[Mesa-dev] [PATCH 2/9] radeonsi: use V_BFE for extracting a sample index

[Tom Stellard]

On Mon, Mar 02, 2015 at 10:14:00PM +0100, Marek Olšák wrote:
> On Mon, Mar 2, 2015 at 10:05 PM, Tom Stellard <tom at stellard.net> wrote:
> > On Mon, Mar 02, 2015 at 12:54:16PM +0100, Marek Olšák wrote:
> >> From: Marek Olšák <marek.olsak at amd.com>
> >>
> >> ---
> >>  src/gallium/drivers/radeonsi/si_shader.c | 22 ++++++++++++++++------
> >>  1 file changed, 16 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/src/gallium/drivers/radeonsi/si_shader.c b/src/gallium/drivers/radeonsi/si_shader.c
> >> index b0417ed..f125483 100644
> >> --- a/src/gallium/drivers/radeonsi/si_shader.c
> >> +++ b/src/gallium/drivers/radeonsi/si_shader.c
> >> @@ -192,6 +192,20 @@ static int get_param_index(unsigned semantic_name, unsigned index,
> >>  }
> >>
> >>  /**
> >> + * BitField Extract: ((value >> rshift) & ((1 << bitwidth) - 1))
> >> + */
> >
> > Ideally, we would just add a pattern for this in the backend and emit generic
> > LLVM IR here.  This would also make it possible to share the code with llvmpipe.
> >
> > I think the best place to do this would be in AMDGPUTargetLowering::performDAGCombine().
> 
> Why not SIInstructions.td?
> 

Because for patterns like this, I think it is important to match them as
early as possible, because there may be another optimization which reduces
the sequence from 5 to 4 instructions which would cause the pattern not to match.

-Tom

> Marek