[Mesa-users] Compiler warning with newer GCC on linux

[Peter Mayo]

Hi users/developers for Mesa,

In compiling some of the mesa project in the chromium situation I ran across
a compile warning which looks valid and easily fixable.

I would like to pass along a solution in case it has value to you, either as
part of the next release, or as a downstream fix for other users.

The problem shows up as a warning from GCC 4.3 and probably up.

MesaLib/src/egl/main/eglconfig.c: In function _eglCompareConfigs:

MesaLib/src/egl/main/eglconfig.h:98: error: array subscript is below
array bounds

This comes about because of the nexted inline functions _eglIndexConfig
and _eglGetConfigKey which although in the case of the call site in
CompareConfigs are called with valid attributes, the call is nested deeply
enough that it appears GCC excercises it's option on not inlining the
functions, and compiled without specific values of arguments in optimized
mode (assert turns empty) they could access element -1 of the Storage array
- hence the warning.

It seems to me that the smallest kind of fix to this is to move the assert
down (since each call to eglIndexConfig is followed by an assert(idx>0) )
and return 0 instead of -1; providing equivalent protection in the debug
case and less random behavior in the non-debug case (-01.patch)

Alternately, if it does not affect performance too much, adding "if (idx <
0) return 0;" between the assert and the return in _eglGetConfigKey would
achieve deterministic performance and no potential illegal accesses.

A third alternative, which is more intrusive/larger but should not affect
performance is to write a macro for the inner portion of the compare loop,
and replace the local array with code that simply lists the attribute values
in place. (see -02.patch)


Alternately, you could disable the warning for the illegal access which
would be a bit of a shame, since with the warning the compiler would warn
you at compile time if you passed an illegal atribute.  About the best time
to get such a warning.

For what it's worth, your mileage may vary, etc,

Peter.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/mesa-users/attachments/20110104/789ed945/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fix-2011-01-03-01.patch
Type: application/octet-stream
Size: 984 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/mesa-users/attachments/20110104/789ed945/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fix-2011-01-03-02.patch
Type: application/octet-stream
Size: 2363 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/mesa-users/attachments/20110104/789ed945/attachment-0001.obj>