Re: [PATCH v3 1/5] LoongArch: Fix usage of LSX and LASX frint/ftint instructions [PR112578]

[Xi Ruoyao <xry111@xry111.site>]

On Fri, 2023-11-24 at 17:46 +0800, chenglulu wrote:
> It's just that I'm confused that the description of rint in n2310, 
> including Joseph's email,

> all say that rint will not set errno, but linux-man says "which might 
> set errno to ERANGE" .

Annex F has:

The C floating types match the IEC 60559 formats as follows:
- The float type matches the IEC 60559 single format.
- The double type matches the IEC 60559 double format.

With these constraints rint and rintf just cannot have a range or domain
error.  So Annex F does not say rint may set errno.  Linux-man says:

ERRORS
       No errors occur.  POSIX.1‐2001 documents a range error  for  overflows,
       but see NOTES.

It's because POSIX does not mandates float/double to match the IEC 60559
formats, thus a range error may happen with some strange floating point
formats.  The NOTES says "this won't happen for IEC 60559 formats".

For lrint, N2310 says:

If the rounded value is outside the range of the return type, the
numeric result is unspecified and a domain error or range error may
occur.

So a EDOM or ERANGE may be set.  On the contrary, the man page says:

       The following errors can occur:

       Domain error: x is a NaN or infinite, or the rounded value is too large
              An invalid floating‐point exception (FE_INVALID) is raised.

       These functions do not set errno.

The last paragraph is Glibc-specific, and it's considered a Glibc bug
(https://sourceware.org/bugzilla/show_bug.cgi?id=6798).

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University