[Mesa-dev] Mesa (master): r600g: Use default mul/mad function for tgsi-to-llvm

[Michel Dänzer]

On Don, 2012-12-06 at 09:05 -0800, Tom Stellard wrote: 
> On Thu, Dec 06, 2012 at 05:08:07PM +0100, Michel Dänzer wrote:
> > On Mit, 2012-12-05 at 09:32 -0800, Vincent Lejeune wrote: 
> > > Module: Mesa
> > > Branch: master
> > > Commit: 0ad1fefd6951aa47ab58a41dc9ee73083cbcf85c
> > > URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=0ad1fefd6951aa47ab58a41dc9ee73083cbcf85c
> > > 
> > > Author: Vincent Lejeune <vljn at ovi.com>
> > > Date:   Wed Nov 28 00:35:55 2012 +0100
> > > 
> > > r600g: Use default mul/mad function for tgsi-to-llvm
> > 
> > This change breaks the piglit tests glsl-{f,v}s-vec4-indexing-temp-src
> > on radeonsi. I suspect the same would be true with r600g as well if it
> > didn't fall back to the non-LLVM backend for this test.
> > 
> > Comparing the generated code, I've noticed two main differences so far:
> > 
> >       * LLVM now optimizes away some TGSI MUL operations with constant
> >         1.0, which previously resulted in V_MUL_LEGACY_F32 (non-IEEE
> >         semantics) instructions. 
> >       * V_MUL_F32 and V_ADD_F32 (IEEE semantics) are used instead of
> >         V_MAD_LEGACY_F32 (non-IEEE semantics) in some places.

BTW, I have a fix so the second bullet at least becomes V_MAD_F32.


> > I suspect the problem is a (non-)IEEE semantics mismatch between TGSI
> > and LLVM introduced by this change.
> 
> What are the semantics of TGSI opcodes?  For MUL and MAD, tgsi_exec uses IEEE
> operations, but it seems like the glsl frontend thinks they are non-IEEE. 

Not sure... FWIW, the tests pass on llvmpipe.

Reverting only the MAD related changes doesn't fix the problem.
Reverting only the MUL related change triggers a shader compile failure
in LLVM.


> Should we revert this and the associated LLVM changes?

I guess it depends on the answer to the above question about the
expected semantics.

Given that these tests seem to use indirect addressing, could it be that
they just happened to pass by accident before?


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer