co);
+  }
+
+  normal_float_to_short_v3(mv->no, no);
+}
+
+void BKE_mesh_calc_normals_poly(MVert *mverts,
+                                float (*r_vertnors)[3],
+                                int numVerts,
+                                const MLoop *mloop,
+                                const MPoly *mpolys,
+                                int UNUSED(numLoops),
+                                int numPolys,
+                                float (*r_polynors)[3],
+                                const bool only_face_normals)
+{
+  float(*pnors)[3] = r_polynors;
+
+  TaskParallelSettings settings;
+  BLI_parallel_range_settings_defaults(&settings);
+  settings.min_iter_per_thread = 1024;
+
+  if (only_face_normals) {
+    BLI_assert((pnors != nullptr) || (numPolys == 0));
+    BLI_assert(r_vertnors == nullptr);
+
+    MeshCalcNormalsData data;
+    data.mpolys = mpolys;
+    data.mloop = mloop;
+    data.mverts = mverts;
+    data.pnors = pnors;
+
+    BLI_task_parallel_range(0, numPolys, &data, mesh_calc_normals_poly_cb, &settings);
+    return;
+  }
+
+  float(*vnors)[3] = r_vertnors;
+  bool free_vnors = false;
+
+  /* first go through and calculate normals for all the polys */
+  if (vnors == nullptr) {
+    vnors = (float(*)[3])MEM_calloc_arrayN((size_t)numVerts, sizeof(*vnors), __func__);
+    free_vnors = true;
+  }
+  else {
+    memset(vnors, 0, sizeof(*vnors) * (size_t)numVerts);
+  }
+
+  MeshCalcNormalsData data;
+  data.mpolys = mpolys;
+  data.mloop = mloop;
+  data.mverts = mverts;
+  data.pnors = pnors;
+  data.vnors = vnors;
+
+  /* Compute poly normals (`pnors`), accumulating them into vertex normals (`vnors`). */
+  BLI_task_parallel_range(0, numPolys, &data, mesh_calc_normals_poly_and_accum_cb, &settings);
+
+  /* Normalize and validate computed vertex normals (`vnors`). */
+  BLI_task_parallel_range(0, numVerts, &data, mesh_calc_normals_poly_finalize_cb, &settings);
+
+  if (free_vnors) {
+    MEM_freeN(vnors);
+  }
+}
+
+void BKE_mesh_ensure_normals(Mesh *mesh)
+{
+  if (mesh->runtime.cd_dirty_vert & CD_MASK_NORMAL) {
+    BKE_mesh_calc_normals(mesh);
+  }
+  BLI_assert((mesh->runtime.cd_dirty_vert & CD_MASK_NORMAL) == 0);
+}
+
+/**
+ * Called after calculating all modifiers.
+ */
+void BKE_mesh_ensure_normals_for_display(Mesh *mesh)
+{
+  switch ((eMeshWrapperType)mesh->runtime.wrapper_type) {
+    case ME_WRAPPER_TYPE_MDATA:
+      /* Run code below. */
+      break;
+    case ME_WRAPPER_TYPE_BMESH: {
+      struct BMEditMesh *em = mesh->edit_mesh;
+      EditMeshData *emd = mesh->runtime.edit_data;
+      if (emd->vertexCos) {
+        BKE_editmesh_cache_ensure_vert_normals(em, emd);
+        BKE_editmesh_cache_ensure_poly_normals(em, emd);
+      }
+      return;
+    }
+  }
+
+  float(*poly_nors)[3] = (float(*)[3])CustomData_get_layer(&mesh->pdata, CD_NORMAL);
+  const bool do_vert_normals = (mesh->runtime.cd_dirty_vert & CD_MASK_NORMAL) != 0;
+  const bool do_poly_normals = (mesh->runtime.cd_dirty_poly & CD_MASK_NORMAL ||
+                                poly_nors == nullptr);
+
+  if (do_vert_normals || do_poly_normals) {
+    const bool do_add_poly_nors_cddata = (poly_nors == nullptr);
+    if (do_add_poly_nors_cddata) {
+      poly_nors = (float(*)[3])MEM_malloc_arrayN(
+          (size_t)mesh->totpoly, sizeof(*poly_nors), __func__);
+    }
+
+    /* calculate poly/vert normals */
+    BKE_mesh_calc_normals_poly(mesh->mvert,
+                               nullptr,
+                               mesh->totvert,
+                               mesh->mloop,
+                               mesh->mpoly,
+                               mesh->totloop,
+                               mesh->totpoly,
+                               poly_nors,
+                               !do_vert_normals);
+
+    if (do_add_poly_nors_cddata) {
+      CustomData_add_layer(&mesh->pdata, CD_NORMAL, CD_ASSIGN, poly_nors, mesh->totpoly);
+    }
+
+    mesh->runtime.cd_dirty_vert &= ~CD_MASK_NORMAL;
+    mesh->runtime.cd_dirty_poly &= ~CD_MASK_NORMAL;
+  }
+}
+
+/* Note that this does not update the CD_NORMAL layer,
+ * but does update the normals in the CD_MVERT layer. */
+void BKE_mesh_calc_normals(Mesh *mesh)
+{
+#ifdef DEBUG_TIME
+  TIMEIT_START_AVERAGED(BKE_mesh_calc_normals);
+#endif
+  BKE_mesh_calc_normals_poly(mesh->mvert,
+                             nullptr,
+                             mesh->totvert,
+                             mesh->mloop,
+                             mesh->mpoly,
+                             mesh->totloop,
+                             mesh->totpoly,
+                             nullptr,
+                             false);
+#ifdef DEBUG_TIME
+  TIMEIT_END_AVERAGED(BKE_mesh_calc_normals);
+#endif
+  mesh->runtime.cd_dirty_vert &= ~CD_MASK_NORMAL;
+}
+
+void BKE_mesh_calc_normals_looptri(MVert *mverts,
+                                   int numVerts,
+                                   const MLoop *mloop,
+                                   const MLoopTri *looptri,
+                                   int looptri_num,
+                                   float (*r_tri_nors)[3])
+{
+  float(*tnorms)[3] = (float(*)[3])MEM_calloc_arrayN((size_t)numVerts, sizeof(*tnorms), "tnorms");
+  float(*fnors)[3] = (r_tri_nors) ? r_tri_nors :
+                                    (float(*)[3])MEM_calloc_arrayN(
+                                        (size_t)looptri_num, sizeof(*fnors), "meshnormals");
+
+  if (!tnorms || !fnors) {
+    goto cleanup;
+  }
+
+  for (int i = 0; i < looptri_num; i++) {
+    const MLoopTri *lt = &looptri[i];
+    float *f_no = fnors[i];
+    const uint vtri[3] = {
+        mloop[lt->tri[0]].v,
+        mloop[lt->tri[1]].v,
+        mloop[lt->tri[2]].v,
+    };
+
+    normal_tri_v3(f_no, mverts[vtri[0]].co, mverts[vtri[1]].co, mverts[vtri[2]].co);
+
+    accumulate_vertex_normals_tri_v3(tnorms[vtri[0]],
+                                     tnorms[vtri[1]],
+                                     tnorms[vtri[2]],
+                                     f_no,
+                                     mverts[vtri[0]].co,
+                                     mverts[vtri[1]].co,
+                                     mverts[vtri[2]].co);
+  }
+
+  /* following Mesh convention; we use vertex coordinate itself for normal in this case */
+  for (int i = 0; i < numVerts; i++) {
+    MVert *mv = &mverts[i];
+    float *no = tnorms[i];
+
+    if (UNLIKELY(normalize_v3(no) == 0.0f)) {
+      normalize_v3_v3(no, mv->co);
+    }
+
+    normal_float_to_short_v3(mv->no, no);
+  }
+
+cleanup:
+  MEM_freeN(tnorms);
+
+  if (fnors != r_tri_nors) {
+    MEM_freeN(fnors);
+  }
+}
+
+void BKE_lnor_spacearr_init(MLoopNorSpaceArray *lnors_spacearr,
+                            const int numLoops,
+                            const char data_type)
+{
+  if (!(lnors_spacearr->lspacearr && lnors_spacearr->loops_pool)) {
+    MemArena *mem;
+
+    if (!lnors_spacearr->mem) {
+      lnors_spacearr->mem = BLI_memarena_new(BLI_MEMARENA_STD_BUFSIZE, __func__);
+    }
+    mem = lnors_spacearr->mem;
+    lnors_spacearr->lspacearr = (MLoopNorSpace **)BLI_memarena_calloc(
+        mem, sizeof(MLoopNorSpace *) * (size_t)numLoops);
+    lnors_spacearr->loops_pool = (LinkNode *)BLI_memarena_alloc(
+        mem, sizeof(LinkNode) * (size_t)numLoops);
+
+    lnors_spacearr->num_spaces = 0;
+  }
+  BLI_assert(ELEM(data_type, MLNOR_SPACEARR_BMLOOP_PTR, MLNOR_SPACEARR_LOOP_INDEX));
+  lnors_spacearr->data_type = data_type;
+}
+
+/**
+ * Utility for multi-threaded calculation that ensures
+ * `lnors_spacearr_tls` doesn't share memory with `lnors_spacearr`
+ * that would cause it not to be thread safe.
+ *
+ * \note This works as long as threads never operate on the same loops at once.
+ */
+void BKE_lnor_spacearr_tls_init(MLoopNorSpaceArray *lnors_spacearr,
+                                MLoopNorSpaceArray *lnors_spacearr_tls)
+{
+  *lnors_spacearr_tls = *lnors_spacearr;
+  lnors_spacearr_tls->mem = BLI_memarena_new(BLI_MEMARENA_STD_BUFSIZE, __func__);
+}
+
+/**
+ * Utility for multi-threaded calculation
+ * that merges `lnors_spacearr_tls` into `lnors_spacearr`.
