On Mon, Apr 27, 2020 at 6:09 PM Matthias Kretz <m.kr...@gsi.de> wrote:
>
> Hi,
>
> This documentation change clarifies the effect of -ffinite-math-only. With the
> current documentation, it is unclear what the presence of NaN and Inf
> representations means if (arithmetic) operations on such values are
> unspecified and even classification functions like isnan are unreliable. If
> the hardware thinks a certain bit pattern is a NaN, but the software assumes a
> NaN value cannot ever exist, it is questionable whether, from a language
> viewpoint, a representation for NaNs really exists. Because, a NaN is defined
> by its behavior. This change also clarifies that isnan(nan) returning false is
> fine.
>
> This relates to PR84949.
>
> * doc/invoke.texi: Clarify the effects of -ffinite-math-only.
> ---
> gcc/doc/invoke.texi | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index a37a2ee9c19..9e76ab057a9 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -11619,8 +11619,10 @@ The default is @option{-fno-reciprocal-math}.
>
> @item -ffinite-math-only
> @opindex ffinite-math-only
> -Allow optimizations for floating-point arithmetic that assume
> -that arguments and results are not NaNs or +-Infs.
> +Assume that floating-point types in the language do not have representations
> for
> +NaNs and +-Inf. Whether floating-point hardware supports and acts on NaNs and
> ++-Inf is not affected. The behavior of a program that uses a NaN or +-Inf
> value
> +as function argument, macro argument, or operand is undefined.
Minor nit here - I'd avoid the 'undefined' word which has bad connotation
and use 'unspecified'. Maybe we can even use ISO C language specification
terms but I'm not sure which one is most appropriate here.
Curiously __builtin_nan ("nan") still gets you a NaN representation
but isnan(__builtin_nan("nan")) is resolved to false.
> This option is not turned on by any @option{-O} option since
> it can result in incorrect output for programs that depend on
> --
> ──────────────────────────────────────────────────────────────────────────
> Dr. Matthias Kretz https://mattkretz.github.io
> GSI Helmholtz Centre for Heavy Ion Research https://gsi.de
> std::experimental::simd https://github.com/VcDevel/std-simd
> ──────────────────────────────────────────────────────────────────────────
>
