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To: Paul Biggar <paul.biggar@gmail.com>
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Date: Thu, 4 Jun 2009 17:08:05 -0700
Thread-Topic: Optimizer discussion
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Subject: RE: Optimizer discussion
From: grahamk@facebook.com (Graham Kelly)

Hey,

I always love having input. When you said it was vicious I was expecting mo=
re, 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 w=
orking 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 thi=
s direction and use the data flow to build a basic platform for code analys=
is on which optimizations can be done. For now though, pecl/optimizer is "d=
umb" 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 bu=
t 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 o=
f 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 remove=
d 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 stuf=
f but what your saying makes sense. (Its been awhile since I've looked at t=
he 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 in=
terested can give feedback. At the moment, I'm working on getting the curre=
nt version to a stable state. I'm also still trying to gauge demand for pec=
l/optimizer to maybe help figure out direction for the project. (or if ther=
e 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 =3D=3D 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 !=3D=3D $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