[Mesa-dev] [PATCH] nir: switch fmul to increase chance of optimising it away

[Timothy Arceri]

If one of the inputs to the multiplcation in ffma is the result of
an fmul there is a chance that we can reuse the result of that
fmul in other ffma calls if we do the multiplication in the right
order.

For example it is a fairly common pattern for shaders to do something
similar to this:

  const float a = 0.5;
  in vec4 b;
  in float c;

  ...

  b.x = b.x * c;
  b.y = b.y * c;

  ...

  b.x = b.x * a + a;
  b.y = b.y * a + a;

So by simply detecting that constant a is part of the multiplication
in ffma and switching it with previous fmul that updates b we end up
with:

  ...

  c = a * c;

  ...

  b.x = b.x * c + a;
  b.y = b.y * c + a;

shader-db results BDW:

total instructions in shared programs: 13056473 -> 13038614 (-0.14%)
instructions in affected programs: 2433641 -> 2415782 (-0.73%)
helped: 10114
HURT: 330

total cycles in shared programs: 256515402 -> 256331740 (-0.07%)
cycles in affected programs: 137476650 -> 137292988 (-0.13%)
helped: 10802
HURT: 3871

total spills in shared programs: 14923 -> 14675 (-1.66%)
spills in affected programs: 7976 -> 7728 (-3.11%)
helped: 280
HURT: 27

total fills in shared programs: 20141 -> 19691 (-2.23%)
fills in affected programs: 11408 -> 10958 (-3.94%)
helped: 282
HURT: 27

LOST:   6
GAINED: 1
---
 src/compiler/nir/nir_opt_algebraic.py |  2 ++
 src/compiler/nir/nir_search_helpers.h | 11 +++++++++++
 2 files changed, 13 insertions(+)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index b5974a7..3cddda3 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -331,6 +331,8 @@ optimizations = [
    (('~fadd', '#a', ('fadd', b, '#c')), ('fadd', ('fadd', a, c), b)),
    (('iadd', '#a', ('iadd', b, '#c')), ('iadd', ('iadd', a, c), b)),
 
+   (('fadd', ('fmul(is_used_once)', ('fmul(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), d), ('fadd', ('fmul', ('fmul', a, '#c'), b), d)),
+
    # Misc. lowering
    (('fmod at 32', a, b), ('fsub', a, ('fmul', b, ('ffloor', ('fdiv', a, b)))), 'options->lower_fmod32'),
    (('fmod at 64', a, b), ('fsub', a, ('fmul', b, ('ffloor', ('fdiv', a, b)))), 'options->lower_fmod64'),
diff --git a/src/compiler/nir/nir_search_helpers.h b/src/compiler/nir/nir_search_helpers.h
index ddaff52..f89e417 100644
--- a/src/compiler/nir/nir_search_helpers.h
+++ b/src/compiler/nir/nir_search_helpers.h
@@ -115,6 +115,17 @@ is_zero_to_one(nir_alu_instr *instr, unsigned src, unsigned num_components,
 }
 
 static inline bool
+is_not_const(nir_alu_instr *instr, unsigned src, unsigned num_components)
+{
+   nir_const_value *val = nir_src_as_const_value(instr->src[src].src);
+
+   if (val)
+      return false;
+
+   return true;
+}
+
+static inline bool
 is_used_more_than_once(nir_alu_instr *instr)
 {
    bool zero_if_use = list_empty(&instr->dest.dest.ssa.if_uses);
-- 
2.9.3