[PATCH] D63782: [FPEnv] Add fptosi and fptoui constrained intrinsics

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

> If we say the result of the constrained intrinsic is poison, will the fact that the intrinsic is defined as having side effects keep it from being eliminated?

If a program uses a poison value in certain ways (defined in LangRef), the behavior of the program as a whole is undefined. So the optimizer can generate code that does anything in that case; most likely, it will erase the entire basic block containing the call. Side-effects like modifying the FP exception bits don't matter.

That said, if llvm.experimental.constrained.fptosi.i32.f64 is allowed to raise an unmasked exception, that would keep it from being eliminated: the return value would never be used.


Repository:
  rL LLVM

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  https://reviews.llvm.org/D63782/new/

https://reviews.llvm.org/D63782