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brw: use the right int8/int16 division lowering

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lowering bitsize before lowering idiv is silly, since then it forces us
down the software int32 division path instead of the much faster
int8/int16 lowered path. Relevant CTS tests:

dEQP-VK.spirv_assembly.type.scalar.i16.div_comp,
dEQP-VK.spirv_assembly.type.scalar.i8.rem_comp,

Go from:

SIMD8 shader: 46 instructions. 1 loops. 4716 cycles. 0:0 spills:fills
SIMD8 shader: 1008 instructions. 0 loops. 3600 cycles. 0:0 spills:fills, 8 sends

to:

SIMD8 shader: 17 instructions. 1 loops. 2556 cycles. 0:0 spills:fills
SIMD8 shader: 464 instructions. 0 loops. 1394 cycles. 0:0 spills:fills, 8 sends

No stats change on fossil-db (which has very little int8/int16 and even
less integer division, apparently).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37966>
This commit is contained in:
Alyssa Rosenzweig 2025-10-20 16:49:56 -04:00
parent c2a6fb6419
commit 05481f56a0
1 changed files with 15 additions and 2 deletions
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17
src/intel/compiler/brw/brw_nir.c
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@ -1145,8 +1145,10 @@ brw_nir_optimize(nir_shader *nir,
}
static unsigned
lower_bit_size_callback(const nir_instr *instr, UNUSED void *data)
lower_bit_size_callback(const nir_instr *instr, void *data)
{
const struct brw_compiler *compiler = data;
switch (instr->type) {
case nir_instr_type_alu: {
nir_alu_instr *alu = nir_instr_as_alu(instr);
@ -1180,6 +1182,12 @@ lower_bit_size_callback(const nir_instr *instr, UNUSED void *data)
case nir_op_irem:
case nir_op_udiv:
case nir_op_umod:
/* Gfx12.5+ lacks integer division instructions. As nir_lower_idiv is
* far more efficient for int8/int16 divisions, we do not lower here.
*
* Older platforms have idiv instructions only for int32, so lower.
*/
return compiler->devinfo->verx10 >= 125 ? 0 : 32;
case nir_op_fceil:
case nir_op_ffloor:
case nir_op_ffract:
@ -2222,7 +2230,12 @@ brw_postprocess_nir_opts(nir_shader *nir, const struct brw_compiler *compiler,
const nir_lower_idiv_options options = {
.allow_fp16 = false
};
OPT(nir_lower_idiv, &options);
/* Given an 8-bit integer remainder, nir_lower_idiv will produce new
* 8-bit integer math which needs to be lowered.
*/
if (OPT(nir_lower_idiv, &options))
OPT(nir_lower_bit_size, lower_bit_size_callback, (void *)compiler);
}
if (devinfo->ver >= 30)