[Mesa-dev] [PATCH] nir: Don't fuse fmul into ffma if used by more than 4 fadds.

[Matt Turner]

total instructions in shared programs: 6596689 -> 6595563 (-0.02%)
instructions in affected programs:     103154 -> 102028 (-1.09%)
helped:                                253
HURT:                                  217

It's kind of a wash in terms of programs helped/hurt, but of the
programs helped 169 are by more than 10%.
---
I tried values of 2-6, and 4 seemed to be the best. I can provide
full shader-db result files if other people want to investigate.

 src/glsl/nir/nir_opt_peephole_ffma.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/glsl/nir/nir_opt_peephole_ffma.c b/src/glsl/nir/nir_opt_peephole_ffma.c
index 4f0f0da..3e8a34f 100644
--- a/src/glsl/nir/nir_opt_peephole_ffma.c
+++ b/src/glsl/nir/nir_opt_peephole_ffma.c
@@ -39,7 +39,7 @@ struct peephole_ffma_state {
 };
 
 static inline bool
-are_all_uses_fadd(nir_ssa_def *def)
+are_all_uses_fadd(nir_ssa_def *def, unsigned *num_uses)
 {
    if (!list_empty(&def->if_uses))
       return false;
@@ -53,6 +53,7 @@ are_all_uses_fadd(nir_ssa_def *def)
       nir_alu_instr *use_alu = nir_instr_as_alu(use_instr);
       switch (use_alu->op) {
       case nir_op_fadd:
+         (*num_uses)++;
          break; /* This one's ok */
 
       case nir_op_imov:
@@ -60,7 +61,7 @@ are_all_uses_fadd(nir_ssa_def *def)
       case nir_op_fneg:
       case nir_op_fabs:
          assert(use_alu->dest.dest.is_ssa);
-         if (!are_all_uses_fadd(&use_alu->dest.dest.ssa))
+         if (!are_all_uses_fadd(&use_alu->dest.dest.ssa, num_uses))
             return false;
          break;
 
@@ -101,15 +102,18 @@ get_mul_for_src(nir_alu_src *src, int num_components,
       *abs = true;
       break;
 
-   case nir_op_fmul:
-      /* Only absorb a fmul into a ffma if the fmul is is only used in fadd
-       * operations.  This prevents us from being too aggressive with our
-       * fusing which can actually lead to more instructions.
+   case nir_op_fmul: {
+      /* Only fuse an fmul into an ffma if its result is used by not more than
+       * four fadd operations. This prevents us from too aggressively fusing
+       * operations which can actually lead to more instructions or many ffma
+       * operations performing the same multiply.
        */
-      if (!are_all_uses_fadd(&alu->dest.dest.ssa))
+
+      unsigned num_uses = 0;
+      if (!are_all_uses_fadd(&alu->dest.dest.ssa, &num_uses) || num_uses > 4)
          return NULL;
       break;
-
+   }
    default:
       return NULL;
    }
-- 
2.4.6