https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114761
Bug ID: 114761
Summary: Ignored [[likely]] attribute
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zamazan4ik at tut dot by
Target Milestone: ---
For the following code:
bool foo(int var)
{
if (var == 42) [[unlikely]] return true;
if (var == 322) [[unlikely]] return true;
if (var == 1337) [[likely]] return true;
return false;
}
GCC (trunk) with "-O3 -std=c++20" generates the following:
foo(int):
cmp edi, 322
sete al
cmp edi, 42
sete dl
or eax, edx
cmp edi, 1337
sete dl
or eax, edx
ret
Clang (18) with "-O3 -std=c++20" however, generates a bit different version:
foo(int): # @foo(int)
mov al, 1
cmp edi, 1337
jne .LBB0_1
.LBB0_4:
ret
.LBB0_1:
cmp edi, 42
je .LBB0_4
cmp edi, 322
je .LBB0_4
xor eax, eax
ret
GCC for some reason ignores [[likely]] attribute and doesn't place the branch
with 1337 at the beginning of the function. Clang does it. Placing this branch
at the beginning should be more optimal. I also tested GCC 13.2 (on my Fedora
machine) with __builtin_expect and PGO - the result is the same for GCC: it
ignores such an optimization.
Godbolt link: https://godbolt.org/z/o8KMx8M33
