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Date: Tue, 17 Feb 2015 00:47:54 +0100
CC: PHP Internals <internals@lists.php.net>
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To: Rasmus Lerdorf <rasmus@lerdorf.com>
Subject: Re: [PHP-DEV] Switch jumptable optimization
From: bobwei9@hotmail.com (Bob Weinand)

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> Am 17.02.2015 um 00:30 schrieb Rasmus Lerdorf <rasmus@lerdorf.com>:
>=20
> On 02/16/2015 03:04 PM, Bob Weinand wrote:
>> I'd like to show you my recent work on a jumptable optimization for =
switches.
>>=20
>> https://github.com/php/php-src/pull/1048 =
<https://github.com/php/php-src/pull/1048>
>>=20
>> It is a fully transparent optimization of switches, by putting a new =
ZEND_SWITCH opcode at the top of the switch in case we can build a =
jumptable, which is the case if the cases are only scalars (no doubles) =
or evaluate to them at compile-time.
>>=20
>> Switches tend to be big sometimes with a lot of static (literals or =
constants usually) cases, which was a comparison for each case, and now =
just a simple hashtable lookup is needed, which greatly improves =
performance. In case where the result of the switch can be determined at =
compile-time, it even replaces the comparisons with a simple ZEND_JMP =
opcode.
>>=20
>> In synthetic benchmarks the results are mind blowing, but in =
practice, benefits mostly are stateful userland parsers (often called =
with sometimes big switches), where it makes it a few percent faster. =
(For more concrete numbers see =
https://github.com/php/php-src/pull/1048#issuecomment-73032647)
>>=20
>> As the optimization is only done if there are 3 or more cases, the =
lookup is always faster than the single comparisons. So there's no =
negative performance impact.
>>=20
>> It already works, except with opcache where the CFG optimization pass =
still lacks support. Dmitry or Laruence volunteered to add support for =
opcache.
>>=20
>> I'd like to know if there are any technical objections to the PR. If =
not, they will add opcache support and merge it in.
>=20
> It looks ok to me if Xinchen or Dmitry gets the opcache support =
working
> for it and the feature is stabilized in time. I gave it a try and it =
is
> segfaulting for me (no opcache):
>=20
> =3D=3D5612=3D=3D Command: sapi/cli/php -n jump.php
> =3D=3D5612=3D=3D
> =3D=3D5612=3D=3D Invalid read of size 4
> =3D=3D5612=3D=3D    at 0xA64637: _zend_is_inconsistent =
(zend_hash.c:44)
> =3D=3D5612=3D=3D    by 0xA65749: _zend_hash_add_or_update_i =
(zend_hash.c:427)
> =3D=3D5612=3D=3D    by 0xA65AF9: _zend_hash_add (zend_hash.c:489)
> =3D=3D5612=3D=3D    by 0xA37E28: zend_hash_add_ptr (zend_hash.h:458)
> =3D=3D5612=3D=3D    by 0xA39B0A: zend_hash_add_constant =
(zend_constants.c:486)
> =3D=3D5612=3D=3D    by 0xA39D01: zend_register_constant =
(zend_constants.c:523)
> =3D=3D5612=3D=3D    by 0xA389E9: zend_register_long_constant =
(zend_constants.c:194)
> =3D=3D5612=3D=3D    by 0xA3855C: zend_register_standard_constants
> (zend_constants.c:116)
> =3D=3D5612=3D=3D    by 0xA52D6E: zend_startup (zend.c:671)
> =3D=3D5612=3D=3D    by 0x9C7FAA: php_module_startup (main.c:2099)
> =3D=3D5612=3D=3D    by 0xAFA460: php_cli_startup (php_cli.c:421)
> =3D=3D5612=3D=3D    by 0xAFC4D6: main (php_cli.c:1335)
> =3D=3D5612=3D=3D  Address 0x8 is not stack'd, malloc'd or (recently) =
free'd
>=20
> And jump.php is:
>=20
> <?php
> $a =3D 1;
> switch($a) {
> 	case 0: echo 0; break;
> 	case 1: echo 1; break;
> 	case 2: echo 1; break;
> 	case 3: echo 1; break;
> 	case 4: echo 1; break;
> 	case 5: echo 1; break;
> 	case 6: echo 1; break;
> 	case 7: echo 1; break;
> 	case 8: echo 1; break;
> 	case 9: echo 1; break;
> }
>=20
> -Rasmus

Hey,

this doesn't really look related to my patch. Did you do something wrong =
when cloning my branch? Or forget a make clean or similar?

Locally it works for me and echoes 1 as expected.

Bob=

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