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Date: Mon, 15 Jan 2007 13:50:08 -0800
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To: Sara Golemon <pollita@php.net>
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Subject: Re: Runtime JIT Proposals
From: pollita@php.net (Sara Golemon)

> Option 4:
> Include fetchtype and subelement during runtime JIT callback allowing 
> JIT callback to only do work necessary to prepare for the read/write 
> call being performed.
> 
> e.g.
> 
> int php_example_jit_callback(int str_type, zstr str, int str_len,
>     int stage, zval *container, int fetch_type, zval *element);
> 
> Where str_type/str/str_len indicate the name of the variable being 
> JITed, stage is one of COMPILETIME or RUNTIME, container is the 
> autoglobal itself.
> Fetch_type is ZEND_FETCH_(DIM_|OBJ_)?_(R|W|RW), and element is the 
> specific property/offset (only applicable for DIM/OBJ fetches, NULL for 
> plain fetch.
> 
> Advantages: Gives maximum flexibility to the implementation.  In the 
> case of http request encoding, it allows the decoder to differentiate 
> between requests for a single element and fetches which want to retreive 
> the entire array (e.g. foreach).
> 
> Disadvantages: Adds a lot of complexity to the fetching of autoglobals 
> and qand effectively doubles the amount of callback work being done for 
> autoglobal objects.  Will also confuse implementers on what the 
> difference between this fetch callback is and the 
> (read|write)_(dimension|property) callbacks used by objects.
> 
> 
Okay, in attempting an implementation of this, I got reminded 
none-too-gently by the engine that it's not quite as simple as I'd 
remembered:

<?php
$g = $_GET;
$f = $_POST['foo'];
?>

compiled vars:  !0 = $g, !1 = $f
line # op             fetch     operands
----------------------------------------------
  2   0 FETCH_R        global    $0, '_GET'
      1 ASSIGN                   $1, !0, $0
  3   2 FETCH_R        global    $2, '_POST'
      3 FETCH_DIM_R              $3, $2, 'foo'
      4 ASSIGN                   $4, !1, $3


Autoglobals aren't treated as CVs (as, for some reason, I was thinking 
they were) so at the time of initial fetch, it's difficult to know if 
the whole var is being fetched (as in the case of the line 2 assignment, 
or if it's being fetched so that a subelement can be fetched later (as 
in the case of the line 3 assignment).

The solution I'm tempted to pursue for this is to back up yet another 
step and make autoglobals be CVs by extending the zend_compiled_variable 
struct to contain a flag indicating how the var should be fetched (the 
determination for which happens during fetch_simple_var during the 
compilation.  This would then yield an opcode stack like the following:

compiled vars:  !0* = $_GET, !1 = $g, !2* = $_POST, !3 = $f
line # op             fetch     operands
----------------------------------------------
  2   0 ASSIGN                   $0, !1, !0*
  3   1 FETCH_DIM_R              $1, !2*, 'foo'
      2 ASSIGN                   $2, !3, $1

(The * notation indicating that cv->fetch_type == ZEND_FETCH_GLOBAL)

Once that's applied (basicly as a stand-alone speed improvement, since 
globals are turned into CV fetches), the RT-JIT can be done using the 
plan I'd already formulated for Option 4.  *THEN* we can apply 
runtime-JIT to http input encoding detection.

Just keeping the conversation in the open and hoping anyone which 
critiques will voice them sooner rather than later.

-Sara

Two steps forward, one step back.