[PATCH] D90529: Allow nonnull attribute to accept poison

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

To share my understanding about how nonnull is used in optimizations:

1. It is mainly used to fold away null pointer checks.

  define void @f(i8* nonnull %p) {
    if (icmp eq %p, null) ret void         ; this check can be folded to true
    ...
  }

Note that the new nonnull-or-poison semantics is fine in the above case because if %p is poison we can fold `icmp eq %p, null` into false.

A similar but slightly different case:

  ...
  call void @f(i8* nonnull %p)
  if (icmp eq %p, null) ret void

In this case, the nonnull-or-poison semantics cannot make the comparison after call folded, because f may not use the poison argument at all, making the program well-defined.
But, if f has noundef attribute as well, this is fine because passing null to f is now UB:

  ...
  call void @f(i8* noundef nonnull %p)
  if (icmp eq %p, null) ret void            ; %p is always non-null pointer!



2. It is combined with `dereferenceable_or_null` to infer `dereferenceable`.

This is still fine with the new nonnull-or-poison because passing poison to `dereferenceable_or_null` is still UB, guaranteeing that the pointer is never poison.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D90529/new/

https://reviews.llvm.org/D90529