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[InstCombine] Don't sink dynamic allocas

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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

llvm-svn: 351475
This commit is contained in:
Reid Kleckner 2019-01-17 20:46:53 +00:00
parent 5094e5ef8b
commit edd653bc07
2 changed files with 57 additions and 3 deletions
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8
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
View File

@ -3065,9 +3065,11 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock) {
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.

52
llvm/test/Transforms/InstCombine/sink-alloca.ll Normal file
View File

@ -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 }