[Mesa-dev] [PATCH] nir: evaluate if condition uses inside the if branches

[Ian Romanick]

On 06/17/2018 05:15 PM, Timothy Arceri wrote:
> Since we know what side of the branch we ended up on we can just
> replace the use with a constant.
> 
> All helped shaders are from Unreal Engine 4 besides one shader from
> Dirt Showdown.
> 
> shader-db results SKL:
> 
> total instructions in shared programs: 13219725 -> 13219643 (<.01%)
> instructions in affected programs: 28917 -> 28835 (-0.28%)
> helped: 45
> HURT: 0
> 
> total cycles in shared programs: 529335971 -> 529334604 (<.01%)
> cycles in affected programs: 216209 -> 214842 (-0.63%)
> helped: 45
> HURT: 4
> ---
>  src/compiler/nir/nir_opt_if.c | 84 +++++++++++++++++++++++++++++++----
>  1 file changed, 76 insertions(+), 8 deletions(-)
> 
> diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
> index 863ca630fbd..2666c5da395 100644
> --- a/src/compiler/nir/nir_opt_if.c
> +++ b/src/compiler/nir/nir_opt_if.c
> @@ -350,8 +350,53 @@ opt_if_loop_terminator(nir_if *nif)
>     return true;
>  }
>  
> +static void
> +replace_if_condition_use_with_const(nir_src *use, nir_instr *instr,
> +                                    unsigned nir_boolean, void *mem_ctx)
> +{
> +   /* Create const */
> +   nir_load_const_instr *load = nir_load_const_instr_create(mem_ctx, 1, 32);
> +   load->value.u32[0] = nir_boolean;
> +   nir_instr_insert_before(instr,  &load->instr);
> +
> +   /* Rewrite use to use const */
> +   nir_src new_src = nir_src_for_ssa(&load->def);
> +   nir_instr_rewrite_src(use->parent_instr, use, new_src);
> +}
> +
>  static bool
> -opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
> +opt_if_evaluate_condition_use(nir_if *nif, void *mem_ctx)
> +{
> +   bool progress = false;
> +
> +   /* Evaluate any uses of the if condition inside the if branches */
> +   nir_foreach_use_safe(use_src, nif->condition.ssa) {

Does this iterate both the uses by regular instructions and the uses by
other if-statements?  I was working on something similar (though using a
different approach) because I came across some shaders that, after loop
unrolling, had some neat stuff like

    if (ssa_111) {
        ...
        if (ssa_111) {
            ...
            if (ssa_111) {
                ...
                if (ssa_111) {
                    ...
                    if (ssa_111) {
                        ...
                        if (ssa_111) {
                            ...

> +      nir_block *first_then = nir_if_first_then_block(nif);
> +      if (use_src->parent_instr->block->index > first_then->index) {
> +         nir_block *first_else = nir_if_first_else_block(nif);
> +         if (use_src->parent_instr->block->index < first_else->index) {
> +            nir_instr *instr = nir_block_first_instr(first_then);
> +            replace_if_condition_use_with_const(use_src, instr,
> +                                                NIR_TRUE, mem_ctx);
> +
> +            progress = true;
> +         } else if (use_src->parent_instr->block->index <
> +                    nir_if_last_else_block(nif)->index) {
> +            nir_instr *instr = nir_block_first_instr(first_else);
> +            replace_if_condition_use_with_const(use_src, instr,
> +                                                NIR_FALSE, mem_ctx);
> +
> +            progress = true;
> +         }
> +      }
> +   }
> +
> +   return progress;
> +}
> +
> +static bool
> +opt_if_cf_list(nir_builder *b, struct exec_list *cf_list, void *mem_ctx,
> +               bool *md_altered)
>  {
>     bool progress = false;
>     foreach_list_typed(nir_cf_node, cf_node, node, cf_list) {
> @@ -361,17 +406,30 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
>  
>        case nir_cf_node_if: {
>           nir_if *nif = nir_cf_node_as_if(cf_node);
> -         progress |= opt_if_cf_list(b, &nif->then_list);
> -         progress |= opt_if_cf_list(b, &nif->else_list);
> -         progress |= opt_if_loop_terminator(nif);
> -         progress |= opt_if_simplification(b, nif);
> +         progress |= opt_if_cf_list(b, &nif->then_list, mem_ctx, md_altered);
> +         progress |= opt_if_cf_list(b, &nif->else_list, mem_ctx, md_altered);
> +
> +         if (*md_altered == false)
> +            progress |= opt_if_evaluate_condition_use(nif, mem_ctx);
> +
> +         bool lp_term_progress = opt_if_loop_terminator(nif);
> +         bool if_simp_progress = opt_if_simplification(b, nif);
> +         if (lp_term_progress || if_simp_progress) {
> +            *md_altered = true;
> +            progress = true;
> +         }
> +
>           break;
>        }
>  
>        case nir_cf_node_loop: {
>           nir_loop *loop = nir_cf_node_as_loop(cf_node);
> -         progress |= opt_if_cf_list(b, &loop->body);
> -         progress |= opt_peel_loop_initial_if(loop);
> +         progress |= opt_if_cf_list(b, &loop->body, mem_ctx, md_altered);
> +         bool lp_peel_progress = opt_peel_loop_initial_if(loop);
> +         if (lp_peel_progress) {
> +            *md_altered = true;
> +            progress = true;
> +         }
>           break;
>        }
>  
> @@ -395,7 +453,17 @@ nir_opt_if(nir_shader *shader)
>        nir_builder b;
>        nir_builder_init(&b, function->impl);
>  
> -      if (opt_if_cf_list(&b, &function->impl->body)) {
> +      nir_metadata_require(function->impl, nir_metadata_block_index);
> +
> +      /* opt_if_evaluate_condition_use() depends on nir_metadata_block_index
> +       * if one of the other opts has altered the block in some way we need to
> +       * make sure we skip this opt.
> +       */
> +      bool index_metadata_altered = false;
> +
> +      void *mem_ctx = ralloc_parent(function->impl);
> +      if (opt_if_cf_list(&b, &function->impl->body, mem_ctx,
> +                         &index_metadata_altered)) {
>           nir_metadata_preserve(function->impl, nir_metadata_none);
>  
>           /* If that made progress, we're no longer really in SSA form.  We
>