[Mesa-dev] [PATCH 2/5] nir: shuffle constants to the top

[Timothy Arceri]

If one of the inputs to and mul/add  is the result of an another
mul/add there is a chance that we can reuse the result of that
mul/add in other calls if we do the multiplication in the right
order.

Also by attempting to move all constants to the top we increase
the chance of constant folding.

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: 13007456 -> 12964294 (-0.33%)
instructions in affected programs: 4117749 -> 4074587 (-1.05%)
helped: 17740
HURT: 1342

total cycles in shared programs: 246765094 -> 246454280 (-0.13%)
cycles in affected programs: 167300168 -> 166989354 (-0.19%)
helped: 18541
HURT: 7955

total spills in shared programs: 14937 -> 14560 (-2.52%)
spills in affected programs: 9331 -> 8954 (-4.04%)
helped: 284
HURT: 33

total fills in shared programs: 20211 -> 19671 (-2.67%)
fills in affected programs: 12586 -> 12046 (-4.29%)
helped: 286
HURT: 33

LOST:   40
GAINED: 34

Some of the hurt will go away when we shuffle things back down to the
bottom in the following patch. It's also noteworthy that almost all of the
spill changes as in Deus Ex both hurt and helped.
---
 src/compiler/nir/nir_opt_algebraic.py |  8 +++++++-
 src/compiler/nir/nir_search_helpers.h | 12 ++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index 2c8ad64..797fc4b 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -326,8 +326,14 @@ optimizations = [
    (('fmul', ('fneg', a), b), ('fneg', ('fmul', a, b))),
    (('imul', ('ineg', a), b), ('ineg', ('imul', a, b))),
 
+   # Propagate constants up multiplication chains
+   (('fmul(is_used_once)', ('fmul(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('fmul', ('fmul', a, c), b)),
+   (('imul(is_used_once)', ('imul(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('imul', ('imul', a, c), b)),
+   (('fadd(is_used_once)', ('fadd(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('fadd', ('fadd', a, c), b)),
+   (('iadd(is_used_once)', ('iadd(is_used_once)', 'a(is_not_const)', 'b(is_not_const)'), '#c'), ('iadd', ('iadd', a, c), b)),
+
    # Reassociate constants in add/mul chains so they can be folded together.
-   # For now, we only handle cases where the constants are separated by
+   # For now, we mostly only handle cases where the constants are separated by
    # a single non-constant.  We could do better eventually.
    (('~fmul', '#a', ('fmul', b, '#c')), ('fmul', ('fmul', a, c), b)),
    (('imul', '#a', ('imul', b, '#c')), ('imul', ('imul', a, c), b)),
diff --git a/src/compiler/nir/nir_search_helpers.h b/src/compiler/nir/nir_search_helpers.h
index 05bd317..09ae137 100644
--- a/src/compiler/nir/nir_search_helpers.h
+++ b/src/compiler/nir/nir_search_helpers.h
@@ -115,6 +115,18 @@ 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,
+             const uint8_t *swizzle)
+{
+   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