[Mesa-dev] [PATCH 2/2] nv50/ir: improve POW lowering

[Roland Scheidegger]

Am 22.06.2017 um 21:28 schrieb Karol Herbst:
> If the exponent is a small integer immediate value, we can lower POW to
> MULs instead to save a few instructions. Also MUL instructions execute
> faster than what we lower POW in the default case to.
> 
> score change for GpuTest /test=pixmark_piano /benchmark /no_scorebox
> /msaa=0 /benchmark_duration_ms=60000 /width=1024 /height=640:
> 1045 -> 1060
> 
> changes in shader-db:
> total instructions in shared programs : 4350261 -> 4349451 (-0.02%)
> total gprs used in shared programs    : 525853 -> 525861 (0.00%)
> total local used in shared programs   : 30081 -> 30081 (0.00%)
> total bytes used in shared programs   : 39865176 -> 39857712 (-0.02%)
> 
>                 local        gpr       inst      bytes
>     helped           0           4         313         313
>       hurt           0          12           1           1
> 
> Signed-off-by: Karol Herbst <karolherbst at gmail.com>
> ---
>  .../drivers/nouveau/codegen/nv50_ir_build_util.cpp | 49 ++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir_build_util.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir_build_util.cpp
> index 5756e1b4d4..b31dcec1ab 100644
> --- a/src/gallium/drivers/nouveau/codegen/nv50_ir_build_util.cpp
> +++ b/src/gallium/drivers/nouveau/codegen/nv50_ir_build_util.cpp
> @@ -640,6 +640,55 @@ bool
>  BuildUtil::lowerPOW(Instruction *i)
>  {
>     LValue *val = getScratch();
> +   ImmediateValue imm;
> +
> +   if (i->src(1).getImmediate(imm)) {
> +      Value *src = i->getSrc(0);
> +      if (imm.isInteger(0)) {
> +         i->op = OP_MOV;
> +         i->setSrc(0, loadImm(NULL, 1));
> +         i->setSrc(1, NULL);
> +         return true;
> +      } else
> +      if (imm.isInteger(1)) {
> +         i->op = OP_MOV;
> +         i->setSrc(1, NULL);
> +         return true;
> +      } else
> +      if (imm.isInteger(2)) {
> +         i->op = OP_MUL;
> +         i->setSrc(1, src);
> +         return true;
> +      } else
> +      if (imm.isInteger(3)) {
> +         mkOp2(OP_MUL, i->dType, val, src, src);
> +         i->op = OP_MUL;
> +         i->setSrc(1, val);
> +         return true;
> +      } else
> +      if (imm.isInteger(4)) {
> +         mkOp2(OP_MUL, i->dType, val, src, src);
> +         i->op = OP_MUL;
> +         i->setSrc(0, val);
> +         i->setSrc(1, val);
> +         return true;
> +      } else
> +      if (imm.isInteger(5)) {
> +         mkOp2(OP_MUL, i->dType, val, src, src);
> +         mkOp2(OP_MUL, i->dType, val, val, val);
> +         i->op = OP_MUL;
> +         i->setSrc(1, val);
> +         return true;
> +      } else
> +      if (imm.isInteger(8)) {
> +         mkOp2(OP_MUL, i->dType, val, src, src);
> +         mkOp2(OP_MUL, i->dType, val, val, val);
> +         i->op = OP_MUL;
> +         i->setSrc(0, val);
> +         i->setSrc(1, val);
> +         return true;
> +      }
> +   }
>  
>     mkOp1(OP_LG2, TYPE_F32, val, i->getSrc(0));
>     mkOp2(OP_MUL, TYPE_F32, val, i->getSrc(1), val)->dnz = 1;
> 

You could do integer 6 with just 3 muls too :-).
But more in general, this looks like it would be useful outside of
nouveau too.

Roland