+ */
+void BKE_lnor_spacearr_tls_join(MLoopNorSpaceArray *lnors_spacearr,
+                                MLoopNorSpaceArray *lnors_spacearr_tls)
+{
+  BLI_assert(lnors_spacearr->data_type == lnors_spacearr_tls->data_type);
+  BLI_assert(lnors_spacearr->mem != lnors_spacearr_tls->mem);
+  lnors_spacearr->num_spaces += lnors_spacearr_tls->num_spaces;
+  BLI_memarena_merge(lnors_spacearr->mem, lnors_spacearr_tls->mem);
+  BLI_memarena_free(lnors_spacearr_tls->mem);
+  lnors_spacearr_tls->mem = nullptr;
+  BKE_lnor_spacearr_clear(lnors_spacearr_tls);
+}
+
+void BKE_lnor_spacearr_clear(MLoopNorSpaceArray *lnors_spacearr)
+{
+  lnors_spacearr->num_spaces = 0;
+  lnors_spacearr->lspacearr = nullptr;
+  lnors_spacearr->loops_pool = nullptr;
+  if (lnors_spacearr->mem != nullptr) {
+    BLI_memarena_clear(lnors_spacearr->mem);
+  }
+}
+
+void BKE_lnor_spacearr_free(MLoopNorSpaceArray *lnors_spacearr)
+{
+  lnors_spacearr->num_spaces = 0;
+  lnors_spacearr->lspacearr = nullptr;
+  lnors_spacearr->loops_pool = nullptr;
+  BLI_memarena_free(lnors_spacearr->mem);
+  lnors_spacearr->mem = nullptr;
+}
+
+MLoopNorSpace *BKE_lnor_space_create(MLoopNorSpaceArray *lnors_spacearr)
+{
+  lnors_spacearr->num_spaces++;
+  return (MLoopNorSpace *)BLI_memarena_calloc(lnors_spacearr->mem, sizeof(MLoopNorSpace));
+}
+
+/* This threshold is a bit touchy (usual float precision issue), this value seems OK. */
+#define LNOR_SPACE_TRIGO_THRESHOLD (1.0f - 1e-4f)
+
+/* Should only be called once.
+ * Beware, this modifies ref_vec and other_vec in place!
+ * In case no valid space can be generated, ref_alpha and ref_beta are set to zero
+ * (which means 'use auto lnors').
+ */
+void BKE_lnor_space_define(MLoopNorSpace *lnor_space,
+                           const float lnor[3],
+                           float vec_ref[3],
+                           float vec_other[3],
+                           BLI_Stack *edge_vectors)
+{
+  const float pi2 = (float)M_PI * 2.0f;
+  float tvec[3], dtp;
+  const float dtp_ref = dot_v3v3(vec_ref, lnor);
+  const float dtp_other = dot_v3v3(vec_other, lnor);
+
+  if (UNLIKELY(fabsf(dtp_ref) >= LNOR_SPACE_TRIGO_THRESHOLD ||
+               fabsf(dtp_other) >= LNOR_SPACE_TRIGO_THRESHOLD)) {
+    /* If vec_ref or vec_other are too much aligned with lnor, we can't build lnor space,
+     * tag it as invalid and abort. */
+    lnor_space->ref_alpha = lnor_space->ref_beta = 0.0f;
+
+    if (edge_vectors) {
+      BLI_stack_clear(edge_vectors);
+    }
+    return;
+  }
+
+  copy_v3_v3(lnor_space->vec_lnor, lnor);
+
+  /* Compute ref alpha, average angle of all available edge vectors to lnor. */
+  if (edge_vectors) {
+    float alpha = 0.0f;
+    int nbr = 0;
+    while (!BLI_stack_is_empty(edge_vectors)) {
+      const float *vec = (const float *)BLI_stack_peek(edge_vectors);
+      alpha += saacosf(dot_v3v3(vec, lnor));
+      BLI_stack_discard(edge_vectors);
+      nbr++;
+    }
+    /* NOTE: In theory, this could be 'nbr > 2',
+     *       but there is one case where we only have two edges for two loops:
+     *       a smooth vertex with only two edges and two faces (our Monkey's nose has that, e.g.).
+     */
+    BLI_assert(nbr >= 2); /* This piece of code shall only be called for more than one loop... */
+    lnor_space->ref_alpha = alpha / (float)nbr;
+  }
+  else {
+    lnor_space->ref_alpha = (saacosf(dot_v3v3(vec_ref, lnor)) +
+                             saacosf(dot_v3v3(vec_other, lnor))) /
+                            2.0f;
+  }
+
+  /* Project vec_ref on lnor's ortho plane. */
+  mul_v3_v3fl(tvec, lnor, dtp_ref);
+  sub_v3_v3(vec_ref, tvec);
+  normalize_v3_v3(lnor_space->vec_ref, vec_ref);
+
+  cross_v3_v3v3(tvec, lnor, lnor_space->vec_ref);
+  normalize_v3_v3(lnor_space->vec_ortho, tvec);
+
+  /* Project vec_other on lnor's ortho plane. */
+  mul_v3_v3fl(tvec, lnor, dtp_other);
+  sub_v3_v3(vec_other, tvec);
+  normalize_v3(vec_other);
+
+  /* Beta is angle between ref_vec and other_vec, around lnor. */
+  dtp = dot_v3v3(lnor_space->vec_ref, vec_other);
+  if (LIKELY(dtp < LNOR_SPACE_TRIGO_THRESHOLD)) {
+    const float beta = saacos(dtp);
+    lnor_space->ref_beta = (dot_v3v3(lnor_space->vec_ortho, vec_other) < 0.0f) ? pi2 - beta : beta;
+  }
+  else {
+    lnor_space->ref_beta = pi2;
+  }
+}
+
+/**
+ * Add a new given loop to given lnor_space.
+ * Depending on \a lnor_space->data_type, we expect \a bm_loop to be a pointer to BMLoop struct
+ * (in case of BMLOOP_PTR), or nullptr (in case of LOOP_INDEX), loop index is then stored in
+ * pointer. If \a is_single is set, the BMLoop or loop index is directly stored in \a
+ * lnor_space->loops pointer (since there is only one loop in this fan), else it is added to the
+ * linked list of loops in the fan.
+ */
+void BKE_lnor_space_add_loop(MLoopNorSpaceArray *lnors_spacearr,
+                             MLoopNorSpace *lnor_space,
+                             const int ml_index,
+                             void *bm_loop,
+                             const bool is_single)
+{
+  BLI_assert((lnors_spacearr->data_type == MLNOR_SPACEARR_LOOP_INDEX && bm_loop == nullptr) ||
+             (lnors_spacearr->data_type == MLNOR_SPACEARR_BMLOOP_PTR && bm_loop != nullptr));
+
+  lnors_spacearr->lspacearr[ml_index] = lnor_space;
+  if (bm_loop == nullptr) {
+    bm_loop = POINTER_FROM_INT(ml_index);
+  }
+  if (is_single) {
+    BLI_assert(lnor_space->loops == nullptr);
+    lnor_space->flags |= MLNOR_SPACE_IS_SINGLE;
+    lnor_space->loops = (LinkNode *)bm_loop;
+  }
+  else {
+    BLI_assert((lnor_space->flags & MLNOR_SPACE_IS_SINGLE) == 0);
+    BLI_linklist_prepend_nlink(&lnor_space->loops, bm_loop, &lnors_spacearr->loops_pool[ml_index]);
+  }
+}
+
+MINLINE float unit_short_to_float(const short val)
+{
+  return (float)val / (float)SHRT_MAX;
+}
+
+MINLINE short unit_float_to_short(const float val)
+{
+  /* Rounding... */
+  return (short)floorf(val * (float)SHRT_MAX + 0.5f);
+}
+
+void BKE_lnor_space_custom_data_to_normal(MLoopNorSpace *lnor_space,
+                                          const short clnor_data[2],
+                                          float r_custom_lnor[3])
+{
+  /* NOP custom normal data or invalid lnor space, return. */
+  if (clnor_data[0] == 0 || lnor_space->ref_alpha == 0.0f || lnor_space->ref_beta == 0.0f) {
+    copy_v3_v3(r_custom_lnor, lnor_space->vec_lnor);
+    return;
+  }
+
+  {
+    /* TODO: Check whether using #sincosf() gives any noticeable benefit
+     * (could not even get it working under linux though)! */
+    const float pi2 = (float)(M_PI * 2.0);
+    const float alphafac = unit_short_to_float(clnor_data[0]);
+    const float alpha = (alphafac > 0.0f ? lnor_space->ref_alpha : pi2 - lnor_space->ref_alpha) *
+                        alphafac;
+    const float betafac = unit_short_to_float(clnor_data[1]);
+
+    mul_v3_v3fl(r_custom_lnor, lnor_space->vec_lnor, cosf(alpha));
+
+    if (betafac == 0.0f) {
+      madd_v3_v3fl(r_custom_lnor, lnor_space->vec_ref, sinf(alpha));
+    }
+    else {
+      const float sinalpha = sinf(alpha);
+      const float beta = (betafac > 0.0f ? lnor_space->ref_beta : pi2 - lnor_space->ref_beta) *
+                         betafac;
+      madd_v3_v3fl(r_custom_lnor, lnor_space->vec_ref, sinalpha * cosf(beta));
+      madd_v3_v3fl(r_custom_lnor, lnor_space->vec_ortho, sinalpha * sinf(beta));
+    }
+  }
+}
+
+void BKE_lnor_space_custom_normal_to_data(MLoopNorSpace *lnor_space,
+                                          const float custom_lnor[3],
+                                          short r_clnor_data[2])
+{
+  /* We use nullptr vector as NOP custom normal (can be simpler than giving auto-computed `lnor`).
