[PATCH] D136201: [InstCombine] Handle PHI nodes when eliminating constant memcpy

Mon Oct 31 14:29:27 PDT 2022

gandhi21299 added a comment.

In D136201#3896724 <https://reviews.llvm.org/D136201#3896724>, @nikic wrote:

>> I believe the change to isOnlyCopiedFromConstantMemory() is fine
>
> I think I spoke too soon ... I can't test this right now, but probably this minor variant is going to be miscompiled:
>
>   define i32 @test1(i1 %c, ptr %ptr) {
>   entry:
>     %a = alloca [32 x i8]
>     br i1 %c, label %if, label %join
>   
>   if:
>     br label %join
>   
>   join:
>     %phi = phi ptr [ %a, %if ], [ %ptr, %entry ]
>     call void @llvm.memcpy.p0.p0.i64(ptr %phi, ptr @g1, i64 32, i1 false)
>     %v = load i32, ptr %phi
>     ret i32 %v
>   }
>
> While we can replace uses of `%a` with `@g1`, dropping the memcpy would be incorrect, as it would also drop the potential write to `%ptr`, which is not covered by the replacement.

It compiles correctly. The output does not differ from the input at all.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136201/new/

https://reviews.llvm.org/D136201