13 Mar
2019
13 Mar
'19
12:43 a.m.
Does anyone know if GCC will replace power of 2 multiplications/divisions of
unsigned integers with bit shifts?
--
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
13 Mar
13 Mar
1:09 a.m.
Will Godfrey:
Does anyone know if GCC will replace power of 2
multiplications/divisions of
unsigned integers with bit shifts?
Test on your system:
$ cat a.c
#include <stdint.h>
int main(int argc, char *argv[]) {
uint16_t b = argc * 2;
return b;
}
$ gcc -S a.c
$ cat a.s
...
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl %edi, -20(%rbp)
movq %rsi, -32(%rbp)
movl -20(%rbp), %eax
addl %eax, %eax
movw %ax, -2(%rbp)
movzwl -2(%rbp), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
...
$
It seems it just adds the value with itself here.
Regards,
/Karl Hammar
16 Mar
16 Mar
11:45 p.m.
On Wed, 13 Mar 2019 00:09:17 +0100 (CET)
karl(a)aspodata.se wrote:
Will Godfrey:
Thanks, that seems to hold true for a mixture of AMD and Intel machines.
--
It wasn't me! (Well actually, it probably was)
... the hard part is not dodging what life throws at you,
but trying to catch the good bits.
Does anyone know if GCC will replace power of 2
multiplications/divisions of
unsigned integers with bit shifts?
Test on your system:
$ cat a.c
#include <stdint.h>
int main(int argc, char *argv[]) {
uint16_t b = argc * 2;
return b;
}
$ gcc -S a.c
$ cat a.s
...
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl %edi, -20(%rbp)
movq %rsi, -32(%rbp)
movl -20(%rbp), %eax
addl %eax, %eax
movw %ax, -2(%rbp)
movzwl -2(%rbp), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
...
$
It seems it just adds the value with itself here.
Regards,
/Karl Hammar
17 Mar
17 Mar
12:11 a.m.
Will Godfrey:
On Wed, 13 Mar 2019 00:09:17 +0100 (CET)
karl(a)aspodata.se wrote:
>Will Godfrey:
>> Does anyone know if GCC will replace power of 2 multiplications/divisions of
>> unsigned integers with bit shifts?
...
>$ cat a.c
>#include <stdint.h>
>
>int main(int argc, char *argv[]) {
> uint16_t b = argc * 2;
>
> return b;
>}
>$ gcc -S a.c
,,,
>It seems it just adds the value with itself here.
...
Thanks, that seems to hold true for a mixture of AMD
and Intel machines.
BTW, testing with 4, 8 and 16 instead of 2, it does actually do bit
shifts:
$ diff a.02.s a.04.s
16c16
< addl %eax, %eax
---
sall $2, %eax
$ diff a.02.s a.08.s
16c16
< addl %eax, %eax
---
sall $3, %eax
$ diff a.02.s a.16.s
16c16
< addl %eax, %eax
---
sall $4, %eax
Regards,
/Karl Hammar
6:57 p.m.
That is likely to change depending on GCC optimization setting, no?
J.E.B.
On Sat, Mar 16, 2019, 5:12 PM <karl(a)aspodata.se> wrote:
Will Godfrey:
On Wed, 13 Mar 2019 00:09:17 +0100 (CET)
karl(a)aspodata.se wrote:
>Will Godfrey:
>> Does anyone know if GCC will replace power of 2
multiplications/divisions
of
>> unsigned integers with bit shifts?
...
>$ cat a.c
>#include <stdint.h>
>
>int main(int argc, char *argv[]) {
> uint16_t b = argc * 2;
>
> return b;
>}
>$ gcc -S a.c
,,,
>It seems it just adds the value with itself
here.
...
Thanks, that seems to hold true for a mixture of
AMD and Intel machines.
BTW, testing with 4, 8 and 16 instead of 2, it does actually do bit
shifts:
$ diff a.02.s a.04.s
16c16
< addl %eax, %eax
---
sall $2, %eax
$ diff a.02.s
a.08.s
16c16
< addl %eax, %eax
---
sall $3, %eax
$ diff a.02.s
a.16.s
16c16
< addl %eax, %eax
---
sall $4, %eax
Regards,
/Karl Hammar
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9:12 p.m.
On 3/17/19 5:56 PM, Jonathan Brickman wrote:
That is likely to change depending on GCC optimization
setting, no?
Not usually. It depends on the target architecture more than anything.
Even with -O0, gcc translates integer addition and multiplications into
a combination of bitwise and arithmetic operations if that is
appropriate for the target CPU.
On 3/12/19 11:43 PM, Will Godfrey wrote:
Does anyone know if GCC will replace power of 2
multiplications/divisions of unsigned integers with bit shifts?
Not only power of
two, and it depends. In some cases the compiler
doesn't replace it because the resulting code is not faster. Many modern
CPU already implement integer multiplication using bitwise operations in
microcode.
I don't know if this your question is academic, but if you plan to
manually optimize code, I highly recommend against that.
Write semantically! If you mean a multiplication, use (a * 2), if you
mean bit-shift use (a << 1). Do not use bitwise-operations when you
really do integer-arithmetics.
Your future self and code-contributers will thank you for it; as will
compilers targeting future CPU architectures.
</rant>
2c,
robin
10:12 p.m.
On Sun, 17 Mar 2019 20:12:04 +0100
Robin Gareus <robin(a)gareus.org> wrote:
On 3/17/19 5:56 PM, Jonathan Brickman wrote:
Thanks Robin. Very clear. It's the direction I was beginning to think in. It's
sometimes difficult to imagine how far compilers have advanced over the years.
--
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
That is likely to change depending on GCC
optimization setting, no?
Not usually. It depends on the target architecture more than anything.
Even with -O0, gcc translates integer addition and multiplications into
a combination of bitwise and arithmetic operations if that is
appropriate for the target CPU.
On 3/12/19 11:43 PM, Will Godfrey wrote:
Does anyone know if GCC will replace power of 2
multiplications/divisions of unsigned integers with bit shifts?
Not only power of
two, and it depends. In some cases the compiler
doesn't replace it because the resulting code is not faster. Many modern
CPU already implement integer multiplication using bitwise operations in
microcode.
I don't know if this your question is academic, but if you plan to
manually optimize code, I highly recommend against that.
Write semantically! If you mean a multiplication, use (a * 2), if you
mean bit-shift use (a << 1). Do not use bitwise-operations when you
really do integer-arithmetics.
Your future self and code-contributers will thank you for it; as will
compilers targeting future CPU architectures.
</rant>
2c,
robin
2106
days inactive
2111
days old
linux-audio-dev@lists.linuxaudio.org
8 comments
6 participants
participants (6)
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Gordonjcp -
Jonathan Brickman -
karl＠aspodata.se -
Robin Gareus -
Will Godfrey -
Will J Godfrey