Mesa (10.2): Revert "i965/vec4: Change vec4_visitor:: emit_lrp to use MAC for gen<6"

[Ian Romanick]

Module: Mesa
Branch: 10.2
Commit: bc4b9467af170dc5bce3335d9766bc95613bd544
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=bc4b9467af170dc5bce3335d9766bc95613bd544

Author: Matt Turner <mattst88 at gmail.com>
Date:   Thu May 22 09:38:24 2014 -0700

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

This reverts commit 2dfbbeca50b95ccdd714d9baa4411c779f6a20d9 with the
comment about MAC and implicit accumulator removed.

Why this code didn't work in all circumstances is unknown and without a
working Ironlake simulator (which uses a different AUB format) we'll
probably never know, short of a lot of experimentation, and spending a
bunch of time to try to optimize a few instructions on Ironlake is not
time well spent.

Moreover, for mix(vec4, vec4, vec4) using the accumulator introduces a
dependence between the otherwise independent per-component calculations.
Not using the accumulator, even if it means an extra instruction per
component might be preferable. We don't know, we don't have data, and
we don't have the necessary register on Ironlake for shader_time to tell
us.

Cc: "10.2" <mesa-stable at lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77703
Acked-by: Kenneth Graunke <kenneth at whitecape.org>
(cherry picked from commit db42dd89527a42def720c3dd78a6e14402a1a2a2)

---

 src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp |   16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index 7bad81c..54a40dd 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -1171,13 +1171,17 @@ vec4_visitor::emit_lrp(const dst_reg &dst,
       /* Earlier generations don't support three source operations, so we
        * need to emit x*(1-a) + y*a.
        */
-      dst_reg one_minus_a   = dst_reg(this, glsl_type::vec4_type);
-      one_minus_a.writemask = dst.writemask;
-
+      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;
+
+      emit(MUL(y_times_a, y, a));
       emit(ADD(one_minus_a, negate(a), src_reg(1.0f)));
-      vec4_instruction *mul = emit(MUL(dst_null_f(), y, a));
-      mul->writes_accumulator = true;
-      emit(MAC(dst, x, src_reg(one_minus_a)));
+      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)));
    }
 }