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Date: Thu, 10 Jan 2008 15:46:03 -0800
Message-ID: <698DE66518E7CA45812BD18E807866CE011C7C78@us-ex1.zend.net>
Thread-Topic: [PHP-DEV] php-5.2.3: zend_opcode.c: pass_two() question
Thread-Index: AchT4ZmYy5CpBNS3T8GcO9WScZ6tFAAAR1mA
References: <300A9932AE9C0145A146B5A4E5CCE96901921EDC@sf2pmxb01.thefacebook.com>
To: "Kannan Muthukkaruppan" <Kannan@facebook.com>,
	<internals@lists.php.net>
Subject: RE: [PHP-DEV] php-5.2.3: zend_opcode.c: pass_two() question
From: andi@zend.com ("Andi Gutmans")

Op arrays are read-only. This is a design goal and is especially
important for byte code caches where several processes are looking at
the same op array. Therefore setting IS_CONST to is_ref=3D1/refcount=3D2 =
we
are ensuring that they will not be changed.
The copy happens only the first time and afterwards that zval may be
used, incremented, etc...

Net net, it is intended behavior and very important.
Andi


> -----Original Message-----
> From: Kannan Muthukkaruppan [mailto:Kannan@facebook.com]
> Sent: Thursday, January 10, 2008 3:36 PM
> To: internals@lists.php.net
> Subject: [PHP-DEV] php-5.2.3: zend_opcode.c: pass_two() question
>=20
> An example assignment such as:
>         $x =3D 10;
> doesn't seem to get handled as a simple ref-count incrementing
> assignment (in zend_assign_to_variable()). Instead, $x ends up getting
> a
> new "zval" into which a
> copy of the RHS (10) is made.
>=20
> This seems to be because the zval/znode corresponding to the literal
10
> is an  IS_CONST operand of the assignment opcode. And, during the end
> of
> compilation of a function, the IS_CONST operands in the function's
> op_array are patched up as "references" (is_ref is set to 1).
>=20
> zend_opcode.c:pass_two()
>   ...
>  while (opline < end) {
>   if (opline->op1.op_type =3D=3D IS_CONST) {
>    opline->op1.u.constant.is_ref =3D 1;
>    opline->op1.u.constant.refcount =3D 2; /* Make sure is_ref won't be
> reset */
>   }
>   if (opline->op2.op_type =3D=3D IS_CONST) {
>    opline->op2.u.constant.is_ref =3D 1;
>    opline->op2.u.constant.refcount =3D 2;
>   }
>    ...
>=20
> And zend_assign_to_variable(), when it sees the RHS to be a reference
> ("is_ref =3D=3D 1") ends up doing a copy rather than a ref-count
> incrementing assignment.
>=20
> It seems like performance would be better if we didn't mark constant
> operands as "references" in pass_two(). But presumably it is required
> to
> handle some cases that'll otherwise break? I am trying to understand
> what those cases are.  If someone can shed further light on this
issue,
> it is much appreciated...
>=20
> regards,
> Kannan Muthukkaruppan