[Mesa-dev] [PATCH] Fix strict-aliasing violations in GLSL shader list implementation
davmac at davmac.org
Wed Jun 24 14:05:11 PDT 2015
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
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.
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