[Mesa-dev] [PATCH] Fix strict-aliasing violations in GLSL shader list implementation

Thomas Helland thomashelland90 at gmail.com
Tue Jun 23 14:36:44 PDT 2015

2015-06-24 23:05 GMT+02:00 Davin McCall <davmac at davmac.org>:
> Hi - I'm new here.
> I've recently started poking the Mesa codebase for little reason other than
> personal interest. In the "help wanted" section of the website it mentions
> aliasing violations as a target for newcomers to fix, so with that in mind
> I've attached a patch (against git head) which resolves a few of them, by
> targeting the linked list implementation (list.h) used in the GLSL
> compiler/optimizers. This change slightly increases the storage requirements
> for a list (adds one word) but resolves the blatant aliasing violation that
> was caused by the trick used to conserve that word in the first place.
> (I toyed with another approach - using a single sentinel node for both the
> head and tail of a list - but this was much more invasive, and meant that
> you could no longer check whether a particular node was a sentinel node
> unless you had a reference to the list, so I gave up and went with this
> simpler approach).
> The most essential change is in the 'exec_list' structure. Three fields
> 'head', 'tail' and 'tail_pred' are removed, and two separate sentinel nodes
> are inserted in their place. The old 'head' is replaced by
> 'head_sentinel.next', 'tail_pred' by 'tail_sentinel.prev', and tail (always
> NULL) by 'head_sentinel.prev' and 'tail_sentinel.next' (both always NULL).
> With this patch, I can build a working (though perhaps not 100% bug-free)
> Mesa without using -fno-strict-aliasing during compilation. Before the
> patch, applications using Mesa would hang at runtime.
> I don't really know the process you have in place so if I need to do
> anything else to get this patch into the codebase please let me know. Any
> comments and criticisms welcome.
> Davin

Hi Davin,

Welcome aboard the Mesa-boat!
I haven't really looked at your patch
(I'll leave that to someone who knows the code),
but here's a few pointers on how things work around here.
(I see Ilia already answered you)

Have a look at http://www.mesa3d.org/devinfo.html.
This has most of the info you need on how to send patches
and how your source code should be formatted.

We try to limit lines to aprox 78 chars (no more than 80).
Patches also should have a "location" for the change in the subject.
(In your patch: [PATCH] glsl: Fix strict-aliasing.......)
And subjects in present tense, but you got that right =)

Until you get commit access someone else will have to
push the patches to upstream for you (usually the reviewer).

That's a few points of the top of my head.
If there's anything else feel free to ask.
I'm not the most experienced guy in here, but I guess I'm at
least able to answer most of your practical questions =)


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