[Mesa-dev] [PATCH v2] nir: add support for removing redundant stores to copy prop var

[Jason Ekstrand]

On November 12, 2018 19:21:42 Timothy Arceri <tarceri at itsqueeze.com> wrote:

> For example the following type of thing is seen in TCS from
> a number of Vulkan and DXVK games:
>
>
> vec1 32 ssa_557 = deref_var &oPatch (shader_out float)
> vec1 32 ssa_558 = intrinsic load_deref (ssa_557) ()
> vec1 32 ssa_559 = deref_var &oPatch at 42 (shader_out float)
> vec1 32 ssa_560 = intrinsic load_deref (ssa_559) ()
> vec1 32 ssa_561 = deref_var &oPatch at 43 (shader_out float)
> vec1 32 ssa_562 = intrinsic load_deref (ssa_561) ()
> intrinsic store_deref (ssa_557, ssa_558) (1) /* wrmask=x */
> intrinsic store_deref (ssa_559, ssa_560) (1) /* wrmask=x */
> intrinsic store_deref (ssa_561, ssa_562) (1) /* wrmask=x */
>
>
> No shader-db changes on i965 (SKL).
>
>
> vkpipeline-db results RADV (VEGA):
>
>
> Totals from affected shaders:
> SGPRS: 7832 -> 7728 (-1.33 %)
> VGPRS: 6476 -> 6740 (4.08 %)
> 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: 469572 -> 456596 (-2.76 %) bytes
> LDS: 0 -> 0 (0.00 %) blocks
> Max Waves: 989 -> 960 (-2.93 %)
> Wait states: 0 -> 0 (0.00 %)
>
>
> The Max Waves and VGPRS changes here are misleading. What is
> happening is a bunch of TCS outputs are being optimised away as
> they are now recognised as unused. This results in more varyings
> being compacted via nir_compact_varyings() which can result in
> more register pressure when they are not packed in an optimal way.
> This is an existing problem independent of this patch. I've run
> some benchmarks and haven't noticed any performance regressions
> in affected games.
>
>
> V2: check all ssa values in case another store has altered the
>    value of a component.
> ---
> src/compiler/nir/nir_opt_copy_prop_vars.c | 41 +++++++++++++++++------
> 1 file changed, 31 insertions(+), 10 deletions(-)
>
>
> diff --git a/src/compiler/nir/nir_opt_copy_prop_vars.c 
> b/src/compiler/nir/nir_opt_copy_prop_vars.c
> index 7a21ad56c79..83ec9189159 100644
> --- a/src/compiler/nir/nir_opt_copy_prop_vars.c
> +++ b/src/compiler/nir/nir_opt_copy_prop_vars.c
> @@ -86,6 +86,17 @@ struct copy_prop_var_state {
>    bool progress;
> };
>
> +static bool
> +value_equals_ssa(struct value *value, nir_intrinsic_instr *intrin)
> +{
> +   for (unsigned i = 0; i < intrin->num_components; i++) {
> +      if (value->ssa[i] != intrin->src[1].ssa)

It's per-component, not per-source.

> +         return false;
> +   }
> +
> +   return true;
> +}
> +
> static struct vars_written *
> create_vars_written(struct copy_prop_var_state *state)
> {
> @@ -676,18 +687,28 @@ copy_prop_vars_block(struct copy_prop_var_state *state,
>       }
>
>       case nir_intrinsic_store_deref: {
> -         struct value value = {
> -            .is_ssa = true
> -         };
> -
> -         for (unsigned i = 0; i < intrin->num_components; i++)
> -            value.ssa[i] = intrin->src[1].ssa;
> -
>          nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
> -         unsigned wrmask = nir_intrinsic_write_mask(intrin);
>          struct copy_entry *entry =
> -            get_entry_and_kill_aliases(copies, dst, wrmask);
> -         store_to_entry(state, entry, &value, wrmask);
> +            lookup_entry_for_deref(copies, dst, nir_derefs_equal_bit);
> +         if (entry && value_equals_ssa(&entry->src, intrin)) {
> +            /* If we are storing the value from a load of the same var the
> +             * store is redundant so remove it.
> +             */
> +            nir_instr_remove(instr);
> +         } else {
> +            struct value value = {
> +               .is_ssa = true
> +            };
> +
> +            for (unsigned i = 0; i < intrin->num_components; i++)
> +               value.ssa[i] = intrin->src[1].ssa;
> +
> +            unsigned wrmask = nir_intrinsic_write_mask(intrin);
> +            struct copy_entry *entry =
> +               get_entry_and_kill_aliases(copies, dst, wrmask);
> +            store_to_entry(state, entry, &value, wrmask);
> +         }
> +
>          break;
>       }
>
> --
> 2.19.1