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brw/algebraic: Don't optimize SEL.L.SAT or SEL.G.SAT

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This optimization was added in October 2013, and the error was only just
now discovered. Removing the SEL.G.SAT optimization affected zero
shader-db shaders, and it affected 9 fossil-db shaders for instruction
size only.

I haven't checked to see if any of the hurt shaders are helped by
!39987.

shader-db:

All Intel platforms had similar results. (Lunar Lake shown)
total instructions in shared programs: 17093041 -> 17093055 (<.01%)
instructions in affected programs: 2072 -> 2086 (0.68%)
helped: 0 / HURT: 8

total cycles in shared programs: 876739578 -> 876739154 (<.01%)
cycles in affected programs: 18946 -> 18522 (-2.24%)
helped: 2 / HURT: 6

fossil-db:

Lunar Lake
Totals:
Instrs: 906230557 -> 906240487 (+0.00%); split: -0.00%, +0.00%
CodeSize: 14498856128 -> 14499003168 (+0.00%); split: -0.00%, +0.00%
Send messages: 40667184 -> 40667205 (+0.00%); split: -0.00%, +0.00%
Cycle count: 104068494103 -> 104068561943 (+0.00%); split: -0.00%, +0.00%
Max live registers: 189570192 -> 189570204 (+0.00%); split: -0.00%, +0.00%
Max dispatch width: 48157648 -> 48157552 (-0.00%)
Non SSA regs after NIR: 139823587 -> 139823016 (-0.00%); split: -0.00%, +0.00%

Totals from 9172 (0.46% of 1985212) affected shaders:
Instrs: 10774709 -> 10784639 (+0.09%); split: -0.00%, +0.09%
CodeSize: 177868384 -> 178015424 (+0.08%); split: -0.08%, +0.17%
Send messages: 311154 -> 311175 (+0.01%); split: -0.00%, +0.01%
Cycle count: 232471392 -> 232539232 (+0.03%); split: -0.15%, +0.18%
Max live registers: 1243549 -> 1243561 (+0.00%); split: -0.00%, +0.01%
Max dispatch width: 196672 -> 196576 (-0.05%)
Non SSA regs after NIR: 509663 -> 509092 (-0.11%); split: -0.19%, +0.08%

Test: piglit!1100
Reported-by: Georg Lehmann
Fixes: ca675b73d3 ("i965/fs: Optimize saturating SEL.L(E) with imm val >= 1.0.")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
(cherry picked from commit 6c6c6ce054)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40359>
This commit is contained in:
Ian Romanick 2026-03-06 14:44:47 -08:00 • committed by Eric Engestrom
parent 63a6e0ffc9
commit 829e5ccc84
3 changed files with 22 additions and 34 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
.pick_status.json
View file

@ -1144,7 +1144,7 @@
"description": "brw/algebraic: Don't optimize SEL.L.SAT or SEL.G.SAT",
"nominated": true,
"nomination_type": 2,
"resolution": 0,
"resolution": 1,
"main_sha": null,
"because_sha": "ca675b73d3ac2e1b57ec385c2c80b05b6382f6b6",
"notes": null

33
src/intel/compiler/brw/brw_opt_algebraic.cpp
View file

@ -553,39 +553,6 @@ brw_opt_algebraic(brw_shader &s)
inst->predicate_inverse = false;
inst->conditional_mod = BRW_CONDITIONAL_NONE;
progress = true;
} else if (inst->saturate && inst->src[1].file == IMM) {
switch (inst->conditional_mod) {
case BRW_CONDITIONAL_LE:
case BRW_CONDITIONAL_L:
switch (inst->src[1].type) {
case BRW_TYPE_F:
if (inst->src[1].f >= 1.0f) {
inst = brw_transform_inst(s, inst, BRW_OPCODE_MOV);
inst->conditional_mod = BRW_CONDITIONAL_NONE;
progress = true;
}
break;
default:
break;
}
break;
case BRW_CONDITIONAL_GE:
case BRW_CONDITIONAL_G:
switch (inst->src[1].type) {
case BRW_TYPE_F:
if (inst->src[1].f <= 0.0f) {
inst = brw_transform_inst(s, inst, BRW_OPCODE_MOV);
inst->conditional_mod = BRW_CONDITIONAL_NONE;
progress = true;
}
break;
default:
break;
}
break;
default:
break;
}
}
break;
case BRW_OPCODE_CSEL:

21
src/intel/compiler/brw/test_opt_algebraic.cpp
View file

@ -59,6 +59,27 @@ TEST_F(algebraic_test, fmax_a_a)
EXPECT_NO_PROGRESS(brw_opt_algebraic, bld);
}
TEST_F(algebraic_test, fmin_sat_a_1)
{
brw_builder bld = make_shader(MESA_SHADER_FRAGMENT, 16);
brw_reg dst0 = vgrf(bld, BRW_TYPE_F);
brw_reg src0 = vgrf(bld, BRW_TYPE_F);
/* This is NaN. The definition of NAN in math.h has a scary comment about
* possibly raising an exception outside static initializers.
*/
bld.MOV(retype(src0, BRW_TYPE_UD), brw_imm_ud(0x7fc00000));
bld.emit_minmax(dst0, src0, brw_imm_f(1.0), BRW_CONDITIONAL_L)
->saturate = true;
/* SEL.L of NaN should produce 1.0, and the .SAT should occur
* afterwards. This means the SEL.L.SAT cannot be optimized to a simple
* MOV.SAT since that would saturate NaN to 0.0 instead.
*/
EXPECT_NO_PROGRESS(brw_opt_algebraic, bld);
}
TEST_F(algebraic_test, mad_with_all_immediates)
{
brw_builder bld = make_shader();