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i965/vec4: Change vec4_visitor::emit_lrp to use MAC for gen<6 

This allows us to emit ADD/MUL/MAC instead of MUL/ADD/MUL/ADD,
saving one instruction and two temporary registers.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
This commit is contained in:
Juha-Pekka Heikkila 2014-03-28 15:28:31 +02:00 committed by Matt Turner
parent 0974706671
commit 2dfbbeca50
1 changed files with 5 additions and 15 deletions
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20
src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
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@ -1165,24 +1165,14 @@ vec4_visitor::emit_lrp(const dst_reg &dst,
} else {
/* Earlier generations don't support three source operations, so we
* need to emit x*(1-a) + y*a.
*
* A better way to do this would be:
* ADD one_minus_a, negate(a), 1.0f
* MUL null, y, a
* MAC dst, x, one_minus_a
* but we would need to support MAC and implicit accumulator.
*/
dst_reg y_times_a = dst_reg(this, glsl_type::vec4_type);
dst_reg one_minus_a = dst_reg(this, glsl_type::vec4_type);
dst_reg x_times_one_minus_a = dst_reg(this, glsl_type::vec4_type);
y_times_a.writemask = dst.writemask;
one_minus_a.writemask = dst.writemask;
x_times_one_minus_a.writemask = dst.writemask;
dst_reg one_minus_a = dst_reg(this, glsl_type::vec4_type);
one_minus_a.writemask = dst.writemask;
emit(MUL(y_times_a, y, a));
emit(ADD(one_minus_a, negate(a), src_reg(1.0f)));
emit(MUL(x_times_one_minus_a, x, src_reg(one_minus_a)));
emit(ADD(dst, src_reg(x_times_one_minus_a), src_reg(y_times_a)));
vec4_instruction *mul = emit(MUL(dst_null_f(), y, a));
mul->writes_accumulator = true;
emit(MAC(dst, x, src_reg(one_minus_a)));
}
}