17 Mar
2019
17 Mar
'19
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