[Mesa-dev] [PATCH v3] nir: allow nir_lower_phis_to_scalar() on more src types

[Timothy Arceri]

Rather than only lowering if all srcs are scalarizable we instead
check that at least one src is scalarizable.

We remove undef type from the is_phi_src_scalarizable() as using
this as a check just cause regressions when it is the only
scalarizable src.

total instructions in shared programs: 13219105 -> 13024547 (-1.47%)
instructions in affected programs: 1153797 -> 959239 (-16.86%)
helped: 581
HURT: 74

total cycles in shared programs: 333968972 -> 324807922 (-2.74%)
cycles in affected programs: 129809402 -> 120648352 (-7.06%)
helped: 571
HURT: 131

total spills in shared programs: 57947 -> 29130 (-49.73%)
spills in affected programs: 53364 -> 24547 (-54.00%)
helped: 351
HURT: 0

total fills in shared programs: 51310 -> 25468 (-50.36%)
fills in affected programs: 44882 -> 19040 (-57.58%)
helped: 351
HURT: 0
---
 src/compiler/nir/nir_lower_phis_to_scalar.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/compiler/nir/nir_lower_phis_to_scalar.c b/src/compiler/nir/nir_lower_phis_to_scalar.c
index 16001f73685..b2ca89d3513 100644
--- a/src/compiler/nir/nir_lower_phis_to_scalar.c
+++ b/src/compiler/nir/nir_lower_phis_to_scalar.c
@@ -75,7 +75,6 @@ is_phi_src_scalarizable(nir_phi_src *src,
       return should_lower_phi(nir_instr_as_phi(src_instr), state);
 
    case nir_instr_type_load_const:
-   case nir_instr_type_ssa_undef:
       /* These are trivially scalarizable */
       return true;
 
@@ -150,11 +149,16 @@ should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
     */
    entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
 
-   bool scalarizable = true;
+   bool scalarizable = false;
 
    nir_foreach_phi_src(src, phi) {
+      /* This loop ignores srcs that are not scalarizable because its likely
+       * still worth copying to temps if another phi source is scalarizable.
+       * This reduces register spilling by a huge amount in the i965 driver for
+       * Deus Ex: MD.
+       */
       scalarizable = is_phi_src_scalarizable(src, state);
-      if (!scalarizable)
+      if (scalarizable)
          break;
    }
 
-- 
2.20.1