+   */
+  if (is_zero_v3(custom_lnor) || compare_v3v3(lnor_space->vec_lnor, custom_lnor, 1e-4f)) {
+    r_clnor_data[0] = r_clnor_data[1] = 0;
+    return;
+  }
+
+  {
+    const float pi2 = (float)(M_PI * 2.0);
+    const float cos_alpha = dot_v3v3(lnor_space->vec_lnor, custom_lnor);
+    float vec[3], cos_beta;
+    float alpha;
+
+    alpha = saacosf(cos_alpha);
+    if (alpha > lnor_space->ref_alpha) {
+      /* Note we could stick to [0, pi] range here,
+       * but makes decoding more complex, not worth it. */
+      r_clnor_data[0] = unit_float_to_short(-(pi2 - alpha) / (pi2 - lnor_space->ref_alpha));
+    }
+    else {
+      r_clnor_data[0] = unit_float_to_short(alpha / lnor_space->ref_alpha);
+    }
+
+    /* Project custom lnor on (vec_ref, vec_ortho) plane. */
+    mul_v3_v3fl(vec, lnor_space->vec_lnor, -cos_alpha);
+    add_v3_v3(vec, custom_lnor);
+    normalize_v3(vec);
+
+    cos_beta = dot_v3v3(lnor_space->vec_ref, vec);
+
+    if (cos_beta < LNOR_SPACE_TRIGO_THRESHOLD) {
+      float beta = saacosf(cos_beta);
+      if (dot_v3v3(lnor_space->vec_ortho, vec) < 0.0f) {
+        beta = pi2 - beta;
+      }
+
+      if (beta > lnor_space->ref_beta) {
+        r_clnor_data[1] = unit_float_to_short(-(pi2 - beta) / (pi2 - lnor_space->ref_beta));
+      }
+      else {
+        r_clnor_data[1] = unit_float_to_short(beta / lnor_space->ref_beta);
+      }
+    }
+    else {
+      r_clnor_data[1] = 0;
+    }
+  }
+}
+
+#define LOOP_SPLIT_TASK_BLOCK_SIZE 1024
+
+struct LoopSplitTaskData {
+  /* Specific to each instance (each task). */
+
+  /** We have to create those outside of tasks, since #MemArena is not thread-safe. */
+  MLoopNorSpace *lnor_space;
+  float (*lnor)[3];
+  const MLoop *ml_curr;
+  const MLoop *ml_prev;
+  int ml_curr_index;
+  int ml_prev_index;
+  /** Also used a flag to switch between single or fan process! */
+  const int *e2l_prev;
+  int mp_index;
+
+  /** This one is special, it's owned and managed by worker tasks,
+   * avoid to have to create it for each fan! */
+  BLI_Stack *edge_vectors;
+
+  char pad_c;
+};
+
+struct LoopSplitTaskDataCommon {
+  /* Read/write.
+   * Note we do not need to protect it, though, since two different tasks will *always* affect
+   * different elements in the arrays. */
+  MLoopNorSpaceArray *lnors_spacearr;
+  float (*loopnors)[3];
+  short (*clnors_data)[2];
+
+  /* Read-only. */
+  const MVert *mverts;
+  const MEdge *medges;
+  const MLoop *mloops;
+  const MPoly *mpolys;
+  int (*edge_to_loops)[2];
+  int *loop_to_poly;
+  const float (*polynors)[3];
+
+  int numEdges;
+  int numLoops;
+  int numPolys;
+};
+
+#define INDEX_UNSET INT_MIN
+#define INDEX_INVALID -1
+/* See comment about edge_to_loops below. */
+#define IS_EDGE_SHARP(_e2l) (ELEM((_e2l)[1], INDEX_UNSET, INDEX_INVALID))
+
+static void mesh_edges_sharp_tag(LoopSplitTaskDataCommon *data,
+                                 const bool check_angle,
+                                 const float split_angle,
+                                 const bool do_sharp_edges_tag)
+{
+  const MVert *mverts = data->mverts;
+  const MEdge *medges = data->medges;
+  const MLoop *mloops = data->mloops;
+
+  const MPoly *mpolys = data->mpolys;
+
+  const int numEdges = data->numEdges;
+  const int numPolys = data->numPolys;
+
+  float(*loopnors)[3] = data->loopnors; /* NOTE: loopnors may be nullptr here. */
+  const float(*polynors)[3] = data->polynors;
+
+  int(*edge_to_loops)[2] = data->edge_to_loops;
+  int *loop_to_poly = data->loop_to_poly;
+
+  BLI_bitmap *sharp_edges = do_sharp_edges_tag ? BLI_BITMAP_NEW(numEdges, __func__) : nullptr;
+
+  const MPoly *mp;
+  int mp_index;
+
+  const float split_angle_cos = check_angle ? cosf(split_angle) : -1.0f;
+
+  for (mp = mpolys, mp_index = 0; mp_index < numPolys; mp++, mp_index++) {
+    const MLoop *ml_curr;
+    int *e2l;
+    int ml_curr_index = mp->loopstart;
+    const int ml_last_index = (ml_curr_index + mp->totloop) - 1;
+
+    ml_curr = &mloops[ml_curr_index];
+
+    for (; ml_curr_index <= ml_last_index; ml_curr++, ml_curr_index++) {
+      e2l = edge_to_loops[ml_curr->e];
+
+      loop_to_poly[ml_curr_index] = mp_index;
+
+      /* Pre-populate all loop normals as if their verts were all-smooth,
+       * this way we don't have to compute those later!
+       */
+      if (loopnors) {
+        normal_short_to_float_v3(loopnors[ml_curr_index], mverts[ml_curr->v].no);
+      }
+
+      /* Check whether current edge might be smooth or sharp */
+      if ((e2l[0] | e2l[1]) == 0) {
+        /* 'Empty' edge until now, set e2l[0] (and e2l[1] to INDEX_UNSET to tag it as unset). */
+        e2l[0] = ml_curr_index;
+        /* We have to check this here too, else we might miss some flat faces!!! */
+        e2l[1] = (mp->flag & ME_SMOOTH) ? INDEX_UNSET : INDEX_INVALID;
+      }
+      else if (e2l[1] == INDEX_UNSET) {
+        const bool is_angle_sharp = (check_angle &&
+                                     dot_v3v3(polynors[loop_to_poly[e2l[0]]], polynors[mp_index]) <
+                                         split_angle_cos);
+
+        /* Second loop using this edge, time to test its sharpness.
+         * An edge is sharp if it is tagged as such, or its face is not smooth,
+         * or both poly have opposed (flipped) normals, i.e. both loops on the same edge share the
+         * same vertex, or angle between both its polys' normals is above split_angle value.
+         */
+        if (!(mp->flag & ME_SMOOTH) || (medges[ml_curr->e].flag & ME_SHARP) ||
+            ml_curr->v == mloops[e2l[0]].v || is_angle_sharp) {
+          /* NOTE: we are sure that loop != 0 here ;). */
+          e2l[1] = INDEX_INVALID;
+
+          /* We want to avoid tagging edges as sharp when it is already defined as such by
+           * other causes than angle threshold... */
+          if (do_sharp_edges_tag && is_angle_sharp) {
+            BLI_BITMAP_SET(sharp_edges, ml_curr->e, true);
+          }
+        }
+        else {
+          e2l[1] = ml_curr_index;
+        }
+      }
+      else if (!IS_EDGE_SHARP(e2l)) {
+        /* More than two loops using this edge, tag as sharp if not yet done. */
+        e2l[1] = INDEX_INVALID;
+
+        /* We want to avoid tagging edges as sharp when it is already defined as such by
+         * other causes than angle threshold... */
+        if (do_sharp_edges_tag) {
+          BLI_BITMAP_SET(sharp_edges, ml_curr->e, false);
+        }
+      }
+      /* Else, edge is already 'disqualified' (i.e. sharp)! */
+    }
+  }
+
+  /* If requested, do actual tagging of edges as sharp in another loop. */
+  if (do_sharp_edges_tag) {
+    MEdge *me;
+    int me_index;
+    for (me = (MEdge *)medges, me_index = 0; me_index < numEdges; me++, me_index++) {
+      if (BLI_BITMAP_TEST(sharp_edges, me_index)) {
+        me->flag |= ME_SHARP;
+      }
+    }
+
+    MEM_freeN(sharp_edges);
+  }
+}
+
+/**
+ * Define sharp edges as needed to mimic 'autosmooth' from angle threshold.
+ *
+ * Used when defining an empty custom loop normals data layer,
+ * to keep same shading as with auto-smooth!
+ */
+void BKE_edges_sharp_from_angle_set(const struct MVert *mverts,
+                                    const int UNUSED(numVerts),
+                                    struct MEdge *medges,
+                                    const int numEdges,
+                                    struct MLoop *mloops,
+                                    const int numLoops,
+                                    struct MPoly *mpolys,
+                                    const float (*polynors)[3],
+                                    const int numPolys,
+                                    const float split_angle)
+{
+  if (split_angle >= (float)M_PI) {
+    /* Nothing to do! */
+    return;
+  }
+
+  /* Mapping edge -> loops. See BKE_mesh_normals_loop_split() for details. */
+  int(*edge_to_loops)[2] = (int(*)[2])MEM_calloc_arrayN(
+      (size_t)numEdges, sizeof(*edge_to_loops), __func__);
+
+  /* Simple mapping from a loop to its polygon index. */
+  int *loop_to_poly = (int *)MEM_malloc_arrayN((size_t)numLoops, sizeof(*loop_to_poly), __func__);
+
+  LoopSplitTaskDataCommon common_data = {};
+  common_data.mverts = mverts;
+  common_data.medges = medges;
+  common_data.mloops = mloops;
+  common_data.mpolys = mpolys;
+  common_data.edge_to_loops = edge_to_loops;
+  common_data.loop_to_poly = loop_to_poly;
+  common_data.polynors = polynors;
+  common_data.numEdges = numEdges;
+  common_data.numPolys = numPolys;
+
+  mesh_edges_sharp_tag(&common_data, true, split_angle, true);
+
+  MEM_freeN(edge_to_loops);
+  MEM_freeN(loop_to_poly);
+}
+
+void BKE_mesh_loop_manifold_fan_around_vert_next(const MLoop *mloops,
+                                                 const MPoly *mpolys,
+                                                 const int *loop_to_poly,
+                                                 const int *e2lfan_curr,
+                                                 const uint mv_pivot_index,
+                                                 const MLoop **r_mlfan_curr,
+                                                 int *r_mlfan_curr_index,
+                                                 int *r_mlfan_vert_index,
+                                                 int *r_mpfan_curr_index)
+{
+  const MLoop *mlfan_next;
+  const MPoly *mpfan_next;
+
+  /* Warning! This is rather complex!
