x86 branches vs conditional moves

[Michael Clark]

Hi,

Curious about this codegen:

- https://godbolt.org/g/5XxP5S

Why does gcc branch on _Bool, but emits a conditional move for an integer? can it emit cmovne instead of branching? also curious where one would change this to learn about GCC internals.

It’s not a bug, but it is a performance issue (*1).

I was just curious under which conditions the ternary operator is lowered to cmov on x86 and found this difference in lowering.

Michael

[1] https://github.com/xiadz/cmov


#include <stdbool.h>

extern int a;
extern int b;
extern int c;
extern _Bool C;

int select_int()
{
	return c ? a : b;
}

_Bool select_bool()
{
	return C ? a : b;
}

_Bool a_bool()
{
	return 2;
}

select_int():
        mov     eax, DWORD PTR c[rip]
        test    eax, eax
        mov     eax, DWORD PTR a[rip]
        cmove   eax, DWORD PTR b[rip]
        ret
select_bool():
        cmp     BYTE PTR C[rip], 0
        jne     .L8
        mov     eax, DWORD PTR b[rip]
        test    eax, eax
        setne   al
        ret
.L8:
        mov     edx, DWORD PTR a[rip]
        test    edx, edx
        setne   al
        ret
a_bool():
        mov     eax, 1
        ret