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

[Kevin P. Neal via Phabricator via llvm-commits]

kpn added a comment.
Herald added subscribers: wuzish, MaskRay.

In D63782#1561585 <https://reviews.llvm.org/D63782#1561585>, @andrew.w.kaylor wrote:

> I might be opening a can of worms here and I'm not a language expert, but it isn't clear to me from reading the C99 standard that defining fptosi/fptoui as returning poison values in the unrepresentable case allows correct implementation of the C standard. That is, it doesn't seem to me that the standard actually says this is undefined behavior. It just says the resulting value is unspecified, and the exception behavior is explicitly defined. On the other hand, C++ does say clearly that it is undefined behavior, right?
>
> I understand that in the unconstrained case LLVM doesn't care about FP exceptions and that we would like the LLVM IR definition to be more precise than the C standard. I'm just trying to get my head wrapped around why we're doing what we are in that case and what we need to do to correctly implement strict FP semantics.
>
> If we say that the constrained version returns undef in the unrepresentable case and clearly emphasize how this differs from the standard fptosi/fptoui instructions, would that have the effect of keeping the operation around so that it can raise the exception while still giving us well-defined IR semantics?


Ping? Can anyone address this above comment?


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