[llvm] [DSE] Enable initializes improvement (PR #119116)

[Nikita Popov via llvm-commits]

nikic wrote:

For reference, the IR is:
```llvm

@g = global [12 x i8] zeroinitializer, align 1
@str = private constant [13 x i8] [i8 97, i8 98, i8 99, i8 100, i8 101, i8 102, i8 103, i8 104, i8 105, i8 106, i8 107, i8 108, i8 0], align 1

define void @copyGto(ptr initializes((0, 12)) %agg.result) nounwind {
entry:
  tail call void @llvm.memmove.p0.p0.i32(ptr noundef nonnull align 1 dereferenceable(12) %agg.result, ptr noundef nonnull align 1 dereferenceable(12) @g, i32 12, i1 false)
  ret void
}

define i16 @main() {
entry:
  tail call void @llvm.memcpy.p0.p0.i32(ptr noundef nonnull align 1 dereferenceable(12) @g, ptr noundef nonnull align 1 dereferenceable(12) @str, i32 12, i1 false)
  tail call void @copyGto(ptr @g)
  call void @check(ptr @g)
  ret i16 0
}

declare void @llvm.memmove.p0.p0.i32(ptr nocapture writeonly, ptr nocapture readonly, i32, i1 immarg)
declare void @llvm.memcpy.p0.p0.i32(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i32, i1 immarg)
declare void @check(ptr noundef)
```

I believe that the `initializes` attribute on copyGto is correct, at least per the current spec:

> This attribute only holds for the memory accessed via this pointer parameter. Other arbitrary accesses to the same memory via other pointers are allowed.

I think the problem here is that https://github.com/llvm/llvm-project/blob/14dcf8214f9c66172d17c1cfaec6aec0030748e0/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp#L2265 only looks at the call arguments, but doesn't account for the fact that the pointer may also be read via escaped memory (prior to initialization).

https://github.com/llvm/llvm-project/pull/119116