[PATCH] D133730: [SimplifyCFG] Don't hoist inalloca allocas

[Arthur Eubanks via Phabricator via llvm-commits]

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D129370 <https://reviews.llvm.org/D129370> started hoisting inalloca allocas across stacksave/stackrestore
boundaries which is wrong.

I looked into treating inalloca allocas implying
`I->mayHaveSideEffects()`, but that breaks various things like MemorySSA
and SLPVectorizer so I stuck with the status quo and special cased this.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133730

Files:
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Transforms/SimplifyCFG/hoist-common-skip.ll


Index: llvm/test/Transforms/SimplifyCFG/hoist-common-skip.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/hoist-common-skip.ll
+++ llvm/test/Transforms/SimplifyCFG/hoist-common-skip.ll
@@ -392,19 +392,21 @@
 define void @f10(i1 %cond) {
 ; CHECK-LABEL: @f10(
 ; CHECK-NEXT:    [[SS:%.*]] = call ptr @llvm.stacksave()
-; CHECK-NEXT:    [[SS2:%.*]] = call ptr @llvm.stacksave()
-; CHECK-NEXT:    [[I2:%.*]] = alloca inalloca i64, align 8
 ; CHECK-NEXT:    br i1 [[COND:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
 ; CHECK:       bb1:
 ; CHECK-NEXT:    [[I1:%.*]] = alloca inalloca i32, align 4
+; CHECK-NEXT:    [[SS2:%.*]] = call ptr @llvm.stacksave()
+; CHECK-NEXT:    [[I2:%.*]] = alloca inalloca i64, align 8
 ; CHECK-NEXT:    call void @inalloca_i64(ptr inalloca(i64) [[I2]])
 ; CHECK-NEXT:    call void @llvm.stackrestore(ptr [[SS2]])
 ; CHECK-NEXT:    call void @inalloca_i32(ptr inalloca(i32) [[I1]])
 ; CHECK-NEXT:    br label [[END:%.*]]
 ; CHECK:       bb2:
 ; CHECK-NEXT:    [[I3:%.*]] = alloca inalloca i64, align 8
-; CHECK-NEXT:    [[TMP1:%.*]] = call ptr @inalloca_i64(ptr inalloca(i64) [[I2]])
-; CHECK-NEXT:    call void @llvm.stackrestore(ptr [[SS2]])
+; CHECK-NEXT:    [[SS3:%.*]] = call ptr @llvm.stacksave()
+; CHECK-NEXT:    [[I4:%.*]] = alloca inalloca i64, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = call ptr @inalloca_i64(ptr inalloca(i64) [[I4]])
+; CHECK-NEXT:    call void @llvm.stackrestore(ptr [[SS3]])
 ; CHECK-NEXT:    [[TMP2:%.*]] = call ptr @inalloca_i64(ptr inalloca(i64) [[I3]])
 ; CHECK-NEXT:    br label [[END]]
 ; CHECK:       end:
Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1442,7 +1442,11 @@
   unsigned Flags = 0;
   if (I->mayReadFromMemory())
     Flags |= SkipReadMem;
-  if (I->mayHaveSideEffects())
+  // For the purposes of hoisting instructions, inalloca allocas are treated as
+  // having side effects to avoid moving them across stacksave/stackrestore
+  // boundaries.
+  if (I->mayHaveSideEffects() ||
+      (isa<AllocaInst>(I) && cast<AllocaInst>(I)->isUsedWithInAlloca()))
     Flags |= SkipSideEffect;
   if (!isGuaranteedToTransferExecutionToSuccessor(I))
     Flags |= SkipImplicitControlFlow;


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