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Date: Mon, 17 Aug 2015 14:38:33 +0300
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To: Matt Wilmas <php_lists@realplain.com>
Cc: PHP Internals <internals@lists.php.net>
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Subject: Re: Marking error, etc. functions as cold?
From: dmitry@zend.com (Dmitry Stogov)

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This is good idea.
Some times ago I played with "hot" attribute, and didn't get any
improvement, but we really may get something with "cold".

Thanks. Dmitry.

On Mon, Aug 17, 2015 at 1:26 PM, Matt Wilmas <php_lists@realplain.com>
wrote:

> Hi Dmitry, all,
>
> Has it been considered to use __attribute__((cold)) on, for example,
> error-type functions?  I happened to notice this part about it in the GCC
> docs [1] a couple days ago: "The paths leading to calls of cold functions
> within code are marked as unlikely by the branch prediction mechanism."
>
> A while ago I figured that, technically, many if () conditions for errors
> could use UNEXPECTED(), but that would be overkill for each possible tiny
> advantage.  But that sort of happens automatically for conditions leading
> to "cold" functions! :^)
>
> I was thinking of widely-used functions like php_error_docref*, zend_error
> (and related); also noticed exception/throwing functions, or even
> zend_accel_error in opcache.
>
> I didn't do any profiling, just simply tried marking *only*
> php_error_docref0 cold.  With GCC 4.8, it reduced the size by almost 4 KB
> (-O2, --disable-all CLI).  And checking through the code, it did indeed
> move the calls around (e.g. out of fast path) in many cases compared to
> default (some cases didn't optimize more).
>
> Looks like more zend_error calls in VM that aren't already part of
> UNEXPECTED() conditions could be moved "out of the way" if marked cold...
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
>
> Thoughts?
>
>
> Thanks,
> Matt
>

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