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

[Ian Romanick]

Module: Mesa
Branch: 10.2
Commit: 03e93f6079a0f87902b3ec3926dad46045b4b185
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=03e93f6079a0f87902b3ec3926dad46045b4b185

Author: Matt Turner <mattst88 at gmail.com>
Date:   Thu May 22 09:39:13 2014 -0700

Revert "i965/fs: Change fs_visitor::emit_lrp to use MAC for gen<6"

This reverts commit a6860100b87415ab510d0d210cabfeeccebc9a0a.

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=77707
Acked-by: Kenneth Graunke <kenneth at whitecape.org>
(cherry picked from commit c2c639ecf667b4b7cf17cfe33dfe710432f2c43a)

---

 src/mesa/drivers/dri/i965/brw_fs_visitor.cpp |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index d2dc5fa..bece45f 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -221,15 +221,18 @@ fs_visitor::emit_lrp(const fs_reg &dst, const fs_reg &x, const fs_reg &y,
        !y.is_valid_3src() ||
        !a.is_valid_3src()) {
       /* We can't use the LRP instruction.  Emit x*(1-a) + y*a. */
+      fs_reg y_times_a           = fs_reg(this, glsl_type::float_type);
       fs_reg one_minus_a         = fs_reg(this, glsl_type::float_type);
+      fs_reg x_times_one_minus_a = fs_reg(this, glsl_type::float_type);
+
+      emit(MUL(y_times_a, y, a));
 
       fs_reg negative_a = a;
       negative_a.negate = !a.negate;
-
       emit(ADD(one_minus_a, negative_a, fs_reg(1.0f)));
-      fs_inst *mul = emit(MUL(reg_null_f, y, a));
-      mul->writes_accumulator = true;
-      emit(MAC(dst, x, one_minus_a));
+      emit(MUL(x_times_one_minus_a, x, one_minus_a));
+
+      emit(ADD(dst, x_times_one_minus_a, y_times_a));
    } else {
       /* The LRP instruction actually does op1 * op0 + op2 * (1 - op0), so
        * we need to reorder the operands.