intel/fs: Emit better code for b2f(inot(a)) and b2i(inot(a))

Since Boolean values are either -1 (true) or 0 (false), b2f(inot(a))
maps -1 => 0.0 and 0 => 1.0.  This is equivalent to 1.0 +
float(boolBitsToInt(a)).  On Intel GPUs, ADD is one of the few
instructions that can type-convert during write to destination, so we
can achieve this in a single instruction:

    add    g47F, g26D, 1D

v2: Fix swizzles.

v3: Fix typos in comments.  Noticed by Ken.

All Gen6+ platforms had similar results. (Skylake shown)
Skylake
total instructions in shared programs: 15185583 -> 15184683 (<.01%)
instructions in affected programs: 239389 -> 238489 (-0.38%)
helped: 899
HURT: 1
helped stats (abs) min: 1 max: 2 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.15% max: 1.85% x̄: 0.49% x̃: 0.44%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.09% max: 0.09% x̄: 0.09% x̃: 0.09%
95% mean confidence interval for instructions value: -1.01 -0.99
95% mean confidence interval for instructions %-change: -0.51% -0.48%
Instructions are helped.

total cycles in shared programs: 370964249 -> 370961508 (<.01%)
cycles in affected programs: 1487586 -> 1484845 (-0.18%)
helped: 420
HURT: 268
helped stats (abs) min: 1 max: 232 x̄: 22.41 x̃: 6
helped stats (rel) min: 0.05% max: 22.60% x̄: 1.30% x̃: 0.41%
HURT stats (abs)   min: 1 max: 230 x̄: 24.90 x̃: 10
HURT stats (rel)   min: <.01% max: 21.60% x̄: 1.45% x̃: 0.52%
95% mean confidence interval for cycles value: -7.61 -0.36
95% mean confidence interval for cycles %-change: -0.44% -0.02%
Cycles are helped.

No changes on Iron Lake or GM45.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

diff --git a/src/intel/compiler/brw_fs.h b/src/intel/compiler/brw_fs.h
index df34c69663b60e10fe458371b185e67243d7e345..97956003973054e0259405e50de3626e7bfa9a16 100644 (file)
--- a/src/intel/compiler/brw_fs.h
+++ b/src/intel/compiler/brw_fs.h
@@ -205,6 +205,8 @@ public:
    void nir_emit_block(nir_block *block);
    void nir_emit_instr(nir_instr *instr);
    void nir_emit_alu(const brw::fs_builder &bld, nir_alu_instr *instr);
+   bool try_emit_b2fi_of_inot(const brw::fs_builder &bld, fs_reg result,
+                              nir_alu_instr *instr);
    void nir_emit_load_const(const brw::fs_builder &bld,
                             nir_load_const_instr *instr);
    void nir_emit_vs_intrinsic(const brw::fs_builder &bld,

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index 110473bfa8506f2c4198b0c30138b9ad22a31bc0..9ecfad99939c9b7cbaef3788b1a816120af107b5 100644 (file)
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -753,6 +753,42 @@ fs_visitor::resolve_inot_sources(const fs_builder &bld, nir_alu_instr *instr,
    }
 }
 
+bool
+fs_visitor::try_emit_b2fi_of_inot(const fs_builder &bld,
+                                  fs_reg result,
+                                  nir_alu_instr *instr)
+{
+   if (devinfo->gen < 6 || devinfo->gen >= 12)
+      return false;
+
+   nir_alu_instr *const inot_instr = nir_src_as_alu_instr(&instr->src[0].src);
+
+   if (inot_instr == NULL || inot_instr->op != nir_op_inot)
+      return false;
+
+   /* HF is also possible as a destination on BDW+.  For nir_op_b2i, the set
+    * of valid size-changing combinations is a bit more complex.
+    *
+    * The source restriction is just because I was lazy about generating the
+    * constant below.
+    */
+   if (nir_dest_bit_size(instr->dest.dest) != 32 ||
+       nir_src_bit_size(inot_instr->src[0].src) != 32)
+      return false;
+
+   /* b2[fi](inot(a)) maps a=0 => 1, a=-1 => 0.  Since a can only be 0 or -1,
+    * this is float(1 + a).
+    */
+   fs_reg op;
+
+   prepare_alu_destination_and_sources(bld, inot_instr, &op, false);
+
+   bld.ADD(result, op, brw_imm_d(1));
+   assert(!instr->dest.saturate);
+
+   return true;
+}
+
 void
 fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
 {
@@ -844,6 +880,8 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
    case nir_op_b2f16:
    case nir_op_b2f32:
    case nir_op_b2f64:
+      if (try_emit_b2fi_of_inot(bld, result, instr))
+         break;
       op[0].type = BRW_REGISTER_TYPE_D;
       op[0].negate = !op[0].negate;
       /* fallthrough */