[Mesa-dev] [PATCH 1/1] nir: add a pass that removes continue blocks

[Samuel Pitoiset]

28717 shaders in 14931 tests
Totals:
SGPRS: 1267317 -> 1267549 (0.02 %)
VGPRS: 896876 -> 895920 (-0.11 %)
Spilled SGPRs: 24701 -> 26367 (6.74 %)
Code Size: 48379452 -> 48507880 (0.27 %) bytes
Max Waves: 241159 -> 241190 (0.01 %)

Totals from affected shaders:
SGPRS: 23584 -> 23816 (0.98 %)
VGPRS: 25908 -> 24952 (-3.69 %)
Spilled SGPRs: 503 -> 2169 (331.21 %)
Code Size: 2471392 -> 2599820 (5.20 %) bytes
Max Waves: 586 -> 617 (5.29 %)

The codesize increases is related to Wolfenstein II, maybe it has
too many continue blocks and we should add an arbitrary limit.

This gives +10% FPS with Doom on my Vega56.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
---
 src/compiler/nir/nir_opt_if.c | 87 +++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)

diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
index bc128f79f3c..47a8a65aad3 100644
--- a/src/compiler/nir/nir_opt_if.c
+++ b/src/compiler/nir/nir_opt_if.c
@@ -890,6 +890,92 @@ opt_if_loop_last_continue(nir_loop *loop)
    return true;
 }
 
+/**
+ * This optimization allows to remove continue blocks by moving the rest of the
+ * loop to the other side. This is only applied if continue blocks are in the
+ * top-level control flow of the loop. This turns:
+ *
+ *    if (cond) {
+ *       ... code ...
+ *       continue
+ *    } else {
+ *    }
+ *    ... code ...
+ *
+ * into:
+ *
+ *    if (cond) {
+ *       ... code ...
+ *       continue
+ *    } else {
+ *       ... code ...
+ *    }
+ */
+static bool
+opt_if_loop_remove_continue(nir_loop *loop)
+{
+   nir_block *header_block = nir_loop_first_block(loop);
+   nir_block *bottom_block = nir_loop_last_block(loop);
+   nir_block *prev_block =
+      nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
+
+   /* It would be insane if this were not true */
+   assert(_mesa_set_search(header_block->predecessors, prev_block));
+
+   /* The loop must have, at least, two continue blocks (including the normal
+    * continue at the end of loop).
+    */
+   if (header_block->predecessors->entries < 3)
+      return false;
+
+   /* Scan the control flow of the loop from the last to the first node, and
+    * optimize when an if block contains a continue.
+    */
+   nir_cf_node *last_node = &bottom_block->cf_node;
+   nir_cf_node *first_node = &header_block->cf_node;
+
+   while (last_node != first_node) {
+      last_node = nir_cf_node_prev(last_node);
+      if (last_node->type != nir_cf_node_if)
+         continue;
+
+      nir_if *nif = nir_cf_node_as_if(last_node);
+      nir_block *then_block = nir_if_last_then_block(nif);
+
+      /* Nothing to do if the block isn't a continue block. */
+      nir_instr *instr = nir_block_last_instr(then_block);
+      if (!instr ||
+          instr->type != nir_instr_type_jump ||
+          nir_instr_as_jump(instr)->type != nir_jump_continue)
+         continue;
+
+      /* Get the block immediately following the if statement. */
+      nir_block *after_if_block =
+         nir_cf_node_as_block(nir_cf_node_next(&nif->cf_node));
+
+      /* Nothing to do if the block following the if statement is empty. */
+      if (is_block_empty(after_if_block))
+         continue;
+
+      nir_cf_node *if_first_node = &after_if_block->cf_node;
+      nir_cf_node *if_last_node = &bottom_block->cf_node;
+      nir_block *else_block = nir_if_last_else_block(nif);
+
+      /* Move all nodes following the if statement to the last else block, in
+       * case there is complicated control flow.
+       */
+      nir_cf_list tmp;
+      nir_cf_extract(&tmp, nir_before_cf_node(if_first_node),
+                           nir_after_cf_node(if_last_node));
+      nir_cf_reinsert(&tmp, nir_after_block(else_block));
+      nir_cf_delete(&tmp);
+
+      return true;
+   }
+
+   return false;
+}
+
 /* Walk all the phis in the block immediately following the if statement and
  * swap the blocks.
  */
@@ -1355,6 +1441,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
          progress |= opt_simplify_bcsel_of_phi(b, loop);
          progress |= opt_peel_loop_initial_if(loop);
          progress |= opt_if_loop_last_continue(loop);
+         progress |= opt_if_loop_remove_continue(loop);
          break;
       }
 
-- 
2.21.0