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Date: Fri, 14 Sep 2007 18:01:13 +0100
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Subject: RFC: mark functions as const for possible optimizations
From: nlopess@php.net ("Nuno Lopes")

Hi,

Following my last post on bytecode optimization 
(http://news.php.net/php.internals/32113), I would like to discuss a few 
more things.
The patch in that e-mail allowed the engine itself to do transformations 
like, e.g.:
$a = 1*2*3*4*5+1-0;   -->  $a = 121;

However the patch is now a bit dead because Stas would like to have more 
advanced things that I didn't have time yet to look at (because the proposal 
introduces several difficult ambiguities in the grammar).

So, let's move on for now :P

My proposal is the following:
some functions when fed with constant arguments always return a constant 
value, too. e.g.:
strlen('abcd') === 4.

This means that an optimizer can and should do this transformation. (and 
others like: $a='abcd'; strlen($a); -> 4). I think Ilia created a list of 
such functions in his optimizer. But I would like to have that list in the 
core, so that everybody can benefit from it.
Basically the only thing needed to do is to change PHP_FE() to 
PHP_CONST_FE() in the function entry tables.

The patch: http://web.ist.utl.pt/nuno.lopes/zend_const_function_entry.txt 
(includes a few function entries changed as an example)


Nuno

P.S.: the university semester has already started, so don't worry with the 
e-mail traffic I'm generating because it'll end pretty soon (until the next 
vacations :P)