+   * We have to find our next edge around the vertex (fan mode).
+   * First we find the next loop, which is either previous or next to mlfan_curr_index, depending
+   * whether both loops using current edge are in the same direction or not, and whether
+   * mlfan_curr_index actually uses the vertex we are fanning around!
+   * mlfan_curr_index is the index of mlfan_next here, and mlfan_next is not the real next one
+   * (i.e. not the future mlfan_curr)...
+   */
+  *r_mlfan_curr_index = (e2lfan_curr[0] == *r_mlfan_curr_index) ? e2lfan_curr[1] : e2lfan_curr[0];
+  *r_mpfan_curr_index = loop_to_poly[*r_mlfan_curr_index];
+
+  BLI_assert(*r_mlfan_curr_index >= 0);
+  BLI_assert(*r_mpfan_curr_index >= 0);
+
+  mlfan_next = &mloops[*r_mlfan_curr_index];
+  mpfan_next = &mpolys[*r_mpfan_curr_index];
+  if (((*r_mlfan_curr)->v == mlfan_next->v && (*r_mlfan_curr)->v == mv_pivot_index) ||
+      ((*r_mlfan_curr)->v != mlfan_next->v && (*r_mlfan_curr)->v != mv_pivot_index)) {
+    /* We need the previous loop, but current one is our vertex's loop. */
+    *r_mlfan_vert_index = *r_mlfan_curr_index;
+    if (--(*r_mlfan_curr_index) < mpfan_next->loopstart) {
+      *r_mlfan_curr_index = mpfan_next->loopstart + mpfan_next->totloop - 1;
+    }
+  }
+  else {
+    /* We need the next loop, which is also our vertex's loop. */
+    if (++(*r_mlfan_curr_index) >= mpfan_next->loopstart + mpfan_next->totloop) {
+      *r_mlfan_curr_index = mpfan_next->loopstart;
+    }
+    *r_mlfan_vert_index = *r_mlfan_curr_index;
+  }
+  *r_mlfan_curr = &mloops[*r_mlfan_curr_index];
+  /* And now we are back in sync, mlfan_curr_index is the index of mlfan_curr! Pff! */
+}
+
+static void split_loop_nor_single_do(LoopSplitTaskDataCommon *common_data, LoopSplitTaskData *data)
+{
+  MLoopNorSpaceArray *lnors_spacearr = common_data->lnors_spacearr;
+  const short(*clnors_data)[2] = common_data->clnors_data;
+
+  const MVert *mverts = common_data->mverts;
+  const MEdge *medges = common_data->medges;
+  const float(*polynors)[3] = common_data->polynors;
+
+  MLoopNorSpace *lnor_space = data->lnor_space;
+  float(*lnor)[3] = data->lnor;
+  const MLoop *ml_curr = data->ml_curr;
+  const MLoop *ml_prev = data->ml_prev;
+  const int ml_curr_index = data->ml_curr_index;
+#if 0 /* Not needed for 'single' loop. */
+  const int ml_prev_index = data->ml_prev_index;
+  const int *e2l_prev = data->e2l_prev;
+#endif
+  const int mp_index = data->mp_index;
+
+  /* Simple case (both edges around that vertex are sharp in current polygon),
+   * this loop just takes its poly normal.
+   */
+  copy_v3_v3(*lnor, polynors[mp_index]);
+
+#if 0
+  printf("BASIC: handling loop %d / edge %d / vert %d / poly %d\n",
+         ml_curr_index,
+         ml_curr->e,
+         ml_curr->v,
+         mp_index);
+#endif
+
+  /* If needed, generate this (simple!) lnor space. */
+  if (lnors_spacearr) {
+    float vec_curr[3], vec_prev[3];
+
+    const uint mv_pivot_index = ml_curr->v; /* The vertex we are "fanning" around! */
+    const MVert *mv_pivot = &mverts[mv_pivot_index];
+    const MEdge *me_curr = &medges[ml_curr->e];
+    const MVert *mv_2 = (me_curr->v1 == mv_pivot_index) ? &mverts[me_curr->v2] :
+                                                          &mverts[me_curr->v1];
+    const MEdge *me_prev = &medges[ml_prev->e];
+    const MVert *mv_3 = (me_prev->v1 == mv_pivot_index) ? &mverts[me_prev->v2] :
+                                                          &mverts[me_prev->v1];
+
+    sub_v3_v3v3(vec_curr, mv_2->co, mv_pivot->co);
+    normalize_v3(vec_curr);
+    sub_v3_v3v3(vec_prev, mv_3->co, mv_pivot->co);
+    normalize_v3(vec_prev);
+
+    BKE_lnor_space_define(lnor_space, *lnor, vec_curr, vec_prev, nullptr);
+    /* We know there is only one loop in this space,
+     * no need to create a linklist in this case... */
+    BKE_lnor_space_add_loop(lnors_spacearr, lnor_space, ml_curr_index, nullptr, true);
+
+    if (clnors_data) {
+      BKE_lnor_space_custom_data_to_normal(lnor_space, clnors_data[ml_curr_index], *lnor);
+    }
+  }
+}
+
+static void split_loop_nor_fan_do(LoopSplitTaskDataCommon *common_data, LoopSplitTaskData *data)
+{
+  MLoopNorSpaceArray *lnors_spacearr = common_data->lnors_spacearr;
+  float(*loopnors)[3] = common_data->loopnors;
+  short(*clnors_data)[2] = common_data->clnors_data;
+
+  const MVert *mverts = common_data->mverts;
+  const MEdge *medges = common_data->medges;
+  const MLoop *mloops = common_data->mloops;
+  const MPoly *mpolys = common_data->mpolys;
+  const int(*edge_to_loops)[2] = common_data->edge_to_loops;
+  const int *loop_to_poly = common_data->loop_to_poly;
+  const float(*polynors)[3] = common_data->polynors;
+
+  MLoopNorSpace *lnor_space = data->lnor_space;
+#if 0 /* Not needed for 'fan' loops. */
+  float(*lnor)[3] = data->lnor;
+#endif
+  const MLoop *ml_curr = data->ml_curr;
+  const MLoop *ml_prev = data->ml_prev;
+  const int ml_curr_index = data->ml_curr_index;
+  const int ml_prev_index = data->ml_prev_index;
+  const int mp_index = data->mp_index;
+  const int *e2l_prev = data->e2l_prev;
+
+  BLI_Stack *edge_vectors = data->edge_vectors;
+
+  /* Gah... We have to fan around current vertex, until we find the other non-smooth edge,
+   * and accumulate face normals into the vertex!
+   * Note in case this vertex has only one sharp edges, this is a waste because the normal is the
+   * same as the vertex normal, but I do not see any easy way to detect that (would need to count
+   * number of sharp edges per vertex, I doubt the additional memory usage would be worth it,
+   * especially as it should not be a common case in real-life meshes anyway).
+   */
+  const uint mv_pivot_index = ml_curr->v; /* The vertex we are "fanning" around! */
+  const MVert *mv_pivot = &mverts[mv_pivot_index];
+
+  /* ml_curr would be mlfan_prev if we needed that one. */
+  const MEdge *me_org = &medges[ml_curr->e];
+
+  const int *e2lfan_curr;
+  float vec_curr[3], vec_prev[3], vec_org[3];
+  const MLoop *mlfan_curr;
+  float lnor[3] = {0.0f, 0.0f, 0.0f};
+  /* mlfan_vert_index: the loop of our current edge might not be the loop of our current vertex! */
+  int mlfan_curr_index, mlfan_vert_index, mpfan_curr_index;
+
+  /* We validate clnors data on the fly - cheapest way to do! */
+  int clnors_avg[2] = {0, 0};
+  short(*clnor_ref)[2] = nullptr;
+  int clnors_nbr = 0;
+  bool clnors_invalid = false;
+
+  /* Temp loop normal stack. */
+  BLI_SMALLSTACK_DECLARE(normal, float *);
+  /* Temp clnors stack. */
+  BLI_SMALLSTACK_DECLARE(clnors, short *);
+
+  e2lfan_curr = e2l_prev;
+  mlfan_curr = ml_prev;
+  mlfan_curr_index = ml_prev_index;
+  mlfan_vert_index = ml_curr_index;
+  mpfan_curr_index = mp_index;
+
+  BLI_assert(mlfan_curr_index >= 0);
+  BLI_assert(mlfan_vert_index >= 0);
+  BLI_assert(mpfan_curr_index >= 0);
+
+  /* Only need to compute previous edge's vector once, then we can just reuse old current one! */
+  {
+    const MVert *mv_2 = (me_org->v1 == mv_pivot_index) ? &mverts[me_org->v2] : &mverts[me_org->v1];
+
+    sub_v3_v3v3(vec_org, mv_2->co, mv_pivot->co);
+    normalize_v3(vec_org);
+    copy_v3_v3(vec_prev, vec_org);
+
+    if (lnors_spacearr) {
+      BLI_stack_push(edge_vectors, vec_org);
+    }
+  }
+
+  // printf("FAN: vert %d, start edge %d\n", mv_pivot_index, ml_curr->e);
+
+  while (true) {
+    const MEdge *me_curr = &medges[mlfan_curr->e];
+    /* Compute edge vectors.
+     * NOTE: We could pre-compute those into an array, in the first iteration, instead of computing
+     *       them twice (or more) here. However, time gained is not worth memory and time lost,
+     *       given the fact that this code should not be called that much in real-life meshes...
