messing with branch prediction View as Markdown

Looks like an interesting patch, if it does not cost us any
performance and only offers either no gain or a small gain I'd say it
would be worth inclusion. Perhaps branch prediction will work better
with newer versions of gcc like 4.0 or 4.1

Hello,

Yesterday I was playing with branch prediction in the Zend VM
executor. Although I didn't have any noticeable performance
improvement within the bench.php script, I've put the patch on-
line, because someone else might want to mess with it too :) (but
with care..)

patch: http://mega.ist.utl.pt/~ncpl/zend_branch_prediction.txt
gcc docs: http://gcc.gnu.org/onlinedocs/gcc/Other-
Builtins.html#index-g_t_005f_005fbuiltin_005fexpect-2440

the zend.h part of the patch is based on the similar macros (with
the same name, too) found in the Linux kernel.

I made & tested the patch in a Centrino laptop with gcc 3.4, so it
might be the cause of getting no relevant performance increase. On
CPUs with longer pipelines (like Pentium 4/NetBurst architecture),
the performance increase might be noticeable.

Happy hacking :)
Nuno

Ilia Alshanetsky

Definitely looks nice. I actually like how it makes the code more
readable hinting which branches are rare.

Zeev

At 16:49 18/06/2006, Nuno Lopes wrote:

Hello,

Yesterday I was playing with branch prediction in the Zend VM
executor. Although I didn't have any noticeable performance
improvement within the bench.php script, I've put the patch on-line,
because someone else might want to mess with it too :) (but with care..)

patch: http://mega.ist.utl.pt/~ncpl/zend_branch_prediction.txt
gcc docs:
http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-g_t_005f_005fbuiltin_005fexpect-2440

the zend.h part of the patch is based on the similar macros (with
the same name, too) found in the Linux kernel.

I made & tested the patch in a Centrino laptop with gcc 3.4, so it
might be the cause of getting no relevant performance increase. On
CPUs with longer pipelines (like Pentium 4/NetBurst architecture),
the performance increase might be noticeable.

Happy hacking :)
Nuno

The patch looks good.

I already did some experiments with __builtin_expect() in the past, but
didn't get any visible speedup.

As I understand __builtin_expect() helps only in case if branch is not in
BTB and static branch prediction is used.
In case if branch instruction was already executed (and didn't go out from
BTB), __builtin_expect() has no effect.
It just reorders basic-blocks according to static branch prediction
algorithm.

Thanks. Dmitry.

-----Original Message-----
From: Nuno Lopes [mailto:nlopess@php.net]
Sent: Sunday, June 18, 2006 5:50 PM
To: PHPdev
Cc: andi@php.net; dmitry@php.net
Subject: messing with branch prediction

Hello,

Yesterday I was playing with branch prediction in the Zend VM
executor.
Although I didn't have any noticeable performance improvement
within the
bench.php script, I've put the patch on-line, because someone
else might
want to mess with it too :) (but with care..)

patch: http://mega.ist.utl.pt/~ncpl/zend_branch_prediction.txt
gcc docs:
http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-g_
t_005f_005fbuiltin_005fexpect-2440

the zend.h part of the patch is based on the similar macros
(with the same
name, too) found in the Linux kernel.

I made & tested the patch in a Centrino laptop with gcc 3.4,
so it might be
the cause of getting no relevant performance increase. On
CPUs with longer
pipelines (like Pentium 4/NetBurst architecture), the
performance increase
might be noticeable.

Happy hacking :)
Nuno