[PATCH] D64746: Add constrained intrinsics for lrint and lround

[Ulrich Weigand via Phabricator via llvm-commits]

uweigand added a comment.

In D64746#3324418 <https://reviews.llvm.org/D64746#3324418>, @nlopes wrote:

> In D64746#3324270 <https://reviews.llvm.org/D64746#3324270>, @uweigand wrote:
>
>> This matches my understanding.   (Though I must admit I'm not 100% confident I fully understand the precise distinction between "undefined behavior", a poison value, and an undef value ...  It's certainly one of those :-)  But from my reading of the IR reference, "poison" does indeed look correct here.)
>
> OK, thank you!
> It's poison. UB would be too strong. I've put on my todo list to fix LangRef as well.

The reason why I was hesitating about poison vs. UB is exceptions.   This is clearer when talking about the exception metadata as opposed to the rounding mode metadata:  if exception metadata promises that FP exceptions are disabled, but they are actually enabled at runtime, compiler optimizations are free to generate code triggering spurious exceptions that might kill the program outright - that seems more like UB than poison to me.

With the rounding mode, this is much less obvious to me, but I guess in theory it could happen that if the actual rounding mode does not match the mode promised by the metadata, a compiler optimization might possibly generate code that could now e.g. introduce an underflow where the unoptimized code would not have one, and if in addition exception are also enabled, that extra underflow could now trigger an extra exception.   However, I'm not sure if that can actually ever occur in practice ...


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