Gcc puts the section markers incorrect, resulting in a basically dummy effect
Take this simple example:
void simple_test(int X)
{
if (__builtin_expect((X==3),0))
call_slowpath_function();
else
call_fastpath_function();
}
this leads to the following assembly with
gcc -O2 -freorder-blocks-and-parittions -s test.c
.file "test.c"
.section .text.unlikely,"ax",@progbits
.LCOLDB0:
.text
.LHOTB0:
.p2align 4,,15
.globl simple_test
.type simple_test, @function
simple_test:
.LFB2:
cmpl $3, %edi
je .L7
xorl %eax, %eax
jmp call_fastpath_function
.L7:
xorl %eax, %eax
jmp call_slowpath_function
.LFE2:
.size simple_test, .-simple_test
.section .text.unlikely
.LCOLDE0:
.text
.LHOTE0:
as you can see, the .text.unlikely is put after the slowpath code, not when the
slowpath code starts.... which is less than useful.
(it happens for much bigger functions/slowpaths as well so it's not just due to
the small size of the unlikely part here)
--
Summary: -freorder-blocks-and-partition is a dud
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: arjanv at redhat dot com
GCC host triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26493
