[Mesa-dev] [PATCH] nir: propagate known constant values into the if-then branch

[Timothy Arceri]

On 23/11/18 11:49 am, Timothy Arceri wrote:
> shader-db results radeonsi (VEGA):
> 
> Totals from affected shaders:
> SGPRS: 7272 -> 7488 (2.97 %)
> VGPRS: 4168 -> 4160 (-0.19 %)
> Spilled SGPRs: 328 -> 327 (-0.30 %)
> Spilled VGPRs: 0 -> 0 (0.00 %)
> Private memory VGPRs: 0 -> 0 (0.00 %)
> Scratch size: 0 -> 0 (0.00 %) dwords per thread
> Code Size: 268212 -> 268656 (0.17 %) bytes
> LDS: 0 -> 0 (0.00 %) blocks
> Max Waves: 452 -> 452 (0.00 %)
> Wait states: 0 -> 0 (0.00 %)
> 
> vkpipeline-db results RADV (VEGA):
> 
> Totals from affected shaders:
> SGPRS: 160 -> 160 (0.00 %)
> VGPRS: 88 -> 88 (0.00 %)
> Spilled SGPRs: 0 -> 0 (0.00 %)
> Spilled VGPRs: 0 -> 0 (0.00 %)
> Private memory VGPRs: 0 -> 0 (0.00 %)
> Scratch size: 0 -> 0 (0.00 %) dwords per thread
> Code Size: 18268 -> 18152 (-0.63 %) bytes
> LDS: 0 -> 0 (0.00 %) blocks
> Max Waves: 26 -> 26 (0.00 %)
> Wait states: 0 -> 0 (0.00 %)
> ---
>   src/compiler/nir/nir_opt_if.c | 60 +++++++++++++++++++++++++++++++++++
>   1 file changed, 60 insertions(+)
> 
> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
> index 8a971c43f2..b1b99b4e40 100644
> --- a/src/compiler/nir/nir_opt_if.c
> +++ b/src/compiler/nir/nir_opt_if.c
> @@ -574,6 +574,65 @@ opt_if_evaluate_condition_use(nir_builder *b, nir_if *nif)
>      return progress;
>   }
>   
> +/* Perform optimisations based on the values we can derive from the evaluation
> + * of if-statement conditions.
> + */
> +static bool
> +opt_for_known_values(nir_builder *b, nir_if *nif)
> +{
> +   bool progress = false;
> +
> +   assert(nif->condition.is_ssa);
> +   nir_ssa_def *if_cond = nif->condition.ssa;
> +
> +   if (if_cond->parent_instr->type != nir_instr_type_alu)
> +      return false;
> +
> +   nir_alu_instr *alu = nir_instr_as_alu(if_cond->parent_instr);
> +   switch (alu->op) {
> +   case nir_op_feq:
> +   case nir_op_ieq: {
> +      nir_load_const_instr *load_const = NULL;
> +      nir_ssa_def *unknown_val = NULL;
> +
> +      nir_ssa_def *src0 = alu->src[0].src.ssa;
> +      nir_ssa_def *src1 = alu->src[1].src.ssa;
> +      if (src0a->parent_instr->type == nir_instr_type_load_const) {

                  ^--- Whoops this is fixed locally