[PATCH] D109548: Make llvm.launder.invariant.group writeonly

[Piotr Padlewski via Phabricator via llvm-commits]

Prazek requested changes to this revision.
Prazek added a comment.
This revision now requires changes to proceed.

Actually, as Krzysztof Pszeniczny pointed out offline, I think we can't mark it as writeonly, because it would let optimizer to optimize:

  %a = launder(%0)
  %b = launder(%0)

As it could assume that both launders store the same value. This could bite us in cases like (https://godbolt.org/z/YaGrrWosj):

  void foo(A *a) {
      auto *b = new(a)B;
      use(b);
      auto *c = new(a)A;
  }

Here we will have both launders use the same pointer argument (`a`), so call like `c->foo()` could be optimzied to `b->foo()` if two launders would be merged.

However, for somre reason LLVM does not do that if I mark the launder as writeonly - probably a missed optimization? (but perhaps not that important).
Does is make sense, or am I misuderstanding the writeonly?


Repository:
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https://reviews.llvm.org/D109548