[Mesa-dev] [PATCH 15/20] nir: detect more induction variables

[Timothy Arceri]

This allows loop analysis to detect inductions variables that
are incremented in both branches of an if rather than in a main
loop block. For example:

   loop {
      block block_1:
      /* preds: block_0 block_7 */
      vec1 32 ssa_8 = phi block_0: ssa_4, block_7: ssa_20
      vec1 32 ssa_9 = phi block_0: ssa_0, block_7: ssa_4
      vec1 32 ssa_10 = phi block_0: ssa_1, block_7: ssa_4
      vec1 32 ssa_11 = phi block_0: ssa_2, block_7: ssa_21
      vec1 32 ssa_12 = phi block_0: ssa_3, block_7: ssa_22
      vec4 32 ssa_13 = vec4 ssa_12, ssa_11, ssa_10, ssa_9
      vec1 32 ssa_14 = ige ssa_8, ssa_5
      /* succs: block_2 block_3 */
      if ssa_14 {
         block block_2:
         /* preds: block_1 */
         break
         /* succs: block_8 */
      } else {
         block block_3:
         /* preds: block_1 */
         /* succs: block_4 */
      }
      block block_4:
      /* preds: block_3 */
      vec1 32 ssa_15 = ilt ssa_6, ssa_8
      /* succs: block_5 block_6 */
      if ssa_15 {
         block block_5:
         /* preds: block_4 */
         vec1 32 ssa_16 = iadd ssa_8, ssa_7
         vec1 32 ssa_17 = load_const (0x3f800000 /* 1.000000*/)
         /* succs: block_7 */
      } else {
         block block_6:
         /* preds: block_4 */
         vec1 32 ssa_18 = iadd ssa_8, ssa_7
         vec1 32 ssa_19 = load_const (0x3f800000 /* 1.000000*/)
         /* succs: block_7 */
      }
      block block_7:
      /* preds: block_5 block_6 */
      vec1 32 ssa_20 = phi block_5: ssa_16, block_6: ssa_18
      vec1 32 ssa_21 = phi block_5: ssa_17, block_6: ssa_4
      vec1 32 ssa_22 = phi block_5: ssa_4, block_6: ssa_19
      /* succs: block_1 */
   }

Unfortunatly GCM could move the addition out of the if for us
(making this patch unrequired) but we still cannot enable the GCM
pass without regressions.

This unrolls a loop in Rise of The Tomb Raider.

vkpipeline-db results (VEGA):

Totals from affected shaders:
SGPRS: 88 -> 96 (9.09 %)
VGPRS: 56 -> 52 (-7.14 %)
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: 2168 -> 4560 (110.33 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 4 -> 4 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Thomas Helland <thomashelland90 at gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32211
---
 src/compiler/nir/nir_loop_analyze.c | 36 +++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/src/compiler/nir/nir_loop_analyze.c b/src/compiler/nir/nir_loop_analyze.c
index ef69422c12..be74105594 100644
--- a/src/compiler/nir/nir_loop_analyze.c
+++ b/src/compiler/nir/nir_loop_analyze.c
@@ -245,6 +245,42 @@ compute_induction_information(loop_info_state *state)
          if (src_var->in_if_branch || src_var->in_nested_loop)
             break;
 
+         /* Detect inductions variables that are incremented in both branches
+          * of an unnested if rather than in a loop block.
+          */
+         if (is_var_phi(src_var)) {
+            nir_phi_instr *src_phi =
+               nir_instr_as_phi(src_var->def->parent_instr);
+
+            nir_op alu_op;
+            nir_ssa_def *alu_srcs[2] = {0};
+            nir_foreach_phi_src(src2, src_phi) {
+               nir_loop_variable *src_var2 =
+                  get_loop_var(src2->src.ssa, state);
+
+               if (!src_var2->in_if_branch || !is_var_alu(src_var2))
+                  break;
+
+               nir_alu_instr *alu =
+                  nir_instr_as_alu(src_var2->def->parent_instr);
+               if (nir_op_infos[alu->op].num_inputs != 2)
+                  break;
+
+               if (alu->src[0].src.ssa == alu_srcs[0] &&
+                   alu->src[1].src.ssa == alu_srcs[1] &&
+                   alu->op == alu_op) {
+                  /* Both branches perform the same calculation so we can use
+                   * one of them to find the induction variable.
+                   */
+                  src_var = src_var2;
+               } else {
+                  alu_srcs[0] = alu->src[0].src.ssa;
+                  alu_srcs[1] = alu->src[1].src.ssa;
+                  alu_op = alu->op;
+               }
+            }
+         }
+
          if (!src_var->in_loop) {
             biv->def_outside_loop = src_var;
          } else if (is_var_alu(src_var)) {
-- 
2.19.2