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Cc: "Dmitry Stogov" <dmitry@zend.com>
Date: Mon, 17 Aug 2015 05:26:30 -0500
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Subject: Marking error, etc. functions as cold?
From: php_lists@realplain.com ("Matt Wilmas")

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