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pan/bi: Optimize abs(derivative) 

We implement fine derivatives as:

	broadcast(x, (lane & ~1) + 1) - broadcast(x, lane & ~1)

Most of the complexity is to get the right sign. If we can ignore the
sign, we can generate the simpler code:

	broadcast(x, lane ^ 1) - lane

This is a particular win on v7+ where the broadcast instruction (CLPER)
can do `lane ^ value` for free. However, even on v6 where we lower to an
explicit XOR instruction, it's still a win.

The limiting case is fwidth. The fragment shader

   gl_FragColor = fwidth(vec4_varying);

has the following results on v6, v7, and v9:

G72 (-26% instructions, -43% cycles):
38 inst, 30 tuples, 5 clauses, 1.166667 cycles, 1.166667 arith, 28 quadwords
28 inst, 19 tuples, 4 clauses, 0.666667 cycles, 0.666667 arith, 19 quadwords

G76 (-37% instructions, -54% cycles):
38 inst, 30 tuples, 5 clauses, 1.166667 cycles, 1.166667 arith, 28 quadwords
24 inst, 16 tuples, 4 clauses, 0.541667 cycles, 0.541667 arith, 18 quadwords

G78 (-40% instructions, -56% cycles):
40 inst, 1.125000 cycles, 0.250000 fma, 0.109375 cvt, 1.125000 sfu, 20 quadwords
24 inst, 0.500000 cycles, 0.250000 fma, 0.015625 cvt, 0.500000 sfu, 12 quadwords

shader-db tells a similar story -- most shaders are unaffected, but a
shader that uses fwidth has a 20% reduction in cycle count:

instructions helped:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 264 -> 262 (-0.76%)
instructions helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 36 -> 28 (-22.22%)
tuples helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 27 -> 22 (-18.52%)
tuples HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 211 -> 212 (0.47%)
clauses HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 32 -> 33 (3.12%)
cycles helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 1 -> 0.79 (-20.83%)
arith helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 1 -> 0.79 (-20.83%)
quadwords helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 31 -> 28 (-9.68%)
quadwords HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 176 -> 178 (1.14%)

total instructions in shared programs: 148370 -> 148360 (<.01%)
instructions in affected programs: 300 -> 290 (-3.33%)
helped: 2
HURT: 0

total tuples in shared programs: 124188 -> 124184 (<.01%)
tuples in affected programs: 238 -> 234 (-1.68%)
helped: 1
HURT: 1
helped stats (abs) min: 5.0 max: 5.0 x̄: 5.00 x̃: 5
helped stats (rel) min: 18.52% max: 18.52% x̄: 18.52% x̃: 18.52%
HURT stats (abs)   min: 1.0 max: 1.0 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.47% max: 0.47% x̄: 0.47% x̃: 0.47%

total clauses in shared programs: 25692 -> 25693 (<.01%)
clauses in affected programs: 32 -> 33 (3.12%)
helped: 0
HURT: 1

total cycles in shared programs: 12132.04 -> 12131.83 (<.01%)
cycles in affected programs: 1 -> 0.79 (-20.83%)
helped: 1
HURT: 0

total arith in shared programs: 4623.75 -> 4623.54 (<.01%)
arith in affected programs: 1 -> 0.79 (-20.83%)
helped: 1
HURT: 0

total quadwords in shared programs: 110386 -> 110385 (<.01%)
quadwords in affected programs: 207 -> 206 (-0.48%)
helped: 1
HURT: 1
helped stats (abs) min: 3.0 max: 3.0 x̄: 3.00 x̃: 3
helped stats (rel) min: 9.68% max: 9.68% x̄: 9.68% x̃: 9.68%
HURT stats (abs)   min: 2.0 max: 2.0 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 1.14% max: 1.14% x̄: 1.14% x̃: 1.14%

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12332>
This commit is contained in:
Alyssa Rosenzweig 2021-10-02 09:02:36 -04:00 committed by Marge Bot
parent 3e8f540753
commit c00e7b729f
2 changed files with 28 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
src/panfrost/bifrost/bifrost_compile.c
View File

@ -1696,6 +1696,23 @@ bi_lower_fsincos_32(bi_builder *b, bi_index dst, bi_index s0, bool cos)
bi_fadd_f32_to(b, dst, I->dest[0], cos ? cosx : sinx, BI_ROUND_NONE);
}
/* The XOR lane op is useful for derivative calculation, but was added in v7.
* Add a safe helper that will do the appropriate lowering on v6 */
static bi_index
bi_clper_xor(bi_builder *b, bi_index s0, bi_index s1)
{
if (b->shader->arch >= 7) {
return bi_clper_i32(b, s0, s1,
BI_INACTIVE_RESULT_ZERO, BI_LANE_OP_XOR,
BI_SUBGROUP_SUBGROUP4);
}
bi_index lane_id = bi_fau(BIR_FAU_LANE_ID, false);
bi_index lane = bi_lshift_xor_i32(b, lane_id, s1, bi_imm_u8(0));
return bi_clper_v6_i32(b, s0, lane);
}
static bi_instr *
bi_emit_alu_bool(bi_builder *b, unsigned sz, nir_op op,
bi_index dst, bi_index s0, bi_index s1, bi_index s2)
@ -2011,6 +2028,14 @@ bi_emit_alu(bi_builder *b, nir_alu_instr *instr)
s0, s1, s0, s1, BI_CMPF_GT);
break;
case nir_op_fddx_must_abs_mali:
case nir_op_fddy_must_abs_mali: {
bi_index bit = bi_imm_u32(instr->op == nir_op_fddx_must_abs_mali ? 1 : 2);
bi_index adjacent = bi_clper_xor(b, s0, bit);
bi_fadd_to(b, sz, dst, adjacent, bi_neg(s0), BI_ROUND_NONE);
break;
}
case nir_op_fddx:
case nir_op_fddy: {
bi_index lane1 = bi_lshift_and_i32(b,

3
src/panfrost/bifrost/bifrost_nir_algebraic.py
View File

@ -36,6 +36,9 @@ algebraic_late = [
(('fmin', ('fmax', a, -1.0), 1.0), ('fsat_signed_mali', a)),
(('fmax', ('fmin', a, 1.0), -1.0), ('fsat_signed_mali', a)),
(('fmax', a, 0.0), ('fclamp_pos_mali', a)),
(('fabs', ('fddx', a)), ('fabs', ('fddx_must_abs_mali', a))),
(('fabs', ('fddy', b)), ('fabs', ('fddy_must_abs_mali', b))),
]