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

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This reverts commit a6860100b8.

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@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77707
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit c2c639ecf6)
This commit is contained in:
Matt Turner 2014-05-22 09:39:13 -07:00 • committed by Ian Romanick
parent bc4b9467af
commit 03e93f6079
1 changed files with 7 additions and 4 deletions
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11
src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
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@ -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.