13 Mar
2019
13 Mar
'19
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