Mikktspace: Reduce number of data queries to caller

The current code for computing tangents is not exactly fast.
This has been a long-standing issue, and recently came up again with T97378.

The main bottleneck is fetching the mesh data, since it's handled through a callback system and each vertex might have its data queried dozens of times.

I've tried a lot of things to optimize `mikktspace.c`, but unfortunately most weren't that useful:
- Vectorizing SVec3 gives a ~5% speedup, but I'm not sure if the additional ~70 lines of code are worth it
- Keeping an internal copy of the data instead of re-querying all the time helps a lot (~50-60% time reduction), but requires a lot of extra memory (~100 byte per face)
- Going C++ and replacing the internal quicksort with std::sort shows no difference
- Restructuring the entire file to be a header-only library so that the callbacks can be inlined gives ~10% reduction, but is a major change and deviation from the original library

In the end, two simple fixes that actually help remain:
- Don't re-query the number of faces in each loop iteration
- Don't bother looking for identical vertices if there's only one vertex with that hash

With this, time for the test case in T97378 goes from 6.64sec to 4.92sec. It's something I guess.

I feel like completely refactoring this library would not be a bad idea at some point, but for now it does the job...

Differential Revision: https://developer.blender.org/D14675

1 files changed, 22 insertions, 18 deletions

diff --git a/intern/mikktspace/mikktspace.c b/intern/mikktspace/mikktspace.c
index 794590d30a4..e39ac4bb6ef 100644
--- a/intern/mikktspace/mikktspace.c
+++ b/intern/mikktspace/mikktspace.c
@@ -565,23 +565,26 @@ static void GenerateSharedVerticesIndexList(int piTriList_in_and_out[],
         break;
     }
 
-    for (int i = blockstart; i < blockend; i++) {
-      int index1 = piTriList_in_and_out[indices[i]];
-      const SVec3 vP = GetPosition(pContext, index1);
-      const SVec3 vN = GetNormal(pContext, index1);
-      const SVec3 vT = GetTexCoord(pContext, index1);
-      for (int i2 = i + 1; i2 < blockend; i2++) {
-        int index2 = piTriList_in_and_out[indices[i2]];
-        if (index1 == index2)
-          continue;
-
-        if (veq(vP, GetPosition(pContext, index2)) && veq(vN, GetNormal(pContext, index2)) &&
-            veq(vT, GetTexCoord(pContext, index2))) {
-          piTriList_in_and_out[indices[i2]] = index1;
-          /* Once i2>i has been identified as a duplicate, we can stop since any
-           * i3>i2>i that is a duplicate of i (and therefore also i2) will also be
-           * compared to i2 and therefore be identified there anyways. */
-          break;
+    /* If there's only one vertex with this hash, we don't have anything to compare. */
+    if (blockend > blockstart + 1) {
+      for (int i = blockstart; i < blockend; i++) {
+        int index1 = piTriList_in_and_out[indices[i]];
+        const SVec3 vP = GetPosition(pContext, index1);
+        const SVec3 vN = GetNormal(pContext, index1);
+        const SVec3 vT = GetTexCoord(pContext, index1);
+        for (int i2 = i + 1; i2 < blockend; i2++) {
+          int index2 = piTriList_in_and_out[indices[i2]];
+          if (index1 == index2)
+            continue;
+
+          if (veq(vP, GetPosition(pContext, index2)) && veq(vN, GetNormal(pContext, index2)) &&
+              veq(vT, GetTexCoord(pContext, index2))) {
+            piTriList_in_and_out[indices[i2]] = index1;
+            /* Once i2>i has been identified as a duplicate, we can stop since any
+             * i3>i2>i that is a duplicate of i (and therefore also i2) will also be
+             * compared to i2 and therefore be identified there anyways. */
+            break;
+          }
         }
       }
     }
@@ -645,7 +648,8 @@ static int GenerateInitialVerticesIndexList(STriInfo pTriInfos[],
 {
   int iTSpacesOffs = 0, f = 0, t = 0;
   int iDstTriIndex = 0;
-  for (f = 0; f < pContext->m_pInterface->m_getNumFaces(pContext); f++) {
+  const int iNrFaces = pContext->m_pInterface->m_getNumFaces(pContext);
+  for (f = 0; f < iNrFaces; f++) {
     const int verts = pContext->m_pInterface->m_getNumVerticesOfFace(pContext, f);
     if (verts != 3 && verts != 4)
       continue;