[Mesa-dev] [PATCH] nir: remove simple dead if detection from nir_opt_dead_cf()

[Timothy Arceri]

This was probably useful when it was first written, however it
looks to be no longer necessary.

As far as I can tell these days dce is smart enough to remove useless
instructions from if branches. Once this is done
nir_opt_peephole_select() will end up removing the empty if.

Removing this support reduces the dolphin uber shader compilation
time by around 60%. Compile time is reduced due to no longer having
to compute the live ssa defs metadata so much.

No shader-db changes on i965 or radeonsi.
---
 src/compiler/nir/nir_opt_dead_cf.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/src/compiler/nir/nir_opt_dead_cf.c b/src/compiler/nir/nir_opt_dead_cf.c
index 14986732096..053c5743527 100644
--- a/src/compiler/nir/nir_opt_dead_cf.c
+++ b/src/compiler/nir/nir_opt_dead_cf.c
@@ -180,7 +180,7 @@ def_not_live_out(nir_ssa_def *def, void *state)
 }
 
 /*
- * Test if a loop node or if node is dead. Such nodes are dead if:
+ * Test if a loop node is dead. Such nodes are dead if:
  *
  * 1) It has no side effects (i.e. intrinsics which could possibly affect the
  * state of the program aside from producing an SSA value, indicated by a lack
@@ -198,7 +198,7 @@ def_not_live_out(nir_ssa_def *def, void *state)
 static bool
 node_is_dead(nir_cf_node *node)
 {
-   assert(node->type == nir_cf_node_loop || node->type == nir_cf_node_if);
+   assert(node->type == nir_cf_node_loop);
 
    nir_block *before = nir_cf_node_as_block(nir_cf_node_prev(node));
    nir_block *after = nir_cf_node_as_block(nir_cf_node_next(node));
@@ -230,11 +230,6 @@ dead_cf_block(nir_block *block)
 {
    nir_if *following_if = nir_block_get_following_if(block);
    if (following_if) {
-      if (node_is_dead(&following_if->cf_node)) {
-         nir_cf_node_remove(&following_if->cf_node);
-         return true;
-      }
-
       if (!nir_src_is_const(following_if->condition))
          return false;
 
-- 
2.20.1