[Mesa-dev] radeonsi: LLVM r187139 broke some piglit tests

[Tom Stellard]

On Thu, Aug 15, 2013 at 11:55:36AM +0200, Michel Dänzer wrote:
> On Fre, 2013-08-02 at 17:58 +0200, Michel Dänzer wrote:
> > On Mit, 2013-07-31 at 08:42 -0700, Tom Stellard wrote:
> > > On Wed, Jul 31, 2013 at 01:04:01PM +0200, Michel Dänzer wrote:
> > > > 
> > > > LLVM revision 187139 ('Allocate local registers in order for optimal
> > > > coloring.') broke some derivative related piglit tests with the radeonsi
> > > > driver. 
> > > > 
> > > > I'm attaching a diff between the bad and good generated code (as printed
> > > > with RADEON_DUMP_SHADERS=1) for the glsl-derivs test. The only
> > > > difference I can see is in which registers are used in which order.
> > > > 
> > > > I wonder if we might be missing S_WAITCNT after DS_READ/WRITE
> > > > instructions in some cases, but I haven't spotted any candidates for
> > > > that in the bad code which aren't there in the good code as well. Can
> > > > anyone else spot something I've missed?
> > > 
> > > Shouldn't we be using the S_BARRIER instruction to keep the threads in sync?
> > 
> > Doesn't seem to help unfortunately, but thanks for the good suggestion.
> 
> I found one thing going wrong: DS_WRITE_B32 ends up using a VGPR
> register number instead of the $gds operand for encoding the GDS field
> (the asm output from llc even shows the VGPR name). If the VGPR number
> happens to be odd (i.e. to have the least significant bit set), the
> shader ends up writing to GDS instead of LDS.
>

Ouch, that's a pretty bad bug.

> But I have no idea why this is happening, or how to fix it. :(
> 
> 

I can take a look at it.

-Tom

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