+     */
+    {
+      const MVert *mv_2 = (me_curr->v1 == mv_pivot_index) ? &mverts[me_curr->v2] :
+                                                            &mverts[me_curr->v1];
+
+      sub_v3_v3v3(vec_curr, mv_2->co, mv_pivot->co);
+      normalize_v3(vec_curr);
+    }
+
+    // printf("\thandling edge %d / loop %d\n", mlfan_curr->e, mlfan_curr_index);
+
+    {
+      /* Code similar to accumulate_vertex_normals_poly_v3. */
+      /* Calculate angle between the two poly edges incident on this vertex. */
+      const float fac = saacos(dot_v3v3(vec_curr, vec_prev));
+      /* Accumulate */
+      madd_v3_v3fl(lnor, polynors[mpfan_curr_index], fac);
+
+      if (clnors_data) {
+        /* Accumulate all clnors, if they are not all equal we have to fix that! */
+        short(*clnor)[2] = &clnors_data[mlfan_vert_index];
+        if (clnors_nbr) {
+          clnors_invalid |= ((*clnor_ref)[0] != (*clnor)[0] || (*clnor_ref)[1] != (*clnor)[1]);
+        }
+        else {
+          clnor_ref = clnor;
+        }
+        clnors_avg[0] += (*clnor)[0];
+        clnors_avg[1] += (*clnor)[1];
+        clnors_nbr++;
+        /* We store here a pointer to all custom lnors processed. */
+        BLI_SMALLSTACK_PUSH(clnors, (short *)*clnor);
+      }
+    }
+
+    /* We store here a pointer to all loop-normals processed. */
+    BLI_SMALLSTACK_PUSH(normal, (float *)(loopnors[mlfan_vert_index]));
+
+    if (lnors_spacearr) {
+      /* Assign current lnor space to current 'vertex' loop. */
+      BKE_lnor_space_add_loop(lnors_spacearr, lnor_space, mlfan_vert_index, nullptr, false);
+      if (me_curr != me_org) {
+        /* We store here all edges-normalized vectors processed. */
+        BLI_stack_push(edge_vectors, vec_curr);
+      }
+    }
+
+    if (IS_EDGE_SHARP(e2lfan_curr) || (me_curr == me_org)) {
+      /* Current edge is sharp and we have finished with this fan of faces around this vert,
+       * or this vert is smooth, and we have completed a full turn around it. */
+      // printf("FAN: Finished!\n");
+      break;
+    }
+
+    copy_v3_v3(vec_prev, vec_curr);
+
+    /* Find next loop of the smooth fan. */
+    BKE_mesh_loop_manifold_fan_around_vert_next(mloops,
+                                                mpolys,
+                                                loop_to_poly,
+                                                e2lfan_curr,
+                                                mv_pivot_index,
+                                                &mlfan_curr,
+                                                &mlfan_curr_index,
+                                                &mlfan_vert_index,
+                                                &mpfan_curr_index);
+
+    e2lfan_curr = edge_to_loops[mlfan_curr->e];
+  }
+
+  {
+    float lnor_len = normalize_v3(lnor);
+
+    /* If we are generating lnor spacearr, we can now define the one for this fan,
+     * and optionally compute final lnor from custom data too!
+     */
+    if (lnors_spacearr) {
+      if (UNLIKELY(lnor_len == 0.0f)) {
+        /* Use vertex normal as fallback! */
+        copy_v3_v3(lnor, loopnors[mlfan_vert_index]);
+        lnor_len = 1.0f;
+      }
+
+      BKE_lnor_space_define(lnor_space, lnor, vec_org, vec_curr, edge_vectors);
+
+      if (clnors_data) {
+        if (clnors_invalid) {
+          short *clnor;
+
+          clnors_avg[0] /= clnors_nbr;
+          clnors_avg[1] /= clnors_nbr;
+          /* Fix/update all clnors of this fan with computed average value. */
+          if (G.debug & G_DEBUG) {
+            printf("Invalid clnors in this fan!\n");
+          }
+          while ((clnor = (short *)BLI_SMALLSTACK_POP(clnors))) {
+            // print_v2("org clnor", clnor);
+            clnor[0] = (short)clnors_avg[0];
+            clnor[1] = (short)clnors_avg[1];
+          }
+          // print_v2("new clnors", clnors_avg);
+        }
+        /* Extra bonus: since small-stack is local to this function,
+         * no more need to empty it at all cost! */
+
+        BKE_lnor_space_custom_data_to_normal(lnor_space, *clnor_ref, lnor);
+      }
+    }
+
+    /* In case we get a zero normal here, just use vertex normal already set! */
+    if (LIKELY(lnor_len != 0.0f)) {
+      /* Copy back the final computed normal into all related loop-normals. */
+      float *nor;
+
+      while ((nor = (float *)BLI_SMALLSTACK_POP(normal))) {
+        copy_v3_v3(nor, lnor);
+      }
+    }
+    /* Extra bonus: since small-stack is local to this function,
+     * no more need to empty it at all cost! */
+  }
+}
+
+static void loop_split_worker_do(LoopSplitTaskDataCommon *common_data,
+                                 LoopSplitTaskData *data,
+                                 BLI_Stack *edge_vectors)
+{
+  BLI_assert(data->ml_curr);
+  if (data->e2l_prev) {
+    BLI_assert((edge_vectors == nullptr) || BLI_stack_is_empty(edge_vectors));
+    data->edge_vectors = edge_vectors;
+    split_loop_nor_fan_do(common_data, data);
+  }
+  else {
+    /* No need for edge_vectors for 'single' case! */
+    split_loop_nor_single_do(common_data, data);
+  }
+}
+
+static void loop_split_worker(TaskPool *__restrict pool, void *taskdata)
+{
+  LoopSplitTaskDataCommon *common_data = (LoopSplitTaskDataCommon *)BLI_task_pool_user_data(pool);
+  LoopSplitTaskData *data = (LoopSplitTaskData *)taskdata;
+
+  /* Temp edge vectors stack, only used when computing lnor spacearr. */
+  BLI_Stack *edge_vectors = common_data->lnors_spacearr ?
+                                BLI_stack_new(sizeof(float[3]), __func__) :
+                                nullptr;
+
+#ifdef DEBUG_TIME
+  TIMEIT_START_AVERAGED(loop_split_worker);
+#endif
+
+  for (int i = 0; i < LOOP_SPLIT_TASK_BLOCK_SIZE; i++, data++) {
+    /* A nullptr ml_curr is used to tag ended data! */
+    if (data->ml_curr == nullptr) {
+      break;
+    }
+
+    loop_split_worker_do(common_data, data, edge_vectors);
+  }
+
+  if (edge_vectors) {
+    BLI_stack_free(edge_vectors);
+  }
+
+#ifdef DEBUG_TIME
+  TIMEIT_END_AVERAGED(loop_split_worker);
+#endif
+}
+
+/**
+ * Check whether given loop is part of an unknown-so-far cyclic smooth fan, or not.
+ * Needed because cyclic smooth fans have no obvious 'entry point',
+ * and yet we need to walk them once, and only once.
+ */
+static bool loop_split_generator_check_cyclic_smooth_fan(const MLoop *mloops,
+                                                         const MPoly *mpolys,
+                                                         const int (*edge_to_loops)[2],
+                                                         const int *loop_to_poly,
+                                                         const int *e2l_prev,
+                                                         BLI_bitmap *skip_loops,
+                                                         const MLoop *ml_curr,
+                                                         const MLoop *ml_prev,
+                                                         const int ml_curr_index,
+                                                         const int ml_prev_index,
+                                                         const int mp_curr_index)
+{
+  const uint mv_pivot_index = ml_curr->v; /* The vertex we are "fanning" around! */
+  const int *e2lfan_curr;
+  const MLoop *mlfan_curr;
+  /* mlfan_vert_index: the loop of our current edge might not be the loop of our current vertex! */
+  int mlfan_curr_index, mlfan_vert_index, mpfan_curr_index;
+
+  e2lfan_curr = e2l_prev;
+  if (IS_EDGE_SHARP(e2lfan_curr)) {
+    /* Sharp loop, so not a cyclic smooth fan... */
+    return false;
+  }
+
+  mlfan_curr = ml_prev;
+  mlfan_curr_index = ml_prev_index;
+  mlfan_vert_index = ml_curr_index;
+  mpfan_curr_index = mp_curr_index;
+
+  BLI_assert(mlfan_curr_index >= 0);
+  BLI_assert(mlfan_vert_index >= 0);
+  BLI_assert(mpfan_curr_index >= 0);
+
+  BLI_assert(!BLI_BITMAP_TEST(skip_loops, mlfan_vert_index));
+  BLI_BITMAP_ENABLE(skip_loops, mlfan_vert_index);
+
+  while (true) {
+    /* Find next loop of the smooth fan. */
+    BKE_mesh_loop_manifold_fan_around_vert_next(mloops,
+                                                mpolys,
+                                                loop_to_poly,
+                                                e2lfan_curr,
+                                                mv_pivot_index,
+                                                &mlfan_curr,
+                                                &mlfan_curr_index,
+                                                &mlfan_vert_index,
+                                                &mpfan_curr_index);
+
+    e2lfan_curr = edge_to_loops[mlfan_curr->e];
+
+    if (IS_EDGE_SHARP(e2lfan_curr)) {
+      /* Sharp loop/edge, so not a cyclic smooth fan... */
+      return false;
+    }
+    /* Smooth loop/edge... */
+    if (BLI_BITMAP_TEST(skip_loops, mlfan_vert_index)) {
+      if (mlfan_vert_index == ml_curr_index) {
+        /* We walked around a whole cyclic smooth fan without finding any already-processed loop,
+         * means we can use initial ml_curr/ml_prev edge as start for this smooth fan. */
+        return true;
+      }
+      /* ... already checked in some previous looping, we can abort. */
+      return false;
+    }
+
+    /* ... we can skip it in future, and keep checking the smooth fan. */
+    BLI_BITMAP_ENABLE(skip_loops, mlfan_vert_index);
+  }
+}
+
+static void loop_split_generator(TaskPool *pool, LoopSplitTaskDataCommon *common_data)
+{
+  MLoopNorSpaceArray *lnors_spacearr = common_data->lnors_spacearr;
+  float(*loopnors)[3] = common_data->loopnors;
+
+  const MLoop *mloops = common_data->mloops;
+  const MPoly *mpolys = common_data->mpolys;
+  const int *loop_to_poly = common_data->loop_to_poly;
+  const int(*edge_to_loops)[2] = common_data->edge_to_loops;
+  const int numLoops = common_data->numLoops;
+  const int numPolys = common_data->numPolys;
+
+  const MPoly *mp;
+  int mp_index;
+
+  const MLoop *ml_curr;
+  const MLoop *ml_prev;
+  int ml_curr_index;
+  int ml_prev_index;
+
+  BLI_bitmap *skip_loops = BLI_BITMAP_NEW(numLoops, __func__);
+
+  LoopSplitTaskData *data_buff = nullptr;
+  int data_idx = 0;
+
+  /* Temp edge vectors stack, only used when computing lnor spacearr
+   * (and we are not multi-threading). */
+  BLI_Stack *edge_vectors = nullptr;
+
+#ifdef DEBUG_TIME
+  TIMEIT_START_AVERAGED(loop_split_generator);
+#endif
+
+  if (!pool) {
+    if (lnors_spacearr) {
+      edge_vectors = BLI_stack_new(sizeof(float[3]), __func__);
+    }
+  }
+
+  /* We now know edges that can be smoothed (with their vector, and their two loops),
+   * and edges that will be hard! Now, time to generate the normals.
