Re: pragma GCC optimize prevents inlining

[Hashan Gayasri <hashan.gayasri@gmail.com>]

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Hi Hao,

Thanks for the suggestion! Yes, - fno-math-errno is definitely more
suitable for enabling globally than -ffast-math.

While I don't particularly remember using errno for math functions in
particularm it's used in non-math functions. So eventhough it seems
reasonable to be enabled globally, still a bit tricky to validate that it
doesn't cause any unintentional side effects with a large codebase with 3rd
parety libs.

Another weird side effect I noticed is GCC still doesn't inline the
function when the function is within a `pragma GCC optimize
("-fno-math-errno") ` region and -ffast-math is enabled globally eventhough
-fno-math-errno is a subset. If you enable both -ffast-math and
-fno-math-errno, globally, the function gets inlined.

I'm not sure if improving that should be considered as a bug-fix or a
feature/enhancement.

Best Regards,
Hashan Gayasri

On Thu, 4 Jan 2024, 8:28 pm LIU Hao, <lh_mouse@126.com> wrote:

> 在 2024/1/4 17:01, Hashan Gayasri via Gcc-help 写道:
> > I wanted the following to be to be optimized:
> >
> > (... ...)
> >
> > So that std::lrint uses the vcvtsd2si instruction on X86 with SSE2.  It
> > does that but prevents the instruction from being inlined. I complied
> with
> > - O3 -march=native -DNDEBUG.
>
> Actually `-ffast-math` is an overkill; `-fno-math-errno` isn't practically
> bad, and can be enabled
> globally:
> (https://gcc.godbolt.org/z/hhfP6cYrr)
>
> ```
> //#pragma GCC push_options
> //#pragma GCC optimize ("-ffast-math")
>
> inline int64_t __attribute__ ((const)) RoundToNearestLong (double v)
> {
>     // assert(fegetround() == FE_TONEAREST);
>      return lrint(v);
> }
>
> //#pragma GCC pop_options
>
> void
> xgset(int64_t& r, double s)
>    {
>      r = RoundToNearestLong(s);
>    }
> ```
>
> results in
> ```
> xgset(long&, double):
>          vcvtsd2si       rax, xmm0
>          mov     QWORD PTR [rdi], rax
>          ret
> ```
>
>
>
> --
> Best regards,
> LIU Hao
>
>