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[Gnash-dev] Property case and AVM2

From: Benjamin Wolsey
Subject: [Gnash-dev] Property case and AVM2
Date: Sat, 03 Jul 2010 11:17:47 +0200

Hi list,

Gnash has a problem with case insensitive properties in SWF6 and below.
I've been working on a fix for this, which corrects most of the

The original implementation relied on a case-insensitive string table.
This worked fine for case-insensitive comparison and lookup, but ruined
property names. An example:

During startup, common strings are added to the table. One such string
is "onLoad". In SWF6, creating a new object with a property ONLOAD will
then work, but the name of the property will always be onLoad, not

I wasn't keen on dropping the string table, so I created a map of all
case-sensitive strings to a caseless string. It's then possible to
compare string_table::keys either with or without case-sensitivity.

This has a time cost when adding strings and a memory cost because all
non-lower-case strings have to be added to the string table in a
lower-case version, and the two keys have to be mapped to each other. My
tests showed no big speed difference, and there's only one string table
so the memory costs are acceptable (I also removed some unnecessary
cruft from the previous string table).

The second stage is to change lookup in the PropertyList. I started by
using a simple list instead of the boost multi-index container. The
disadvantage is that there is no lexicographical ordering, so all
lookups have to iterate through the list until the required key is
found. This was acceptable in most cases, but for objects with many
properties (mostly arrays) the performance hit was huge.

There are two obvious remedies here:

1. Use a special array typing to index array properties.
2. Use multi-index again.

I didn't get a satisfactory result from 1, but in principle it should
work. The disadvantage is that it won't help for non-array objects with
large numbers of properties. I haven't noticed that the pp displays this

Choice 2 increases the memory requirements for all objects, but provides
very quick lookup for all properties, regardless of their type. This
approach increased the speed for versions 7+ to previous levels.

However, the problem of case-insensitive lookup remains. The difficulty
here is that a string_table reference is required to find caseless
string_table::keys for comparison. There's no possibility of using this
to index a multi-index container without storing a caseless key for all

I've currently hacked this so that the performance is acceptable under
most conditions (the iterating lookup is only done if a case-sensitive
one fails and the property name is not lower-case), but a better
solution is needed.

So question 1 is: does anyone have other suggestions or preferences
based on what I've described?

Then there is the second point: removing AVM2 code.

The current AVM2 implementation adds various data members and member
functions to PropertyList, Property, as_object etc. None of this
behaviour is correct, yet it makes AVM1 refactoring and optimization
much harder. 

The AVM2 implementation was started with the fundamentally incorrect
assumption that it could be mixed with AVM1. This results in all sorts
of things (e.g. static properties, classes, super objects) appearing to
work in some cases, when in fact they don't have a snowball's chance in
hell of ever doing things right.

This extends to all the AS3 stubs, which similarly rely on AVM1-style
properties. Again, this will never work and serves no useful purpose.

Because the whole caboodle adds about 10 minutes onto compilation time
(on a fast machine!), adds useless data all over the place, increases
startup time and memory footprint, increases executable size, slows
everything down, makes optimization and improvements to AVM1 difficult,
and blocks a real implementation of AVM2. I propose to drop all the
stubs, to remove all AVM2 code from as_object and the properties, not to
add AVM2 by default, and not to expect any passes in the AVM2 tests. The
AVM2 code in libcore/abc would stay, as it's not completely wrong.

Anyone else for this?

Free Flash, use Gnash

Benjamin Wolsey, Software Developer -
C++ and Open-Source Flash blog -


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