Mesa (master): i965/vec4: Emit smarter code for b2f of a comparison

[Ian Romanick]

Module: Mesa
Branch: master
Commit: 63117ac32948310c87e30f67b475a07f98884633
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=63117ac32948310c87e30f67b475a07f98884633

Author: Ian Romanick <ian.d.romanick at intel.com>
Date:   Tue Jun 10 18:07:50 2014 -0700

i965/vec4: Emit smarter code for b2f of a comparison

Previously we would emit the comparison, emit an AND to mask off extra
bits from the comparison result, then convert the result to float.  Now,
do the comparison, then use a cleverly constructed SEL to pick either
0.0f or 1.0f.

No piglit regressions on Ivybridge.

total instructions in shared programs: 1642311 -> 1639449 (-0.17%)
instructions in affected programs:     136533 -> 133671 (-2.10%)
GAINED:                                0
LOST:                                  0

Programs that are affected appear to save between 1 and 5 instuctions
(just by skimming the output from shader-db report.py.

v2: s/b2i/b2f/ in commit subject (noticed by Chris Forbes).  Remove
extraneous fix_3src_operand (suggested by Matt).  The latter change
required swapping the order of the operands and using predicate_inverse.

Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
Reviewed-by: Matt Turner <mattst88 at gmail.com>

---

 src/mesa/drivers/dri/i965/brw_vec4.h           |    1 +
 src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp |   47 ++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.h b/src/mesa/drivers/dri/i965/brw_vec4.h
index 614ff0b..20d717a 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4.h
+++ b/src/mesa/drivers/dri/i965/brw_vec4.h
@@ -587,6 +587,7 @@ public:
 
    bool try_emit_sat(ir_expression *ir);
    bool try_emit_mad(ir_expression *ir);
+   bool try_emit_b2f_of_compare(ir_expression *ir);
    void resolve_ud_negate(src_reg *reg);
 
    src_reg get_timestamp();
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index 249072c..7fd8c2b 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -1126,6 +1126,48 @@ vec4_visitor::try_emit_mad(ir_expression *ir)
    return true;
 }
 
+bool
+vec4_visitor::try_emit_b2f_of_compare(ir_expression *ir)
+{
+   ir_expression *const cmp = ir->operands[0]->as_expression();
+
+   if (cmp == NULL)
+      return false;
+
+   switch (cmp->operation) {
+   case ir_binop_less:
+   case ir_binop_greater:
+   case ir_binop_lequal:
+   case ir_binop_gequal:
+   case ir_binop_equal:
+   case ir_binop_nequal:
+      break;
+
+   default:
+      return false;
+   }
+
+   cmp->operands[0]->accept(this);
+   const src_reg cmp_src0 = this->result;
+
+   cmp->operands[1]->accept(this);
+   const src_reg cmp_src1 = this->result;
+
+   this->result = src_reg(this, ir->type);
+
+   emit(CMP(dst_reg(this->result), cmp_src0, cmp_src1,
+            brw_conditional_for_comparison(cmp->operation)));
+
+   /* If the comparison is false, this->result will just happen to be zero.
+    */
+   vec4_instruction *const inst = emit(BRW_OPCODE_SEL, dst_reg(this->result),
+                                       this->result, src_reg(1.0f));
+   inst->predicate = BRW_PREDICATE_NORMAL;
+   inst->predicate_inverse = true;
+
+   return true;
+}
+
 void
 vec4_visitor::emit_bool_comparison(unsigned int op,
 				 dst_reg dst, src_reg src0, src_reg src1)
@@ -1202,6 +1244,11 @@ vec4_visitor::visit(ir_expression *ir)
 	 return;
    }
 
+   if (ir->operation == ir_unop_b2f) {
+      if (try_emit_b2f_of_compare(ir))
+	 return;
+   }
+
    for (operand = 0; operand < ir->get_num_operands(); operand++) {
       this->result.file = BAD_FILE;
       ir->operands[operand]->accept(this);