[PATCH] D151644: [InstCombine] Propegating `nocapture` flag to callsites

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D151644#4378838 <https://reviews.llvm.org/D151644#4378838>, @goldstein.w.n wrote:

> It should still work then in the case that the function doesn't interact with the global state after
> the call?
> So we could reverse iterate and while the sequence remains `pure`, it should be appliable?

I don't think it's quite as simple as that. Here's another example:

  define void @test(ptr nocapture %p) {
    %a = alloca ptr
    call void @store_into_alloca(ptr %a, ptr %p)
    ret void
  }

The call-site is not no-capture, even though there are no memory accesses after it.

> My motivation is something like: https://godbolt.org/z/b1Pjf4bjP
>
> where AFAICT there is no way to write a syscall wrapper like that and actually preserve nocapture. Maybe a better approach?

So yeah, the case where we're literally calling another function with the exact same arguments is clearly safe -- it's just not entirely obvious to me what the right way to generalize that would be.


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