https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66890
Bug ID: 66890
Summary: function splitting only works with profile feedback
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: andi-gcc at firstfloor dot org
Target Milestone: ---
Consider this simple example:
volatile int count;
int main()
{
int i;
for (i = 0; i < 100000; i++) {
if (i == 999)
count *= 2;
count++;
}
}
The default EQ is unlikely heuristic in predict.* predicts that the if (i ==
999) is unlikely. So the tracer moves the count *= 2 basic block out of line to
preserve instruction cache.
gcc50 -O2 -S thotcold.c
movl $1, %edx
jmp .L2
.p2align 4,,10
.p2align 3
.L4:
addl $1, %edx
.L2:
cmpl $1000, %edx
movl count(%rip), %eax
je .L6
addl $1, %eax
cmpl $100000, %edx
movl %eax, count(%rip)
jne .L4
xorl %eax, %eax
ret
# out of line code
.L6:
addl %eax, %eax
movl %eax, count(%rip)
movl count(%rip), %eax
addl $1, %eax
movl %eax, count(%rip)
jmp .L4
Now if we enable -freorder-blocks-and-partition I would expect it to be also
put into .text.unlikely to given even better cache layout. But that's what is
not happening. It generates the same code.
Only when I use actual profile feedback and -freorder-blocks-and-partition the
code actually ends up being in a separate section
(it also unrolled the loop, so the code looks a bit different)
gcc -O2 -fprofile-generate -freorder-blocks-and-partition thotcold.c
./a.out
gcc -O2 -fprofile-use -freorder-blocks-and-partition thotcold.c
...
.cfi_endproc
.section .text.unlikely
.cfi_startproc
.L55:
movl count(%rip), %ecx
addl $1, %eax
addl $1, %ecx
cmpl $100000, %eax
movl %ecx, count(%rip)
je .L6
cmpl $1, %edx
je .L5
cmpl $2, %edx
je .L28
cmpl $3, %edx
-freorder-blocks-and-partition should already use the extra section even
without profile feedback.
I tested some larger programs and without profile feedback the unlikely section
is always empty.
The heuristics in predict.* often work quite well and a lot of code would
benefit from moving cold code out of the way of the caches.
This would allow to use the option to improve frontend bound codes without
needing to do full profile feedback.
