[Mesa-dev] [PATCH 4/4] nir: Make nir_opt_remove_phis see through moves.

[Ilia Mirkin]

Sounds like you're doing something similar to nouveau's GlobalCSE
pass. On the off chance it's useful, you can have a look at how we do
it here:

https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp#n3118

On Fri, Jul 29, 2016 at 4:29 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> I found a shader in Tales of Maj'Eyal that contains:
>
>         if ssa_21 {
>                 block block_1:
>                 /* preds: block_0 */
>                 ...instructions that prevent the select peephole...
>                 vec1 32 ssa_23 = imov ssa_4
>                 vec1 32 ssa_24 = imov ssa_4.y
>                 vec1 32 ssa_25 = imov ssa_4.z
>                 /* succs: block_3 */
>         } else {
>                 block block_2:
>                 /* preds: block_0 */
>                 vec1 32 ssa_26 = imov ssa_4
>                 vec1 32 ssa_27 = imov ssa_4.y
>                 vec1 32 ssa_28 = imov ssa_4.z
>                 /* succs: block_3 */
>         }
>         block block_3:
>         /* preds: block_1 block_2 */
>         vec1 32 ssa_29 = phi block_1: ssa_23, block_2: ssa_26
>         vec1 32 ssa_30 = phi block_1: ssa_24, block_2: ssa_27
>         vec1 32 ssa_31 = phi block_1: ssa_25, block_2: ssa_28
>
> Here, copy propagation will bail because phis cannot perform swizzles,
> and CSE won't do anything because there is no dominance relationship
> between the imovs.  By making nir_opt_remove_phis handle identical moves,
> we can eliminate the phis and rewrite everything to use ssa_4 directly,
> so all the moves become dead and get eliminated.
>
> I don't think we need to check "exact" - just the alu sources.
> Presumably phi sources should match in their exactness.
>
> On Broadwell:
>
> total instructions in shared programs: 11639872 -> 11638535 (-0.01%)
> instructions in affected programs: 134222 -> 132885 (-1.00%)
> helped: 338
> HURT: 0
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/compiler/nir/nir_opt_remove_phis.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/src/compiler/nir/nir_opt_remove_phis.c b/src/compiler/nir/nir_opt_remove_phis.c
> index f1e7bb0..ebcc856 100644
> --- a/src/compiler/nir/nir_opt_remove_phis.c
> +++ b/src/compiler/nir/nir_opt_remove_phis.c
> @@ -27,6 +27,26 @@
>
>  #include "nir.h"
>
> +static nir_alu_instr *
> +get_parent_mov(nir_ssa_def *ssa)
> +{
> +   if (ssa->parent_instr->type != nir_instr_type_alu)
> +      return false;
> +
> +   nir_alu_instr *alu = nir_instr_as_alu(ssa->parent_instr);
> +   return (alu->op == nir_op_imov || alu->op == nir_op_fmov) ? alu : NULL;
> +}
> +
> +static bool
> +matching_mov(nir_alu_instr *mov1, nir_ssa_def *ssa)
> +{
> +   if (!mov1)
> +      return false;
> +
> +   nir_alu_instr *mov2 = get_parent_mov(ssa);
> +   return mov2 && nir_alu_srcs_equal(mov1, mov2, 0, 0);
> +}
> +
>  /*
>   * This is a pass for removing phi nodes that look like:
>   * a = phi(b, b, b, ...)
> @@ -54,6 +74,7 @@ remove_phis_block(nir_block *block)
>        nir_phi_instr *phi = nir_instr_as_phi(instr);
>
>        nir_ssa_def *def = NULL;
> +      nir_alu_instr *mov = NULL;
>        bool srcs_same = true;
>
>        nir_foreach_phi_src(src, phi) {
> @@ -75,8 +96,9 @@ remove_phis_block(nir_block *block)
>
>           if (def == NULL) {
>              def  = src->src.ssa;
> +            mov = get_parent_mov(def);
>           } else {
> -            if (src->src.ssa != def) {
> +            if (src->src.ssa != def && !matching_mov(mov, src->src.ssa)) {
>                 srcs_same = false;
>                 break;
>              }
> --
> 2.9.0
>
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