[llvm] [BasicAA] Treat returns_twice functions as clobbering unescaped objects (PR #117902)
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Wed Nov 27 08:29:39 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Nikita Popov (nikic)
<details>
<summary>Changes</summary>
Effectively this models all the accesses that occur between the first and second return as happening at the point of the call.
I left this a generic check, though at least for setjmp and C semantics specifically, we could skip allocas, as these are required to use volatile accesses when modified between setjmp and longjmp.
Fixes https://github.com/llvm/llvm-project/issues/116668.
---
Full diff: https://github.com/llvm/llvm-project/pull/117902.diff
2 Files Affected:
- (modified) llvm/lib/Analysis/BasicAliasAnalysis.cpp (+5-1)
- (modified) llvm/test/Transforms/GVN/setjmp.ll (+2-2)
``````````diff
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 381fb7bbdb5171..e5f71753005f42 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -947,8 +947,12 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
//
// Make sure the object has not escaped here, and then check that none of the
// call arguments alias the object below.
+ //
+ // We model calls that can return twice (setjmp) as clobbering non-escaping
+ // objects, to model any accesses that may occur prior to the second return.
if (!isa<Constant>(Object) && Call != Object &&
- AAQI.CA->isNotCapturedBefore(Object, Call, /*OrAt*/ false)) {
+ AAQI.CA->isNotCapturedBefore(Object, Call, /*OrAt*/ false) &&
+ !Call->hasFnAttr(Attribute::ReturnsTwice)) {
// Optimistically assume that call doesn't touch Object and check this
// assumption in the following loop.
diff --git a/llvm/test/Transforms/GVN/setjmp.ll b/llvm/test/Transforms/GVN/setjmp.ll
index 0277fcfa226ed6..0ebe24879d320c 100644
--- a/llvm/test/Transforms/GVN/setjmp.ll
+++ b/llvm/test/Transforms/GVN/setjmp.ll
@@ -5,7 +5,6 @@ declare i32 @setjmp() returns_twice
declare void @longjmp()
declare ptr @malloc(i64)
-; FIXME: This is a miscompile.
define i32 @test() {
; CHECK-LABEL: define i32 @test() {
; CHECK-NEXT: [[MALLOC:%.*]] = call noalias ptr @malloc(i64 4)
@@ -18,7 +17,8 @@ define i32 @test() {
; CHECK-NEXT: call void @longjmp()
; CHECK-NEXT: unreachable
; CHECK: [[IF_END]]:
-; CHECK-NEXT: ret i32 10
+; CHECK-NEXT: [[RES:%.*]] = load i32, ptr [[MALLOC]], align 4
+; CHECK-NEXT: ret i32 [[RES]]
;
%malloc = call noalias ptr @malloc(i64 4)
store i32 10, ptr %malloc, align 4
``````````
</details>
https://github.com/llvm/llvm-project/pull/117902
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