[llvm] [BasicAA] Treat returns_twice functions as clobbering unescaped objects (PR #117902)

via llvm-commits llvm-commits at lists.llvm.org
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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