[InstCombine] Don't sink dynamic allocas
Summary:
InstCombine's sinking algorithm only thinks about memory. It doesn't
think about non-memory constraints like stack object lifetime. It can
sink dynamic allocas across a stacksave call, which may be used with
stackrestore, which can incorrectly reduce the lifetime of the dynamic
alloca.
Fixes PR40365
Reviewers: hfinkel, efriedma
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D56872
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351475 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index be7d43b..f530ee1 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3065,9 +3065,11 @@
I->isTerminator())
return false;
- // Do not sink alloca instructions out of the entry block.
- if (isa<AllocaInst>(I) && I->getParent() ==
- &DestBlock->getParent()->getEntryBlock())
+ // Do not sink static or dynamic alloca instructions. Static allocas must
+ // remain in the entry block, and dynamic allocas must not be sunk in between
+ // a stacksave / stackrestore pair, which would incorrectly shorten its
+ // lifetime.
+ if (isa<AllocaInst>(I))
return false;
// Do not sink into catchswitch blocks.
diff --git a/test/Transforms/InstCombine/sink-alloca.ll b/test/Transforms/InstCombine/sink-alloca.ll
new file mode 100644
index 0000000..f2de74f
--- /dev/null
+++ b/test/Transforms/InstCombine/sink-alloca.ll
@@ -0,0 +1,52 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i686-unknown-linux-gnu"
+
+; Check that instcombine doesn't sink dynamic allocas across llvm.stacksave.
+
+; Helper to generate branch conditions.
+declare i1 @cond()
+
+declare i32* @use_and_return(i32*)
+
+declare i8* @llvm.stacksave() #0
+
+declare void @llvm.stackrestore(i8*) #0
+
+define void @foo(i32 %x) {
+entry:
+ %c1 = call i1 @cond()
+ br i1 %c1, label %ret, label %nonentry
+
+nonentry: ; preds = %entry
+ %argmem = alloca i32, i32 %x, align 4
+ %sp = call i8* @llvm.stacksave()
+ %c2 = call i1 @cond()
+ br i1 %c2, label %ret, label %sinktarget
+
+sinktarget: ; preds = %nonentry
+ ; Arrange for there to be a single use of %argmem by returning it.
+ %p = call i32* @use_and_return(i32* nonnull %argmem)
+ store i32 13, i32* %p, align 4
+ call void @llvm.stackrestore(i8* %sp)
+ %0 = call i32* @use_and_return(i32* %p)
+ br label %ret
+
+ret: ; preds = %sinktarget, %nonentry, %entry
+ ret void
+}
+
+; CHECK-LABEL: define void @foo(i32 %x)
+; CHECK: nonentry:
+; CHECK: %argmem = alloca i32, i32 %x
+; CHECK: %sp = call i8* @llvm.stacksave()
+; CHECK: %c2 = call i1 @cond()
+; CHECK: br i1 %c2, label %ret, label %sinktarget
+; CHECK: sinktarget:
+; CHECK: %p = call i32* @use_and_return(i32* nonnull %argmem)
+; CHECK: store i32 13, i32* %p
+; CHECK: call void @llvm.stackrestore(i8* %sp)
+; CHECK: %0 = call i32* @use_and_return(i32* %p)
+
+attributes #0 = { nounwind }