Mesa (main): nir: remove unreachable loop terminators

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Module: Mesa
Branch: main
Commit: 0f98ed4afe5f8131fc453dc91db1f4e73efd290c
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=0f98ed4afe5f8131fc453dc91db1f4e73efd290c

Author: Timothy Arceri <tarceri at itsqueeze.com>
Date:   Mon May  9 20:20:26 2022 +1000

nir: remove unreachable loop terminators

Remove the conditional break statements associated with all
terminators that are associated with a fixed iteration count,
except for the one associated with the limiting terminator.

This logic matches similiar functionality that exists in the
old GLSL IR unrolling code.

This change helps a piglit test pass on the r300 driver once
we switch off the old GLSL IR unrolling code.

Shader-db results IRIS (BDW):

total instructions in shared programs: 17538619 -> 17538595 (<.01%)
instructions in affected programs: 216 -> 192 (-11.11%)
helped: 3
HURT: 0
helped stats (abs) min: 7 max: 10 x̄: 8.00 x̃: 7
helped stats (rel) min: 10.00% max: 12.07% x̄: 11.38% x̃: 12.07%

total cycles in shared programs: 858674910 -> 858672810 (<.01%)
cycles in affected programs: 79540 -> 77440 (-2.64%)
helped: 3
HURT: 0
helped stats (abs) min: 620 max: 800 x̄: 700.00 x̃: 680
helped stats (rel) min: 2.45% max: 2.83% x̄: 2.63% x̃: 2.62%

Reviewed-by: Emma Anholt <emma at anholt.net>
Reviewed-by: Jason Ekstrand <jason.ekstrand at collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16399>

---

 src/compiler/nir/nir_opt_loop_unroll.c | 37 ++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/src/compiler/nir/nir_opt_loop_unroll.c b/src/compiler/nir/nir_opt_loop_unroll.c
index e81cebbb338..49c696dc514 100644
--- a/src/compiler/nir/nir_opt_loop_unroll.c
+++ b/src/compiler/nir/nir_opt_loop_unroll.c
@@ -946,6 +946,43 @@ process_loops(nir_shader *sh, nir_cf_node *cf_node, bool *has_nested_loop_out,
     */
    if (!progress && loop->control != nir_loop_control_dont_unroll) {
 
+      /* Remove the conditional break statements associated with all terminators
+       * that are associated with a fixed iteration count, except for the one
+       * associated with the limiting terminator--that one needs to stay, since
+       * it terminates the loop.
+       */
+      if (loop->info->limiting_terminator) {
+         list_for_each_entry_safe(nir_loop_terminator, t,
+                                  &loop->info->loop_terminator_list,
+                                  loop_terminator_link) {
+            if (t->exact_trip_count_unknown)
+               continue;
+
+            if (t != loop->info->limiting_terminator) {
+
+               /* Only delete the if-statement if the continue block is empty.
+                * We trust that nir_opt_if() does its job well enough to
+                * remove all instructions from the continue block when possible.
+                */
+               nir_block *first_continue_from_blk = t->continue_from_then ?
+                  nir_if_first_then_block(t->nif) :
+                  nir_if_first_else_block(t->nif);
+
+               if (!(nir_cf_node_is_last(&first_continue_from_blk->cf_node) &&
+                     exec_list_is_empty(&first_continue_from_blk->instr_list)))
+                  continue;
+
+               /* Now delete the if */
+               nir_cf_node_remove(&t->nif->cf_node);
+
+               /* Also remove it from the terminator list */
+               list_del(&t->loop_terminator_link);
+
+               progress = true;
+            }
+         }
+      }
+
       /* Check for the classic
        *
        *    do {