Mesa (main): nak: Only insert barriers around ifs if they actually re-converge

[GitLab Mirror]

Module: Mesa
Branch: main
Commit: 500955b6cbc367439549a92498201d34b1f3d5d9
URL:    
http://cgit.freedesktop.org/mesa/mesa/commit/?id=500955b6cbc367439549a92498201d34b1f3d5d9

Author: Faith Ekstrand <faith.ekstr...@collabora.com>
Date:   Wed Oct 25 22:03:30 2023 -0500

nak: Only insert barriers around ifs if they actually re-converge

We don't care about patterns like

    loop {
        ...
        if (...) {
            break;
        } else {
            ...
        }
        ...
    }

In that case, we don't need to sync after the if because there's nothing
to re-converge.  Every path except one will end up breaking out of it
anyway.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26382>

---

 src/nouveau/compiler/nak_nir_add_barriers.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/src/nouveau/compiler/nak_nir_add_barriers.c 
b/src/nouveau/compiler/nak_nir_add_barriers.c
index 420987fa4dc..c8b644ae7f3 100644
--- a/src/nouveau/compiler/nak_nir_add_barriers.c
+++ b/src/nouveau/compiler/nak_nir_add_barriers.c
@@ -195,6 +195,27 @@ cf_node_imm_succ_is_sync(nir_cf_node *node)
    }
 }
 
+static bool
+block_is_merge(const nir_block *block)
+{
+   /* If it's unreachable, there is no merge */
+   if (block->imm_dom == NULL)
+      return false;
+
+   unsigned num_preds = 0;
+   set_foreach(block->predecessors, entry) {
+      const nir_block *pred = entry->key;
+
+      /* We don't care about unreachable blocks */
+      if (pred->imm_dom == NULL)
+         continue;
+
+      num_preds++;
+   }
+
+   return num_preds > 1;
+}
+
 static void
 add_barriers_cf_list(struct exec_list *cf_list,
                      struct add_barriers_state *state)
@@ -213,6 +234,7 @@ add_barriers_cf_list(struct exec_list *cf_list,
          nir_if *nif = nir_cf_node_as_if(node);
 
          if (nif->condition.ssa->divergent &&
+             block_is_merge(nir_cf_node_as_block(nir_cf_node_next(node))) &&
              !cf_node_imm_succ_is_sync(&nif->cf_node))
             add_bar_cf_node(&nif->cf_node, state);
 
@@ -243,6 +265,8 @@ static bool
 nak_nir_add_barriers_impl(nir_function_impl *impl,
                           const struct nak_compiler *nak)
 {
+   nir_metadata_require(impl, nir_metadata_dominance);
+
    struct add_barriers_state state = {
       .nak = nak,
       .builder = nir_builder_create(impl),