https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111297
Bug ID: 111297
Summary: missed optimization: [[unlikely]] attribute has no
effect at -O2/-O3/-Ofast
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: moncef.mechri at gmail dot com
Target Milestone: ---
Consider the following code:
extern bool CheckCondition(int i);
extern void DoWork();
extern void DoOtherWork();
void f1()
{
if (CheckCondition(42)) [[likely]]
DoWork();
else
DoOtherWork();
}
void f2()
{
if (CheckCondition(42)) [[unlikely]]
DoWork();
else
DoOtherWork();
}
The [[unlikely]] attribute in f2() seems to have no impact on codegen at -O2,
-O3, and -Ofast:
f1():
sub rsp, 8
mov edi, 42
call CheckCondition(int)
test al, al
je .L2
add rsp, 8
jmp DoWork()
.L2:
add rsp, 8
jmp DoOtherWork()
f2():
sub rsp, 8
mov edi, 42
call CheckCondition(int)
test al, al
je .L6
add rsp, 8
jmp DoWork()
.L6:
add rsp, 8
jmp DoOtherWork()
While the codegen for f1() looks good, the codegen I would have expected for
f2() is:
f2():
sub rsp, 8
mov edi, 42
call CheckCondition(int)
test al, al
jne .L8
add rsp, 8
jmp DoOtherWork()
.L8:
add rsp, 8
jmp DoWork()
Observations:
- All GCC versions since 9.1 (where support for [[likely]] and [[unlikely]] was
first added) seem impacted.
- When f1() is commented out, the issue somehow disappears
- Replacing [[likely]] / [[unlikely]] with __builtin_expect() seems to solve
the issue
- Clang does not suffer from this issue (and neither does GCC at -O1)
https://godbolt.org/z/8o1njKvr1
