[Mesa-dev] [RFC] Mesa 7.9 release criteria

Eric Anholt eric at anholt.net
Wed Sep 1 17:32:54 PDT 2010

On Tue, 31 Aug 2010 19:35:09 -0600, Brian Paul <brianp at vmware.com> wrote:
> On 08/31/2010 02:57 PM, Ian Romanick wrote:
> > Hash: SHA1


> > So... what are our collective criteria for a 7.9 release to happen?
> Ian, I'm still getting up to speed on the new compiler, but it looks 
> like a full 4-element temp/const vector is allocated for each GLSL 
> float.  The previous compiler would try to pack four floats into a 
> single vector whenever possible.  This could dramatically reduced the 
> number of vector temps/consts needed.
> I'm worried that some apps that use large shaders will start failing.

Actually, in my experience that packing of variables into vec4 storage
before register allocation caused all temps allocated to basically be
live for the lifetime of the program, and copy propagation/dead code
elimination at the Mesa IR level to not occur.  Then if a large program
could even fit under the number of temps for register-constrained hw, it
probably didn't have enough instructions for all the dead code.

If we want to codegen from Mesa IR for our drivers without doing
register allocation in the drivers (which itself is quite questionable
if you really want programs to work on register-constrained,
multiple-phase hw like 915), then the register allocator needs to grow
awareness of how to move results around to pack things *after* copy
propagation and dead code elimination.
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