[llvm-dev] [cfe-dev] Intrinsic llvm::isnan (original) (raw)

James Y Knight via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 24 14:25:02 PDT 2021

On Tue, Aug 24, 2021 at 1:53 PM Roman Lebedev via cfe-dev < cfe-dev at lists.llvm.org> wrote:

Regardless of everything, i would like to see a patch that restores the -ffast-math handling, and then the RFC on what the is-nan check should do when -ffast-math is present. It is more than possible that the RFC will succeed, but i don't think a change like that should happen the way it did.

I find the rationale to be convincing, as to the need for a change. But, the scope of the proposal is too narrow. We cannot discuss fast-math semantics changes only for "isnan", it needs to be in the context of the desired behavior for all operations -- the RFC should cover the entire set of changes we want to eventually make, even if isnan is the only thing implemented so far. Discussing this greater scope could result in a different desired implementation, rather than simply adding "llvm.isnan" intrinsic.

Yet, even with that expanded scope, the two halves of the proposal are still going to be closely linked, so I suspect it still makes sense to discuss both the strict-fp and fast-math changes in a single RFC.

Anyhow, for the fast-math section, I believe the proposed semantics ought to be: The -ffinite-math-only and -fno-signed-zeros options do not impact the ability to accurately load, store, copy, or pass or return such values from general function calls. They also do not impact any of the "non-computational" and "quiet-computational" IEEE-754 operations, which includes classification functions (fpclassify, signbit, isinf/isnan/etc), sign-modification (copysign, fabs, and negation -(x)), as well as the totalorder and totalordermag functions. Those correctly handle NaN, Inf, and signed zeros even when the flags are in effect. These flags do affect the behavior of other expressions and math standard-library calls, as well as comparison operations.

I would not expect this to have an actual negative impact on the performance benefit of those flags, since the optimization benefits mainly arise from comparisons and the general computation instructions which are unchanged.

https://gcc.gnu.org/bugzilla/showbug.cgi?id=84949 was mentioned as motivational, but i don't see any resolution there, it's not even in "confirmed" state.

I agree, this is not at all clear evidence as to GCC's position on the matter. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210824/6bf95a8f/attachment.html>

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