[SimpleLoopUnswitch] Forget SCEVs for replaced phis.

Forget SCEVs based on exit phis in case SCEV looked through the phi.
After unswitching, it may not be possible to look through the phi due to
it having multiple incoming values, so it needs to be re-computed.

Fixes #58868

2 files changed, 80 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index f7ecc4794715..f6f3c8fffb8b 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -1114,7 +1114,8 @@ static BasicBlock *buildClonedLoopBlocks(
     const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
     ValueToValueMapTy &VMap,
     SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
-    DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
+    DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU,
+    ScalarEvolution *SE) {
   SmallVector<BasicBlock *, 4> NewBlocks;
   NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
 
@@ -1190,6 +1191,10 @@ static BasicBlock *buildClonedLoopBlocks(
       // We should have a value map between the instruction and its clone.
       assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
 
+      // Forget SCEVs based on exit phis in case SCEV looked through the phi.
+      if (SE && isa<PHINode>(I))
+        SE->forgetValue(&I);
+
       auto *MergePN =
           PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
                           &*MergeBB->getFirstInsertionPt());
@@ -2218,7 +2223,7 @@ static void unswitchNontrivialInvariants(
     VMaps.emplace_back(new ValueToValueMapTy());
     ClonedPHs[SuccBB] = buildClonedLoopBlocks(
         L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
-        DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
+        DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU, SE);
   }
 
   // Drop metadata if we may break its semantics by moving this instr into the
diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/pr58868-scev-invalidation-for-phis.ll b/llvm/test/Transforms/SimpleLoopUnswitch/pr58868-scev-invalidation-for-phis.ll
new file mode 100644
index 000000000000..2a8553c69c40
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/pr58868-scev-invalidation-for-phis.ll
@@ -0,0 +1,73 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes="loop-mssa(loop-deletion,simple-loop-unswitch<nontrivial;trivial>),loop(indvars)" -verify-scev -S %s | FileCheck %s
+
+define void @pr58868(i1 %c.1, ptr %src, ptr %dst) {
+; CHECK-LABEL: @pr58868(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[C_1:%.*]], label [[ENTRY_SPLIT_US:%.*]], label [[ENTRY_SPLIT:%.*]]
+; CHECK:       entry.split.us:
+; CHECK-NEXT:    br label [[OUTER_HEADER_US:%.*]]
+; CHECK:       outer.header.us:
+; CHECK-NEXT:    br label [[OUTER_BB1_US:%.*]]
+; CHECK:       outer.bb1.us:
+; CHECK-NEXT:    br label [[OUTER_BB2_US:%.*]]
+; CHECK:       outer.bb2.us:
+; CHECK-NEXT:    [[LV_US:%.*]] = load i16, ptr [[SRC:%.*]], align 1
+; CHECK-NEXT:    br i1 true, label [[OUTER_HEADER_LOOPEXIT_US:%.*]], label [[OUTER_BB2_SPLIT_SPLIT_US:%.*]]
+; CHECK:       outer.header.loopexit.us:
+; CHECK-NEXT:    br label [[OUTER_HEADER_US]]
+; CHECK:       outer.bb2.split.split.us:
+; CHECK-NEXT:    [[LV_LCSSA_US:%.*]] = phi i16 [ [[LV_US]], [[OUTER_BB2_US]] ]
+; CHECK-NEXT:    br label [[OUTER_BB2_SPLIT:%.*]]
+; CHECK:       entry.split:
+; CHECK-NEXT:    br label [[OUTER_HEADER:%.*]]
+; CHECK:       outer.header.loopexit:
+; CHECK-NEXT:    unreachable
+; CHECK:       outer.header:
+; CHECK-NEXT:    br label [[OUTER_BB2:%.*]]
+; CHECK:       outer.bb2:
+; CHECK-NEXT:    [[LV:%.*]] = load i16, ptr [[SRC]], align 1
+; CHECK-NEXT:    br i1 false, label [[OUTER_HEADER_LOOPEXIT:%.*]], label [[OUTER_BB2_SPLIT_SPLIT:%.*]]
+; CHECK:       outer.bb2.split.split:
+; CHECK-NEXT:    [[LV_LCSSA:%.*]] = phi i16 [ [[LV]], [[OUTER_BB2]] ]
+; CHECK-NEXT:    br label [[OUTER_BB2_SPLIT]]
+; CHECK:       outer.bb2.split:
+; CHECK-NEXT:    [[DOTUS_PHI:%.*]] = phi i16 [ [[LV_LCSSA]], [[OUTER_BB2_SPLIT_SPLIT]] ], [ [[LV_LCSSA_US]], [[OUTER_BB2_SPLIT_SPLIT_US]] ]
+; CHECK-NEXT:    br label [[INNER_HEADER:%.*]]
+; CHECK:       inner.header:
+; CHECK-NEXT:    [[IV:%.*]] = phi i16 [ [[DOTUS_PHI]], [[OUTER_BB2_SPLIT]] ], [ [[IV_NEXT:%.*]], [[INNER_LATCH:%.*]] ]
+; CHECK-NEXT:    br label [[INNER_LATCH]]
+; CHECK:       inner.latch:
+; CHECK-NEXT:    [[IV_NEXT]] = add nsw i16 [[IV]], 1
+; CHECK-NEXT:    store i16 0, ptr [[DST:%.*]], align 1
+; CHECK-NEXT:    [[C_2:%.*]] = icmp ne i16 [[IV]], 0
+; CHECK-NEXT:    br i1 [[C_2]], label [[INNER_HEADER]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  br i1 %c.1, label %outer.bb1, label %outer.bb2
+
+outer.bb1:
+  br label %outer.bb2
+
+outer.bb2:
+  %lv = load i16, ptr %src, align 1
+  br label %inner.header
+
+inner.header:
+  %iv = phi i16 [ %lv, %outer.bb2 ], [ %iv.next, %inner.latch ]
+  br i1 %c.1, label %outer.header, label %inner.latch
+
+inner.latch:
+  %iv.next = add nsw i16 %iv, 1
+  store i16 0, ptr %dst, align 1
+  %c.2 = icmp ne i16 %iv, 0
+  br i1 %c.2, label %inner.header, label %exit
+
+exit:
+  ret void
+}