a small C (naive) program faster with clang than with gcc

a small C (naive) program faster with clang than with gcc

[Basile Starynkevitch]

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Hello all,


Consider the naive program (GPLv3+) to solve the cryptaddition

`NEUF` + `DEUX` = `ONZE`

onhttps://github.com/bstarynk/misc-basile/blob/master/CryptArithm/neuf%2Bdeux%3Donze/naive0.c  (commit0d1bd0e 
<https://github.com/bstarynk/misc-basile/commit/0d1bd0ea8e2708036fcf126cc1f096be888360ab>)


On Linux/x86-64 that source code compiled with gcc-12 -O3 is twice as 
slower as with clang -O3

(Debian/Sid or Ubuntu/22/10)

Feel free to add it to some testsuite!


Thanks


-- 
Basile Starynkevitch<basile@starynkevitch.net>
(only mine opinions / les opinions sont miennes uniquement)
92340 Bourg-la-Reine, France
web page: starynkevitch.net/Basile/ & refpersys.org

Re: a small C (naive) program faster with clang than with gcc

[Andy]

I see it in godbolt
GCC compiles to:
movsx eax, BYTE PTR [rdi+2]
cmp al, 9
ja .L42
Clang:
movzx edx, byte ptr [rdi + 2]
cmp edx, 9
ja .LBB0_40


GCC extend with sign, Clang with zero.
cmp with 32 bit register is apparently faster than 8bit

pon., 24 kwi 2023 o 17:34 Basile Starynkevitch
<basile@starynkevitch.net> napisał(a):
>
> Hello all,
>
>
> Consider the naive program (GPLv3+) to solve the cryptaddition
>
> `NEUF` + `DEUX` = `ONZE`
>
> onhttps://github.com/bstarynk/misc-basile/blob/master/CryptArithm/neuf%2Bdeux%3Donze/naive0.c  (commit0d1bd0e
> <https://github.com/bstarynk/misc-basile/commit/0d1bd0ea8e2708036fcf126cc1f096be888360ab>)
>
>
> On Linux/x86-64 that source code compiled with gcc-12 -O3 is twice as
> slower as with clang -O3
>
> (Debian/Sid or Ubuntu/22/10)
>
> Feel free to add it to some testsuite!
>
>
> Thanks
>
>
> --
> Basile Starynkevitch<basile@starynkevitch.net>
> (only mine opinions / les opinions sont miennes uniquement)
> 92340 Bourg-la-Reine, France
> web page: starynkevitch.net/Basile/ & refpersys.org

Re: a small C (naive) program faster with clang than with gcc

[LIU Hao]

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在 2023/4/26 00:01, Andy via Gcc 写道:
> I see it in godbolt
> GCC compiles to:
> movsx eax, BYTE PTR [rdi+2]
> cmp al, 9
> ja .L42
> Clang:
> movzx edx, byte ptr [rdi + 2]
> cmp edx, 9
> ja .LBB0_40
> 
> 
> GCC extend with sign, Clang with zero.
> cmp with 32 bit register is apparently faster than 8bit

As for extension, it seems to make a difference only if the result is ever written back to memory. 
And for comparison, it makes completely no difference whether the operand is 32-bit or 8-bit, except 
when the operand is an 8-bit ?H register. [1]


[1] https://uops.info/table.html


-- 
Best regards,
LIU Hao


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Re: a small C (naive) program faster with clang than with gcc

[Gabriel Paubert]

On Tue, Apr 25, 2023 at 06:01:22PM +0200, Andy via Gcc wrote:
> I see it in godbolt
> GCC compiles to:
> movsx eax, BYTE PTR [rdi+2]
> cmp al, 9
> ja .L42
> Clang:
> movzx edx, byte ptr [rdi + 2]
> cmp edx, 9
> ja .LBB0_40
> 
> 
> GCC extend with sign, Clang with zero.
> cmp with 32 bit register is apparently faster than 8bit

What happens if you compile with -funsigned-char?

There may be also some alignment issue, after all cmp al,9 is 2 bytes
while cmp edx,9 is 6.

	Gabriel

> 
> pon., 24 kwi 2023 o 17:34 Basile Starynkevitch
> <basile@starynkevitch.net> napisał(a):
> >
> > Hello all,
> >
> >
> > Consider the naive program (GPLv3+) to solve the cryptaddition
> >
> > `NEUF` + `DEUX` = `ONZE`
> >
> > onhttps://github.com/bstarynk/misc-basile/blob/master/CryptArithm/neuf%2Bdeux%3Donze/naive0.c  (commit0d1bd0e
> > <https://urldefense.com/v3/__https://github.com/bstarynk/misc-basile/commit/0d1bd0ea8e2708036fcf126cc1f096be888360ab__;!!D9dNQwwGXtA!TTyEodY6pBOA7p844eEtBYxzayqn8ZjEYjc-4GVTZaYJTYVSspXkfjZTE1oDYSJzVhSTOYPQbo8$ >)
> >
> >
> > On Linux/x86-64 that source code compiled with gcc-12 -O3 is twice as
> > slower as with clang -O3
> >
> > (Debian/Sid or Ubuntu/22/10)
> >
> > Feel free to add it to some testsuite!
> >
> >
> > Thanks
> >
> >
> > --
> > Basile Starynkevitch<basile@starynkevitch.net>
> > (only mine opinions / les opinions sont miennes uniquement)
> > 92340 Bourg-la-Reine, France
> > web page: starynkevitch.net/Basile/ & refpersys.org