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Date: Fri, 5 Jun 2009 00:20:37 +0100
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To: grahamk@facebook.com
Cc: PHP Internals <internals@lists.php.net>, shire <shire@facebook.com>
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Subject: Optimizer discussion
From: paul.biggar@gmail.com (Paul Biggar)

Graham and I are having a brief chat about the work he's going to do
on the PECL optimizer. People have asked me to do this on-list (they
may have meant the PECL list, but optimizations on PHP seem more
relevant here), so here goes.


Hi Graham,

So the general gist of what I have to say is that dataflow
optimizations on PHP are very difficult, and nearly impossible at the
function-local level. Loop-invariant hoisting and other redundant
expression computation liekwise. If you're planning on working on
them, we can go into more detail.


I guess the biggest thing is that I'm wondering what your plans are
for the PECL optimizer? I've spent about 2 years working on the phc
optimizer, (and a bit longer on relevant things) so I hope that my
advice will be relevant.



I've taken a look through the optimizer a few times over the last
while, (and even stolen some ideas from it). Here are my comments on
the current code:

- There is lots of code which reimplements parts of the engine, for
example: ini_bool_decode, optimizer_acosh and friends, optimize_md5,
optimize_crc32, optimize_sha1, optimize_class_exists and friends (to a
lesser extent). There are also lots of constant foldings, like casts
and "0 == false" (etc) in optimize_code_block. I don't understand why
there is logic in the code for that, rather than simply executing the
opcodes, or constructing an eval and executing that.

- is_numeric_result: there has been great effort to figure out numeric
results from pure functions, when it seems straightforward to
optimizer the results straight in. Maybe that is being done elsewhere?
If so, there may need to be some care taken to ensure that all
optimizations terminate.

- File system functions are very iffy. I would be surprised if people
have content that reads from files repeatedly, but where the files do
not change, and who are willing to use that flag.

- Most of the identity optimizations arent safe. $x + 0 !== $x,
unfortunately, due to integer coercions (parallels exist for other
types/operators)

- I think I saw an optimizations converting ("45" + $x) into (45+$x) -
that's a great idea, which I will steal.

- How does runkit (and other weird extensions) affect optimizations on
constants, class_exists, etc?

- The optimization "unsafe: optimize out isset()/empty() ops on
GLOBALS['foo'] into $foo " is not safe, as GLOBALS['foo'] may not be
the same variable as $foo ($GLOBALS may be unset, and indeed, there
may be good reasons to do so).


I'm also wondering what the optimizations are on fcall? I couldn't make it out.


That's quite a lot, but its everything I have on the current PECL optimizer :)


Thanks,
Paul


-- 
Paul Biggar
paul.biggar@gmail.com