[PATCH] Constant expr folding (again), and other things View as Markdown

Hi Matt,

For now I would like to disable "-CONST" constant expression which
started to work after your patch.

Later we are able to implement the complete constant expressions support.

Thanks. Dmitry.

Matt Wilmas wrote:

Hi Dmitry,

----- Original Message -----
From: "Dmitry Stogov"
Sent: Wednesday, July 30, 2008

Hi Matt,

does the following code work with your patch?

<?php
function foo() {
static $a = A + B;
var_dump($a);
}
const A = 1;
const B = 2;
foo();
?>

It would be hard to explain why some of constant expression work, but
others don't.

No, it doesn't. That would of course require additional changes (I don't
know how to do that, didn't even look into it), and was also mentioned with
Nuno's patch. I just included the part I did for something "closer" to what
was talked about last time. The only explanation I know of would be that it
only works with literals, not named constants. But yes, it's hard to
understand. :-)

But anyway, it doesn't really matter to me about that, the main point (like
Nuno's) was the the folding of constant expressions in the other areas
(especially negative numbers). And like I said, it's more useful after
constant substitution with function flags being ORed and things like that...
But that could always be applied at a later date/version since it doesn't
break compatibility, right?

Anyway, it's too late for this patches.
5.3 is going to be frozen.

Well, what about the behavior change I mentioned? Something should be done
about that, I'm sure. That's what the second small patch
("signed_expr_tweaks") was for.

Actually, I wrote the above right after getting your message, but didn't
send it then, because I had only been thinking about the parser, and thought
of a more proper fix in zend_compile.c for the behavior difference (and
something else). I will send the fix shortly in a reply to the previous
thread about constant substitution... :-)

Thanks. Dmitry.

Thanks,
Matt

Matt Wilmas wrote:

Hi Dmitry, all,

I was going to send this patch as a companion one to go with the
compile-time constant substitution change (Nuno sent a message about
this
[1] last Sep. with a patch [2]), BUT I also realized that some
different,
inconsistent behavior is possible because of that change.

First, the folding optimization part, which I don't know if you'll want
to
use (though it seems pretty simple/safe to apply at any time), does the
same
thing as Nuno's patch, but with a different implementation. This is
even
more of an optimization when combined with constant substitution, for
ORing
function flags, etc.

At the time, one of the objections (?) was that it didn't allow the same
constant expressions to initialize static variables and class
members/constants. (Though I don't see much relation: hidden, internal
optimization vs. actual syntax change/enhancement.) So I also added
support
for arithmetic, bitwise, and concatenation operators (not logical or
comparison) in that context. Anyway, should be fairly simple to
understand... :-)

Now, the different, inconsistent behavior that's possible after the
constant
substitution change. This code, for example:

function foo() {
static $a = -PHP_INT_MAX;
}

Would have previously given "Unsupported operand types". Now it can
work
because the value of PHP_INT_MAX may be substituted first. Fine, good;
except that it won't be substituted if it's in a namespace or
ZEND_COMPILE_NO_CONSTANT_SUBSTITUTION is set, and then the error occurs.

I thought the best solution was to disallow constants in a
"static_scalar"
expression (e.g. only literals). This shouldn't break anything unless
someone was using - or + signs on TRUE/FALSE/NULL. :-O That will cause
a
parse error with my changes. Also:

function foo() {
static $a = -'abc'; // 0
static $b = +'abc'; // abc
$c = +'abc'; // 0
}

So I made the static_scalar unary + operator "do something" and behave
like
the regular one. Again, shouldn't break anything unless someone was
applying + to literal strings or arrays!

Full patch with constant folding, etc.
http://realplain.com/php/const_folding.diff
http://realplain.com/php/const_folding_5_3.diff

-OR-, basic patch only containing the fix for signed static scalars, and
only an optimization to save the opcode for unary - and + applied to a
constant value (e.g. negative numbers, primarily):
http://realplain.com/php/signed_expr_tweaks.diff
http://realplain.com/php/signed_expr_tweaks_5_3.diff

Thoughts...? Thanks,
Matt

[1] http://marc.info/?l=php-internals&m=118925033731019&w=2
[2] http://web.ist.utl.pt/nuno.lopes/zend_constant_folding.txt