Optimizer discussion

Hi Graham,

Simple things first:

I'm not sure which optimization you are talking about with the GLOBALS stuff but what your saying makes sense. (Its been awhile since I've looked at the code base myself, I'm just getting back to working on it)

I copied that comment straight from the source, but I can't find it
now that I went looking for it. No matter.

Why not start off with the big stuff, dataflow. I personally believe that working out good data flow for PHP is key to getting good optimizations. But you are right, its a very tricky thing to do and in some cases impossible. Ultimately, I would like to move a lot of the optimizer work more into this direction and use the data flow to build a basic platform for code analysis on which optimizations can be done. For now though, pecl/optimizer is "dumb" about data types :-)

And now the hard stuff. To avoid me repeating myself, let me just pimp
my Tech Talk. Have a look at
http://www.youtube.com/watch?v=kKySEUrP7LA from about the 30:45 mark
until just before the 47:00 mark (slides at
https://www.cs.tcd.ie/~pbiggar/paul_biggar_google_18_mar_2009_notes.pdf).
That highlights most of the problems, and vaguely hints at their
solution. We can go into much greater detail on the solutions after.

Thanks,
Paul

--
Paul Biggar
paul.biggar@gmail.com

Hi,

I'm happy there's some interest in a PHP optimizer :)
I agree with Paul that PECL's optimizer duplicates way too much stuff from
the Zend engine, which is not practic nor maintainable. (compare for example
with the simple constant folder I implemented some years ago:
http://web.ist.utl.pt/nuno.lopes/zend_constant_folding.txt).
About runkit & friends, I wouldn't worry much about them. If you're running
them problably you also don't care about optimizations. If you want to be
able to optimize something, you need to remove as many freedom degrees as
you can..

Anyway, I don't know how much time you're going to invest in this optimizer,
but I'll certainly be more than happy to discuss your ideas.

Nuno

P.S.: I'll try to meet with Paul in PLDI (in a week) and chat about these
kinds of things. Is anyone else comming that wants to join the discussion?

----- Original Message -----
From: "Graham Kelly" grahamk@facebook.com
To: "Paul Biggar" paul.biggar@gmail.com
Cc: "PHP Internals" internals@lists.php.net; "Brian Shire"
shire@facebook.com
Sent: Friday, June 05, 2009 1:08 AM
Subject: [PHP-DEV] RE: Optimizer discussion

Hey,

I always love having input. When you said it was vicious I was expecting
more, in fact I agree completely with you on a lot of things :-)

Anyway, I'm not really sure how much detail you want me to go into (or how
much detail people on internals really want me to get into). So, I'll keep
it brief for now and can expand on anything.

Why not start off with the big stuff, dataflow. I personally believe that
working out good data flow for PHP is key to getting good optimizations. But
you are right, its a very tricky thing to do and in some cases impossible.
Ultimately, I would like to move a lot of the optimizer work more into this
direction and use the data flow to build a basic platform for code analysis
on which optimizations can be done. For now though, pecl/optimizer is "dumb"
about data types :-)

The reimplementations of some engine code is messy and work should probably
be done to try to remove this where possible. Also, I might be mistaken but
the is_numeric_result stuff is partly left over from Turck MMCache which to
my understanding this version of pecl/optimizer was based off of. Some of
the stuff I was doing with building a function table (for optimizable and
some non optimizable functions) was to try and get rid of rudimentary data
type detection like this. Actually folding in values from function calls is
happening over in the optimize_fcr.c file.

I 100% agree with you on the file system functions. They were in there when
I started working on the optimizer and I havent really paid much attention
to them. The latest CVS version of pecl/optimizer has them at least removed
from being candidates for optimization (the code to actually optimize is
still there).

I'm not sure which optimization you are talking about with the GLOBALS stuff
but what your saying makes sense. (Its been awhile since I've looked at the
code base myself, I'm just getting back to working on it)

As far as my future plans for pecl/optimizer I should really gather up all
my ideas and stuff in the next week or so that you or anyone else who is
interested can give feedback. At the moment, I'm working on getting the
current version to a stable state. I'm also still trying to gauge demand for
pecl/optimizer to maybe help figure out direction for the project. (or if
there is really any real interest/or use).

From: Paul Biggar [paul.biggar@gmail.com]
Sent: Thursday, June 04, 2009 4:20 PM
To: Graham Kelly
Cc: PHP Internals; Brian Shire
Subject: Optimizer discussion

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