| Issue |
79137
|
| Summary |
[LoopVectorize] Miscompile with iteration-local scoped alias metadata
|
| Labels |
miscompilation,
vectorization
|
| Assignees |
|
| Reporter |
nikic
|
C reproducer: https://clang.godbolt.org/z/f6rE4jvhM
IR reproducer: https://llvm.godbolt.org/z/7anbPon8K
```llvm
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
define hidden void @test(ptr %arg, i64 %num) {
entry:
%icmp = icmp ult i64 %num, 2
br i1 %icmp, label %exit, label %preheader
preheader:
%arg.1 = getelementptr inbounds i8, ptr %arg, i64 1
%end = add i64 %num, -2
br label %loop
loop:
%prev.ptr = phi ptr [ %cur.ptr, %loop ], [ %arg, %preheader ]
%iv = phi i64 [ %iv.next, %loop ], [ 0, %preheader ]
%cur.ptr = getelementptr inbounds i8, ptr %arg.1, i64 %iv
call void @llvm.experimental.noalias.scope.decl(metadata !0)
call void @llvm.experimental.noalias.scope.decl(metadata !3)
%load.prev = load i8, ptr %prev.ptr, align 1, !alias.scope !0, !noalias !3
%load.cur = load i8, ptr %cur.ptr, align 1, !alias.scope !3
%add = add i8 %load.cur, %load.prev
store i8 %add, ptr %cur.ptr, align 1, !alias.scope !3
%iv.next = add nuw i64 %iv, 1
%cmp = icmp eq i64 %iv, %end
br i1 %cmp, label %exit, label %loop
exit:
ret void
}
; Function Attrs: nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite)
declare void @llvm.experimental.noalias.scope.decl(metadata) #0
attributes #0 = { nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite) }
!0 = !{!1}
!1 = distinct !{!1, !2}
!2 = distinct !{!2}
!3 = !{!4}
!4 = distinct !{!4, !5}
!5 = distinct !{!5}
```
The current and previous value are only noalias within the iteration (per the location of the `@llvm.experimental.noalias.scope.decl`), but the vectorization appears to assume that the underlying objects don't alias.
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