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

[Timothy Arceri]

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) {
+      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
-- 
2.17.1