[Mesa-dev] [PATCH 07/10] nir: add loop unroll support for wrapper loops

[Timothy Arceri]

This adds support for unrolling the classic

    do {
        // ...
    } while (false)

that is used to wrap multi-line macros. GLSL IR also wraps switch
statements in a loop like this.

shader-db results IVB:

total loops in shared programs: 2515 -> 2512 (-0.12%)
loops in affected programs: 33 -> 30 (-9.09%)
helped: 3
HURT: 0
---
 src/compiler/nir/nir_opt_loop_unroll.c | 63 ++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/src/compiler/nir/nir_opt_loop_unroll.c b/src/compiler/nir/nir_opt_loop_unroll.c
index 38901470f96..f715fc8ebda 100644
--- a/src/compiler/nir/nir_opt_loop_unroll.c
+++ b/src/compiler/nir/nir_opt_loop_unroll.c
@@ -516,6 +516,69 @@ process_loops(nir_shader *sh, nir_cf_node *cf_node, bool *has_nested_loop)
     */
    if (!progress) {
 
+      /* Check for the the classic
+       *
+       *    do {
+       *        // ...
+       *    } while (false)
+       *
+       * that is used to wrap multi-line macros. GLSL IR also wraps switch
+       * statements in a loop like this.
+       */
+      if (loop->info->limiting_terminator == NULL &&
+          list_empty(&loop->info->loop_terminator_list) &&
+          !loop->info->complex_loop) {
+
+         nir_block *blk_after_loop =
+            nir_cursor_current_block(nir_after_cf_node(&loop->cf_node));
+
+         /* There may still be some single src phis following the loop that
+          * have not yet been cleaned up by another pass. Tidy those up before
+          * unrolling the loop.
+          */
+         nir_foreach_instr_safe(instr, blk_after_loop) {
+            if (instr->type != nir_instr_type_phi)
+               break;
+
+            nir_phi_instr *phi = nir_instr_as_phi(instr);
+            assert(exec_list_length(&phi->srcs) == 1);
+
+            nir_phi_src *phi_src =
+               exec_node_data(nir_phi_src,
+                              exec_list_get_head(&phi->srcs),
+                              node);
+
+            nir_ssa_def_rewrite_uses(&phi->dest.ssa, phi_src->src);
+            nir_instr_remove(instr);
+
+            progress = true;
+         }
+
+         nir_block *last_loop_blk = nir_loop_last_block(loop);
+         if (nir_block_ends_in_break(last_loop_blk)) {
+
+            /* Remove break at end of the loop */
+            nir_instr *break_instr = nir_block_last_instr(last_loop_blk);
+            nir_instr_remove(break_instr);
+
+            /* Pluck out the loop body. */
+            nir_cf_list loop_body;
+            nir_cf_extract(&loop_body,
+                           nir_before_block(nir_loop_first_block(loop)),
+                           nir_after_block(nir_loop_last_block(loop)));
+
+            /* Reinsert loop body after the loop */
+            nir_cf_reinsert(&loop_body, nir_after_cf_node(&loop->cf_node));
+
+            /* The loop has been unrolled so remove it. */
+            nir_cf_node_remove(&loop->cf_node);
+
+            progress = true;
+         }
+
+         goto exit;
+      }
+
       if (*has_nested_loop || loop->info->limiting_terminator == NULL)
          goto exit;
 
-- 
2.17.1