+   */
+  for (mp = mpolys, mp_index = 0; mp_index < numPolys; mp++, mp_index++) {
+    float(*lnors)[3];
+    const int ml_last_index = (mp->loopstart + mp->totloop) - 1;
+    ml_curr_index = mp->loopstart;
+    ml_prev_index = ml_last_index;
+
+    ml_curr = &mloops[ml_curr_index];
+    ml_prev = &mloops[ml_prev_index];
+    lnors = &loopnors[ml_curr_index];
+
+    for (; ml_curr_index <= ml_last_index; ml_curr++, ml_curr_index++, lnors++) {
+      const int *e2l_curr = edge_to_loops[ml_curr->e];
+      const int *e2l_prev = edge_to_loops[ml_prev->e];
+
+#if 0
+      printf("Checking loop %d / edge %u / vert %u (sharp edge: %d, skiploop: %d)...",
+             ml_curr_index,
+             ml_curr->e,
+             ml_curr->v,
+             IS_EDGE_SHARP(e2l_curr),
+             BLI_BITMAP_TEST_BOOL(skip_loops, ml_curr_index));
+#endif
+
+      /* A smooth edge, we have to check for cyclic smooth fan case.
+       * If we find a new, never-processed cyclic smooth fan, we can do it now using that loop/edge
+       * as 'entry point', otherwise we can skip it. */
+
+      /* NOTE: In theory, we could make #loop_split_generator_check_cyclic_smooth_fan() store
+       * mlfan_vert_index'es and edge indexes in two stacks, to avoid having to fan again around
+       * the vert during actual computation of `clnor` & `clnorspace`.
+       * However, this would complicate the code, add more memory usage, and despite its logical
+       * complexity, #loop_manifold_fan_around_vert_next() is quite cheap in term of CPU cycles,
+       * so really think it's not worth it. */
+      if (!IS_EDGE_SHARP(e2l_curr) && (BLI_BITMAP_TEST(skip_loops, ml_curr_index) ||
+                                       !loop_split_generator_check_cyclic_smooth_fan(mloops,
+                                                                                     mpolys,
+                                                                                     edge_to_loops,
+                                                                                     loop_to_poly,
+                                                                                     e2l_prev,
+                                                                                     skip_loops,
+                                                                                     ml_curr,
+                                                                                     ml_prev,
+                                                                                     ml_curr_index,
+                                                                                     ml_prev_index,
+                                                                                     mp_index))) {
+        // printf("SKIPPING!\n");
+      }
+      else {
+        LoopSplitTaskData *data, data_local;
+
+        // printf("PROCESSING!\n");
+
+        if (pool) {
+          if (data_idx == 0) {
+            data_buff = (LoopSplitTaskData *)MEM_calloc_arrayN(
+                LOOP_SPLIT_TASK_BLOCK_SIZE, sizeof(*data_buff), __func__);
+          }
+          data = &data_buff[data_idx];
+        }
+        else {
+          data = &data_local;
+          memset(data, 0, sizeof(*data));
+        }
+
+        if (IS_EDGE_SHARP(e2l_curr) && IS_EDGE_SHARP(e2l_prev)) {
+          data->lnor = lnors;
+          data->ml_curr = ml_curr;
+          data->ml_prev = ml_prev;
+          data->ml_curr_index = ml_curr_index;
+#if 0 /* Not needed for 'single' loop. */
+          data->ml_prev_index = ml_prev_index;
+          data->e2l_prev = nullptr; /* Tag as 'single' task. */
+#endif
+          data->mp_index = mp_index;
+          if (lnors_spacearr) {
+            data->lnor_space = BKE_lnor_space_create(lnors_spacearr);
+          }
+        }
+        /* We *do not need* to check/tag loops as already computed!
+         * Due to the fact a loop only links to one of its two edges,
+         * a same fan *will never be walked more than once!*
+         * Since we consider edges having neighbor polys with inverted
+         * (flipped) normals as sharp, we are sure that no fan will be skipped,
+         * even only considering the case (sharp curr_edge, smooth prev_edge),
+         * and not the alternative (smooth curr_edge, sharp prev_edge).
+         * All this due/thanks to link between normals and loop ordering (i.e. winding).
+         */
+        else {
+#if 0 /* Not needed for 'fan' loops. */
+          data->lnor = lnors;
+#endif
+          data->ml_curr = ml_curr;
+          data->ml_prev = ml_prev;
+          data->ml_curr_index = ml_curr_index;
+          data->ml_prev_index = ml_prev_index;
+          data->e2l_prev = e2l_prev; /* Also tag as 'fan' task. */
+          data->mp_index = mp_index;
+          if (lnors_spacearr) {
+            data->lnor_space = BKE_lnor_space_create(lnors_spacearr);
+          }
+        }
+
+        if (pool) {
+          data_idx++;
+          if (data_idx == LOOP_SPLIT_TASK_BLOCK_SIZE) {
+            BLI_task_pool_push(pool, loop_split_worker, data_buff, true, nullptr);
+            data_idx = 0;
+          }
+        }
+        else {
+          loop_split_worker_do(common_data, data, edge_vectors);
+        }
+      }
+
+      ml_prev = ml_curr;
+      ml_prev_index = ml_curr_index;
+    }
+  }
+
+  /* Last block of data... Since it is calloc'ed and we use first nullptr item as stopper,
+   * everything is fine. */
+  if (pool && data_idx) {
+    BLI_task_pool_push(pool, loop_split_worker, data_buff, true, nullptr);
+  }
+
+  if (edge_vectors) {
+    BLI_stack_free(edge_vectors);
+  }
+  MEM_freeN(skip_loops);
+
+#ifdef DEBUG_TIME
+  TIMEIT_END_AVERAGED(loop_split_generator);
+#endif
+}
+
+/**
+ * Compute split normals, i.e. vertex normals associated with each poly (hence 'loop normals').
+ * Useful to materialize sharp edges (or non-smooth faces) without actually modifying the geometry
+ * (splitting edges).
+ */
+void BKE_mesh_normals_loop_split(const MVert *mverts,
+                                 const int UNUSED(numVerts),
+                                 MEdge *medges,
+                                 const int numEdges,
+                                 MLoop *mloops,
+                                 float (*r_loopnors)[3],
+                                 const int numLoops,
+                                 MPoly *mpolys,
+                                 const float (*polynors)[3],
+                                 const int numPolys,
+                                 const bool use_split_normals,
+                                 const float split_angle,
+                                 MLoopNorSpaceArray *r_lnors_spacearr,
+                                 short (*clnors_data)[2],
+                                 int *r_loop_to_poly)
+{
+  /* For now this is not supported.
+   * If we do not use split normals, we do not generate anything fancy! */
+  BLI_assert(use_split_normals || !(r_lnors_spacearr));
+
+  if (!use_split_normals) {
+    /* In this case, we simply fill lnors with vnors (or fnors for flat faces), quite simple!
+     * Note this is done here to keep some logic and consistency in this quite complex code,
+     * since we may want to use lnors even when mesh's 'autosmooth' is disabled
+     * (see e.g. mesh mapping code).
+     * As usual, we could handle that on case-by-case basis,
+     * but simpler to keep it well confined here.
+     */
+    int mp_index;
+
+    for (mp_index = 0; mp_index < numPolys; mp_index++) {
+      MPoly *mp = &mpolys[mp_index];
+      int ml_index = mp->loopstart;
+      const int ml_index_end = ml_index + mp->totloop;
+      const bool is_poly_flat = ((mp->flag & ME_SMOOTH) == 0);
+
+      for (; ml_index < ml_index_end; ml_index++) {
+        if (r_loop_to_poly) {
+          r_loop_to_poly[ml_index] = mp_index;
+        }
+        if (is_poly_flat) {
+          copy_v3_v3(r_loopnors[ml_index], polynors[mp_index]);
+        }
+        else {
+          normal_short_to_float_v3(r_loopnors[ml_index], mverts[mloops[ml_index].v].no);
+        }
+      }
+    }
+    return;
+  }
+
+  /**
+   * Mapping edge -> loops.
+   * If that edge is used by more than two loops (polys),
+   * it is always sharp (and tagged as such, see below).
+   * We also use the second loop index as a kind of flag:
+   *
+   * - smooth edge: > 0.
+   * - sharp edge: < 0 (INDEX_INVALID || INDEX_UNSET).
+   * - unset: INDEX_UNSET.
