https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83610
Bug ID: 83610
Summary: __builtin_expect sometimes is ignored
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: bugzi...@poradnik-webmastera.com
Target Milestone: ---
[code]
void f1();
void f2();
void test(int a, int b, int c, int d, int n, int k)
{
int val = a & b;
if (__builtin_expect(!!(n == k), 0))
val &= c;
if (__builtin_expect(!!(n == 10 - k), 0))
val &= d;
if (val)
f1();
else
f2();
}
[/code]
This code compiled with gcc 4.8.5 generates branches as expected:
[asm]
test(int, int, int, int, int, int):
and edi, esi
cmp r8d, r9d
je .L6
.L2:
mov eax, 10
sub eax, r9d
cmp r8d, eax
je .L7
.L3:
test edi, edi
jne .L8
jmp f2()
.L8:
jmp f1()
.L7:
and edi, ecx
jmp .L3
.L6:
and edi, edx
jmp .L2
[/asm]
When this code is compiled with gcc 4.9.0 or higher, it generates branchless
code like below. In my case it is slower than version with branches. I wanted
to convince compiler to generate this version of code by using
__builtin_expect, but for some reason it does not work.
[asm]
test(int, int, int, int, int, int):
and esi, edi
mov eax, 10
and edx, esi
cmp r8d, r9d
cmove esi, edx
sub eax, r9d
and ecx, esi
cmp r8d, eax
cmove esi, ecx
test esi, esi
jne .L6
jmp f2()
.L6:
jmp f1()
[/asm]
