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

[Michel Dänzer]

On Don, 2012-12-06 at 10:52 -0800, Tom Stellard wrote: 
> On Thu, Dec 06, 2012 at 07:25:55PM +0100, Michel Dänzer wrote:
> > 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: 
> > > > 
> > > > 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.
> 
> I just looked at the TGSI->LLVM code, which is shared by r600g,
> radeonsi, and llvmpipe.  For TGSI_OPCODE_MUL, it actually uses both
> the IEEE and non-IEEE definitions depending on what the arguments are.
> If one of the arguments to MUL is the compile time constant zero then it
> uses the non-IEEE definition (0 * anything) = 0, otherwise it produces
> a fmul instruction which has IEEE semantics.  This probably deserves
> it's own thread, because I think newer versions of GLSL require IEEE,
> but older versions and also ARB have different semantics.

Right, also, the code you described doesn't cover cases where a
non-constant value is 0.


> > > 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?
> 
> Yeah, those tests must have been passing by accident.  Are there any other
> failing tests?

No, those are the only two tests that regressed for me. So I guess this
can be ignored for now. It would be interesting to know which tests Alex
was referring to though, which were fixed by switching from MUL_IEEE to
MUL. Maybe those are covered by the code you described above though.


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