+   *
+   * Note that currently we only have two values for second loop of sharp edges.
+   * However, if needed, we can store the negated value of loop index instead of INDEX_INVALID
+   * to retrieve the real value later in code).
+   * Note also that loose edges always have both values set to 0! */
+  int(*edge_to_loops)[2] = (int(*)[2])MEM_calloc_arrayN(
+      (size_t)numEdges, sizeof(*edge_to_loops), __func__);
+
+  /* Simple mapping from a loop to its polygon index. */
+  int *loop_to_poly = r_loop_to_poly ? r_loop_to_poly :
+                                       (int *)MEM_malloc_arrayN(
+                                           (size_t)numLoops, sizeof(*loop_to_poly), __func__);
+
+  /* When using custom loop normals, disable the angle feature! */
+  const bool check_angle = (split_angle < (float)M_PI) && (clnors_data == nullptr);
+
+  MLoopNorSpaceArray _lnors_spacearr = {nullptr};
+
+#ifdef DEBUG_TIME
+  TIMEIT_START_AVERAGED(BKE_mesh_normals_loop_split);
+#endif
+
+  if (!r_lnors_spacearr && clnors_data) {
+    /* We need to compute lnor spacearr if some custom lnor data are given to us! */
+    r_lnors_spacearr = &_lnors_spacearr;
+  }
+  if (r_lnors_spacearr) {
+    BKE_lnor_spacearr_init(r_lnors_spacearr, numLoops, MLNOR_SPACEARR_LOOP_INDEX);
+  }
+
+  /* Init data common to all tasks. */
+  LoopSplitTaskDataCommon common_data;
+  common_data.lnors_spacearr = r_lnors_spacearr;
+  common_data.loopnors = r_loopnors;
+  common_data.clnors_data = clnors_data;
+  common_data.mverts = mverts;
+  common_data.medges = medges;
+  common_data.mloops = mloops;
+  common_data.mpolys = mpolys;
+  common_data.edge_to_loops = edge_to_loops;
+  common_data.loop_to_poly = loop_to_poly;
+  common_data.polynors = polynors;
+  common_data.numEdges = numEdges;
+  common_data.numLoops = numLoops;
+  common_data.numPolys = numPolys;
+
+  /* This first loop check which edges are actually smooth, and compute edge vectors. */
+  mesh_edges_sharp_tag(&common_data, check_angle, split_angle, false);
+
+  if (numLoops < LOOP_SPLIT_TASK_BLOCK_SIZE * 8) {
+    /* Not enough loops to be worth the whole threading overhead... */
+    loop_split_generator(nullptr, &common_data);
+  }
+  else {
+    TaskPool *task_pool = BLI_task_pool_create(&common_data, TASK_PRIORITY_HIGH);
+
+    loop_split_generator(task_pool, &common_data);
+
+    BLI_task_pool_work_and_wait(task_pool);
+
+    BLI_task_pool_free(task_pool);
+  }
+
+  MEM_freeN(edge_to_loops);
+  if (!r_loop_to_poly) {
+    MEM_freeN(loop_to_poly);
+  }
+
+  if (r_lnors_spacearr) {
+    if (r_lnors_spacearr == &_lnors_spacearr) {
+      BKE_lnor_spacearr_free(r_lnors_spacearr);
+    }
+  }
+
+#ifdef DEBUG_TIME
+  TIMEIT_END_AVERAGED(BKE_mesh_normals_loop_split);
+#endif
+}
+
+#undef INDEX_UNSET
+#undef INDEX_INVALID
+#undef IS_EDGE_SHARP
+
+/**
+ * Compute internal representation of given custom normals (as an array of float[2]).
+ * It also makes sure the mesh matches those custom normals, by setting sharp edges flag as needed
+ * to get a same custom lnor for all loops sharing a same smooth fan.
+ * If use_vertices if true, r_custom_loopnors is assumed to be per-vertex, not per-loop
+ * (this allows to set whole vert's normals at once, useful in some cases).
+ * r_custom_loopnors is expected to have normalized normals, or zero ones,
+ * in which case they will be replaced by default loop/vertex normal.
+ */
+static void mesh_normals_loop_custom_set(const MVert *mverts,
+                                         const int numVerts,
+                                         MEdge *medges,
+                                         const int numEdges,
+                                         MLoop *mloops,
+                                         float (*r_custom_loopnors)[3],
+                                         const int numLoops,
+                                         MPoly *mpolys,
+                                         const float (*polynors)[3],
+                                         const int numPolys,
+                                         short (*r_clnors_data)[2],
+                                         const bool use_vertices)
+{
+  /* We *may* make that poor BKE_mesh_normals_loop_split() even more complex by making it handling
+   * that feature too, would probably be more efficient in absolute.
+   * However, this function *is not* performance-critical, since it is mostly expected to be called
+   * by io addons when importing custom normals, and modifier
+   * (and perhaps from some editing tools later?).
+   * So better to keep some simplicity here, and just call BKE_mesh_normals_loop_split() twice!
+   */
+  MLoopNorSpaceArray lnors_spacearr = {nullptr};
+  BLI_bitmap *done_loops = BLI_BITMAP_NEW((size_t)numLoops, __func__);
+  float(*lnors)[3] = (float(*)[3])MEM_calloc_arrayN((size_t)numLoops, sizeof(*lnors), __func__);
+  int *loop_to_poly = (int *)MEM_malloc_arrayN((size_t)numLoops, sizeof(int), __func__);
+  /* In this case we always consider split nors as ON,
+   * and do not want to use angle to define smooth fans! */
+  const bool use_split_normals = true;
+  const float split_angle = (float)M_PI;
+
+  BLI_SMALLSTACK_DECLARE(clnors_data, short *);
+
+  /* Compute current lnor spacearr. */
+  BKE_mesh_normals_loop_split(mverts,
+                              numVerts,
+                              medges,
+                              numEdges,
+                              mloops,
+                              lnors,
+                              numLoops,
+                              mpolys,
+                              polynors,
+                              numPolys,
+                              use_split_normals,
+                              split_angle,
+                              &lnors_spacearr,
+                              nullptr,
+                              loop_to_poly);
+
+  /* Set all given zero vectors to their default value. */
+  if (use_vertices) {
+    for (int i = 0; i < numVerts; i++) {
+      if (is_zero_v3(r_custom_loopnors[i])) {
+        normal_short_to_float_v3(r_custom_loopnors[i], mverts[i].no);
+      }
+    }
+  }
+  else {
+    for (int i = 0; i < numLoops; i++) {
+      if (is_zero_v3(r_custom_loopnors[i])) {
+        copy_v3_v3(r_custom_loopnors[i], lnors[i]);
+      }
+    }
+  }
+
+  BLI_assert(lnors_spacearr.data_type == MLNOR_SPACEARR_LOOP_INDEX);
+
+  /* Now, check each current smooth fan (one lnor space per smooth fan!),
+   * and if all its matching custom lnors are not (enough) equal, add sharp edges as needed.
+   * This way, next time we run BKE_mesh_normals_loop_split(), we'll get lnor spacearr/smooth fans
+   * matching given custom lnors.
+   * Note this code *will never* unsharp edges! And quite obviously,
+   * when we set custom normals per vertices, running this is absolutely useless.
+   */
+  if (!use_vertices) {
+    for (int i = 0; i < numLoops; i++) {
+      if (!lnors_spacearr.lspacearr[i]) {
+        /* This should not happen in theory, but in some rare case (probably ugly geometry)
+         * we can get some nullptr loopspacearr at this point. :/
+         * Maybe we should set those loops' edges as sharp?
+         */
+        BLI_BITMAP_ENABLE(done_loops, i);
+        if (G.debug & G_DEBUG) {
+          printf("WARNING! Getting invalid nullptr loop space for loop %d!\n", i);
+        }
+        continue;
+      }
+
+      if (!BLI_BITMAP_TEST(done_loops, i)) {
+        /* Notes:
+         * * In case of mono-loop smooth fan, we have nothing to do.
+         * * Loops in this linklist are ordered (in reversed order compared to how they were
+         *   discovered by BKE_mesh_normals_loop_split(), but this is not a problem).
+         *   Which means if we find a mismatching clnor,
+         *   we know all remaining loops will have to be in a new, different smooth fan/lnor space.
+         * * In smooth fan case, we compare each clnor against a ref one,
+         *   to avoid small differences adding up into a real big one in the end!
+         */
+        if (lnors_spacearr.lspacearr[i]->flags & MLNOR_SPACE_IS_SINGLE) {
+          BLI_BITMAP_ENABLE(done_loops, i);
+          continue;
+        }
+
+        LinkNode *loops = lnors_spacearr.lspacearr[i]->loops;
+        MLoop *prev_ml = nullptr;
+        const float *org_nor = nullptr;
+
+        while (loops) {
+          const int lidx = POINTER_AS_INT(loops->link);
+          MLoop *ml = &mloops[lidx];
+          const int nidx = lidx;
+          float *nor = r_custom_loopnors[nidx];
+
+          if (!org_nor) {
+            org_nor = nor;
+          }
+          else if (dot_v3v3(org_nor, nor) < LNOR_SPACE_TRIGO_THRESHOLD) {
+            /* Current normal differs too much from org one, we have to tag the edge between
+             * previous loop's face and current's one as sharp.
+             * We know those two loops do not point to the same edge,
+             * since we do not allow reversed winding in a same smooth fan.
+             */
+            const MPoly *mp = &mpolys[loop_to_poly[lidx]];
+            const MLoop *mlp =
+                &mloops[(lidx == mp->loopstart) ? mp->loopstart + mp->totloop - 1 : lidx - 1];
+            medges[(prev_ml->e == mlp->e) ? prev_ml->e : ml->e].flag |= ME_SHARP;
+
+            org_nor = nor;
+          }
+
+          prev_ml = ml;
+          loops = loops->next;
+          BLI_BITMAP_ENABLE(done_loops, lidx);
+        }
+
+        /* We also have to check between last and first loops,
+         * otherwise we may miss some sharp edges here!
+         * This is just a simplified version of above while loop.
+         * See T45984. */
+        loops = lnors_spacearr.lspacearr[i]->loops;
+        if (loops && org_nor) {
+          const int lidx = POINTER_AS_INT(loops->link);
+          MLoop *ml = &mloops[lidx];
+          const int nidx = lidx;
+          float *nor = r_custom_loopnors[nidx];
+
+          if (dot_v3v3(org_nor, nor) < LNOR_SPACE_TRIGO_THRESHOLD) {
+            const MPoly *mp = &mpolys[loop_to_poly[lidx]];
+            const MLoop *mlp =
+                &mloops[(lidx == mp->loopstart) ? mp->loopstart + mp->totloop - 1 : lidx - 1];
+            medges[(prev_ml->e == mlp->e) ? prev_ml->e : ml->e].flag |= ME_SHARP;
+          }
+        }
+      }
+    }
+
+    /* And now, recompute our new auto lnors and lnor spacearr! */
+    BKE_lnor_spacearr_clear(&lnors_spacearr);
+    BKE_mesh_normals_loop_split(mverts,
+                                numVerts,
+                                medges,
+                                numEdges,
+                                mloops,
+                                lnors,
+                                numLoops,
+                                mpolys,
+                                polynors,
+                                numPolys,
+                                use_split_normals,
+                                split_angle,
+                                &lnors_spacearr,
+                                nullptr,
+                                loop_to_poly);
+  }
+  else {
+    BLI_bitmap_set_all(done_loops, true, (size_t)numLoops);
+  }
+
+  /* And we just have to convert plain object-space custom normals to our
+   * lnor space-encoded ones. */
+  for (int i = 0; i < numLoops; i++) {
+    if (!lnors_spacearr.lspacearr[i]) {
+      BLI_BITMAP_DISABLE(done_loops, i);
+      if (G.debug & G_DEBUG) {
+        printf("WARNING! Still getting invalid nullptr loop space in second loop for loop %d!\n",
+               i);
+      }
+      continue;
+    }
+
+    if (BLI_BITMAP_TEST_BOOL(done_loops, i)) {
+      /* Note we accumulate and average all custom normals in current smooth fan,
+       * to avoid getting different clnors data (tiny differences in plain custom normals can
+       * give rather huge differences in computed 2D factors).
+       */
+      LinkNode *loops = lnors_spacearr.lspacearr[i]->loops;
+      if (lnors_spacearr.lspacearr[i]->flags & MLNOR_SPACE_IS_SINGLE) {
+        BLI_assert(POINTER_AS_INT(loops) == i);
+        const int nidx = use_vertices ? (int)mloops[i].v : i;
+        float *nor = r_custom_loopnors[nidx];
+
+        BKE_lnor_space_custom_normal_to_data(lnors_spacearr.lspacearr[i], nor, r_clnors_data[i]);
+        BLI_BITMAP_DISABLE(done_loops, i);
+      }
+      else {
+        int nbr_nors = 0;
+        float avg_nor[3];
+        short clnor_data_tmp[2], *clnor_data;
+
+        zero_v3(avg_nor);
+        while (loops) {
+          const int lidx = POINTER_AS_INT(loops->link);
+          const int nidx = use_vertices ? (int)mloops[lidx].v : lidx;
+          float *nor = r_custom_loopnors[nidx];
+
+          nbr_nors++;
+          add_v3_v3(avg_nor, nor);
+          BLI_SMALLSTACK_PUSH(clnors_data, (short *)r_clnors_data[lidx]);
+
+          loops = loops->next;
+          BLI_BITMAP_DISABLE(done_loops, lidx);
+        }
+
+        mul_v3_fl(avg_nor, 1.0f / (float)nbr_nors);
+        BKE_lnor_space_custom_normal_to_data(lnors_spacearr.lspacearr[i], avg_nor, clnor_data_tmp);
+
+        while ((clnor_data = (short *)BLI_SMALLSTACK_POP(clnors_data))) {
+          clnor_data[0] = clnor_data_tmp[0];
+          clnor_data[1] = clnor_data_tmp[1];
+        }
+      }
+    }
+  }
+
+  MEM_freeN(lnors);
+  MEM_freeN(loop_to_poly);
+  MEM_freeN(done_loops);
+  BKE_lnor_spacearr_free(&lnors_spacearr);
+}
+
+void BKE_mesh_normals_loop_custom_set(const MVert *mverts,
+                                      const int numVerts,
+                                      MEdge *medges,
+                                      const int numEdges,
+                                      MLoop *mloops,
+                                      float (*r_custom_loopnors)[3],
+                                      const int numLoops,
+                                      MPoly *mpolys,
+                                      const float (*polynors)[3],
+                                      const int numPolys,
+                                      short (*r_clnors_data)[2])
+{
+  mesh_normals_loop_custom_set(mverts,
+                               numVerts,
+                               medges,
+                               numEdges,
+                               mloops,
+                               r_custom_loopnors,
+                               numLoops,
+                               mpolys,
+                               polynors,
+                               numPolys,
+                               r_clnors_data,
+                               false);
+}
+
+void BKE_mesh_normals_loop_custom_from_vertices_set(const MVert *mverts,
+                                                    float (*r_custom_vertnors)[3],
+                                                    const int numVerts,
+                                                    MEdge *medges,
+                                                    const int numEdges,
+                                                    MLoop *mloops,
+                                                    const int numLoops,
+                                                    MPoly *mpolys,
+                                                    const float (*polynors)[3],
+                                                    const int numPolys,
+                                                    short (*r_clnors_data)[2])
+{
+  mesh_normals_loop_custom_set(mverts,
+                               numVerts,
+                               medges,
+                               numEdges,
+                               mloops,
+                               r_custom_vertnors,
+                               numLoops,
+                               mpolys,
+                               polynors,
+                               numPolys,
+                               r_clnors_data,
+                               true);
+}
+
+static void mesh_set_custom_normals(Mesh *mesh, float (*r_custom_nors)[3], const bool use_vertices)
+{
+  short(*clnors)[2];
+  const int numloops = mesh->totloop;
+
+  clnors = (short(*)[2])CustomData_get_layer(&mesh->ldata, CD_CUSTOMLOOPNORMAL);
+  if (clnors != nullptr) {
+    memset(clnors, 0, sizeof(*clnors) * (size_t)numloops);
+  }
+  else {
+    clnors = (short(*)[2])CustomData_add_layer(
+        &mesh->ldata, CD_CUSTOMLOOPNORMAL, CD_CALLOC, nullptr, numloops);
+  }
+
+  float(*polynors)[3] = (float(*)[3])CustomData_get_layer(&mesh->pdata, CD_NORMAL);
+  bool free_polynors = false;
+  if (polynors == nullptr) {
+    polynors = (float(*)[3])MEM_mallocN(sizeof(float[3]) * (size_t)mesh->totpoly, __func__);
+    BKE_mesh_calc_normals_poly(mesh->mvert,
+                               nullptr,
+                               mesh->totvert,
+                               mesh->mloop,
+                               mesh->mpoly,
+                               mesh->totloop,
+                               mesh->totpoly,
+                               polynors,
+                               false);
+    free_polynors = true;
+  }
+
+  mesh_normals_loop_custom_set(mesh->mvert,
+                               mesh->totvert,
+                               mesh->medge,
+                               mesh->totedge,
+                               mesh->mloop,
+                               r_custom_nors,
+                               mesh->totloop,
+                               mesh->mpoly,
+                               polynors,
+                               mesh->totpoly,
+                               clnors,
+                               use_vertices);
+
+  if (free_polynors) {
+    MEM_freeN(polynors);
+  }
+}
+
+/**
+ * Higher level functions hiding most of the code needed around call to
+ * #BKE_mesh_normals_loop_custom_set().
+ *
+ * \param r_custom_loopnors: is not const, since code will replace zero_v3 normals there
+ * with automatically computed vectors.
+ */
+void BKE_mesh_set_custom_normals(Mesh *mesh, float (*r_custom_loopnors)[3])
+{
+  mesh_set_custom_normals(mesh, r_custom_loopnors, false);
+}
+
+/**
+ * Higher level functions hiding most of the code needed around call to
+ * #BKE_mesh_normals_loop_custom_from_vertices_set().
+ *
+ * \param r_custom_vertnors: is not const, since code will replace zero_v3 normals there
+ * with automatically computed vectors.
+ */
+void BKE_mesh_set_custom_normals_from_vertices(Mesh *mesh, float (*r_custom_vertnors)[3])
+{
+  mesh_set_custom_normals(mesh, r_custom_vertnors, true);
+}
+
+/**
+ * Computes average per-vertex normals from given custom loop normals.
+ *
+ * \param clnors: The computed custom loop normals.
+ * \param r_vert_clnors: The (already allocated) array where to store averaged per-vertex normals.
+ */
+void BKE_mesh_normals_loop_to_vertex(const int numVerts,
+                                     const MLoop *mloops,
+                                     const int numLoops,
+                                     const float (*clnors)[3],
+                                     float (*r_vert_clnors)[3])
+{
+  int *vert_loops_nbr = (int *)MEM_calloc_arrayN(
+      (size_t)numVerts, sizeof(*vert_loops_nbr), __func__);
+
+  copy_vn_fl((float *)r_vert_clnors, 3 * numVerts, 0.0f);
+
+  int i;
+  const MLoop *ml;
+  for (i = 0, ml = mloops; i < numLoops; i++, ml++) {
+    const uint v = ml->v;
+
+    add_v3_v3(r_vert_clnors[v], clnors[i]);
+    vert_loops_nbr[v]++;
+  }
+
+  for (i = 0; i < numVerts; i++) {
+    mul_v3_fl(r_vert_clnors[i], 1.0f / (float)vert_loops_nbr[i]);
+  }
+
+  MEM_freeN(vert_loops_nbr);
+}
+
+#undef LNOR_SPACE_TRIGO_THRESHOLD
+
+/** \} */