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* [Bug c++/107458] New: std::fma generates slow scalar-call
@ 2022-10-29 23:33 g.peterhoff@t-online.de
2022-10-30 0:00 ` [Bug c++/107458] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: g.peterhoff@t-online.de @ 2022-10-29 23:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107458
Bug ID: 107458
Summary: std::fma generates slow scalar-call
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: g.peterhoff@t-online.de
Target Milestone: ---
Please see https://godbolt.org/z/bxxc9ezeM
thx
Gero
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/107458] std::fma generates slow scalar-call
2022-10-29 23:33 [Bug c++/107458] New: std::fma generates slow scalar-call g.peterhoff@t-online.de
@ 2022-10-30 0:00 ` pinskia at gcc dot gnu.org
2022-10-30 0:02 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-30 0:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107458
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 53790
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53790&action=edit
testcase -march=x86-64-v2 -O3 -std=c++20 -Wall -mno-vzeroupper
Please attach the testcase next time instead of just linking to godbolt.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/107458] std::fma generates slow scalar-call
2022-10-29 23:33 [Bug c++/107458] New: std::fma generates slow scalar-call g.peterhoff@t-online.de
2022-10-30 0:00 ` [Bug c++/107458] " pinskia at gcc dot gnu.org
@ 2022-10-30 0:02 ` pinskia at gcc dot gnu.org
2022-10-30 0:11 ` pinskia at gcc dot gnu.org
2022-10-30 0:13 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-30 0:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107458
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
fmaf80_manually, fmaf64_manually, and fmaf32_manually are not exactly fma.
at least for x86-64-v2 due not doing the add before the rounding of the
multiply.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/107458] std::fma generates slow scalar-call
2022-10-29 23:33 [Bug c++/107458] New: std::fma generates slow scalar-call g.peterhoff@t-online.de
2022-10-30 0:00 ` [Bug c++/107458] " pinskia at gcc dot gnu.org
2022-10-30 0:02 ` pinskia at gcc dot gnu.org
@ 2022-10-30 0:11 ` pinskia at gcc dot gnu.org
2022-10-30 0:13 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-30 0:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107458
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Your definition of fma_manually is broken really.
The following is closer to being correct though there is still an extra
rounding step so it is still not 100%.
inline constexpr float fma_manually(const float a, const float b, const float
c) noexcept
{
double
res = a;
res *= b;
res += c;
return res;
}
inline constexpr double fma_manually(const double a, const double b, const
double
c) noexcept
{
long double
res = a;
res *= b;
res += c;
return res;
}
But GCC does the correct thing for fma and fmaf with x86-64-v3 and use the fma
instruction.
So closing as invalid.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/107458] std::fma generates slow scalar-call
2022-10-29 23:33 [Bug c++/107458] New: std::fma generates slow scalar-call g.peterhoff@t-online.de
` (2 preceding siblings ...)
2022-10-30 0:11 ` pinskia at gcc dot gnu.org
@ 2022-10-30 0:13 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-30 0:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107458
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The C++ standard defines fma as doing (a*b) + c in infinite precision (and then
round), this is why GCC cannot inline if your target does not have a FMA
instruction.
There is still no bug here.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-10-30 0:13 UTC | newest]
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2022-10-29 23:33 [Bug c++/107458] New: std::fma generates slow scalar-call g.peterhoff@t-online.de
2022-10-30 0:00 ` [Bug c++/107458] " pinskia at gcc dot gnu.org
2022-10-30 0:02 ` pinskia at gcc dot gnu.org
2022-10-30 0:11 ` pinskia at gcc dot gnu.org
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