[llvm] [LICM] Invalidate cached SCEV results after reassociation (PR #92655)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Antonio Frighetto (antoniofrighetto)

<details>
<summary>Changes</summary>

After reassociating expressions, LICM is required to invalidate SCEV results, as otherwise it could lead subsequent passes in the pipeline (e.g., IndVars) to miscompile.

Fixes: https://github.com/llvm/llvm-project/issues/91957.

---
Full diff: https://github.com/llvm/llvm-project/pull/92655.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/LICM.cpp (+7-5) 
- (added) llvm/test/Transforms/LICM/update-scev-after-hoist.ll (+24) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 6aa4188d1cc4d..d5716c5cda59d 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -194,7 +194,7 @@ static bool pointerInvalidatedByBlock(BasicBlock &BB, MemorySSA &MSSA,
 static bool hoistArithmetics(Instruction &I, Loop &L,
                              ICFLoopSafetyInfo &SafetyInfo,
                              MemorySSAUpdater &MSSAU, AssumptionCache *AC,
-                             DominatorTree *DT);
+                             DominatorTree *DT, ScalarEvolution *SE);
 static Instruction *cloneInstructionInExitBlock(
     Instruction &I, BasicBlock &ExitBlock, PHINode &PN, const LoopInfo *LI,
     const LoopSafetyInfo *SafetyInfo, MemorySSAUpdater &MSSAU);
@@ -987,7 +987,7 @@ bool llvm::hoistRegion(DomTreeNode *N, AAResults *AA, LoopInfo *LI,
 
       // Try to reassociate instructions so that part of computations can be
       // done out of loop.
-      if (hoistArithmetics(I, *CurLoop, *SafetyInfo, MSSAU, AC, DT)) {
+      if (hoistArithmetics(I, *CurLoop, *SafetyInfo, MSSAU, AC, DT, SE)) {
         Changed = true;
         continue;
       }
@@ -2690,7 +2690,7 @@ static bool isReassociableOp(Instruction *I, unsigned IntOpcode,
 static bool hoistMulAddAssociation(Instruction &I, Loop &L,
                                    ICFLoopSafetyInfo &SafetyInfo,
                                    MemorySSAUpdater &MSSAU, AssumptionCache *AC,
-                                   DominatorTree *DT) {
+                                   DominatorTree *DT, ScalarEvolution *SE) {
   if (!isReassociableOp(&I, Instruction::Mul, Instruction::FMul))
     return false;
   Value *VariantOp = I.getOperand(0);
@@ -2761,6 +2761,8 @@ static bool hoistMulAddAssociation(Instruction &I, Loop &L,
       Mul = Builder.CreateFMulFMF(U->get(), Factor, Ins, "factor.op.fmul");
     U->set(Mul);
   }
+  if (SE)
+    SE->forgetValue(&I);
   I.replaceAllUsesWith(VariantOp);
   eraseInstruction(I, SafetyInfo, MSSAU);
   return true;
@@ -2769,7 +2771,7 @@ static bool hoistMulAddAssociation(Instruction &I, Loop &L,
 static bool hoistArithmetics(Instruction &I, Loop &L,
                              ICFLoopSafetyInfo &SafetyInfo,
                              MemorySSAUpdater &MSSAU, AssumptionCache *AC,
-                             DominatorTree *DT) {
+                             DominatorTree *DT, ScalarEvolution *SE) {
   // Optimize complex patterns, such as (x < INV1 && x < INV2), turning them
   // into (x < min(INV1, INV2)), and hoisting the invariant part of this
   // expression out of the loop.
@@ -2794,7 +2796,7 @@ static bool hoistArithmetics(Instruction &I, Loop &L,
   }
 
   bool IsInt = I.getType()->isIntOrIntVectorTy();
-  if (hoistMulAddAssociation(I, L, SafetyInfo, MSSAU, AC, DT)) {
+  if (hoistMulAddAssociation(I, L, SafetyInfo, MSSAU, AC, DT, SE)) {
     ++NumHoisted;
     if (IsInt)
       ++NumIntAssociationsHoisted;
diff --git a/llvm/test/Transforms/LICM/update-scev-after-hoist.ll b/llvm/test/Transforms/LICM/update-scev-after-hoist.ll
new file mode 100644
index 0000000000000..f0876c3fa00e6
--- /dev/null
+++ b/llvm/test/Transforms/LICM/update-scev-after-hoist.ll
@@ -0,0 +1,24 @@
+; RUN: opt -S -passes='loop-unroll,loop-mssa(licm),print<scalar-evolution>' -unroll-count=4 -disable-output < %s 2>&1 | FileCheck %s --check-prefix=SCEV-EXPR
+
+define i16 @main() {
+; SCEV-EXPR:      Classifying expressions for: @main
+; SCEV-EXPR-NEXT:  %mul = phi i16 [ 1, %entry ], [ %mul.n.3, %loop ]
+; SCEV-EXPR-NEXT:  -->  %mul U: [0,-15) S: [-32768,32753)		Exits: 4096		LoopDispositions: { %loop: Variant }
+; SCEV-EXPR-NEXT:  %div = phi i16 [ 32767, %entry ], [ %div.n.3, %loop ]
+; SCEV-EXPR-NEXT:  -->  %div U: [-2048,-32768) S: [-2048,-32768)		Exits: 7		LoopDispositions: { %loop: Variant }
+; SCEV-EXPR-NEXT:  %mul.n = mul i16 %mul, 8
+; SCEV-EXPR-NEXT:  -->  (8 * %mul) U: [0,-7) S: [-32768,32761)		Exits: -32768		LoopDispositions: { %loop: Variant }
+entry:
+  br label %loop
+
+loop:
+  %mul = phi i16 [ 1, %entry ], [ %mul.n, %loop ]
+  %div = phi i16 [ 32767, %entry ], [ %div.n, %loop ]
+  %mul.n = mul i16 %mul, 2
+  %div.n = sdiv i16 %div, 2
+  %cmp = icmp sgt i16 %div, 0
+  br i1 %cmp, label %loop, label %end
+
+end:
+  ret i16 %mul
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/92655