nir/lower_subgroups: Mask off unused bits in ballot ops
Thanks to VK_EXT_subgroup_size_control, we can end up with
gl_SubgroupSize being as low as 8 on Intel.
Fixes: d10de25309
"anv: Implement VK_EXT_subgroup_size_control"
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4694>
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@ -446,6 +446,32 @@ lower_subgroups_instr(nir_builder *b, nir_instr *instr, void *_options)
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assert(intrin->src[0].is_ssa);
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nir_ssa_def *int_val = ballot_type_to_uint(b, intrin->src[0].ssa,
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options->ballot_bit_size);
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if (intrin->intrinsic != nir_intrinsic_ballot_bitfield_extract &&
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intrin->intrinsic != nir_intrinsic_ballot_find_lsb) {
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/* For OpGroupNonUniformBallotFindMSB, the SPIR-V Spec says:
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*
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* "Find the most significant bit set to 1 in Value, considering
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* only the bits in Value required to represent all bits of the
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* group’s invocations. If none of the considered bits is set to
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* 1, the result is undefined."
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*
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* It has similar text for the other three. This means that, in case
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* the subgroup size is less than 32, we have to mask off the unused
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* bits. If the subgroup size is fixed and greater than or equal to
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* 32, the mask will be 0xffffffff and nir_opt_algebraic will delete
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* the iand.
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*
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* We only have to worry about this for BitCount and FindMSB because
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* FindLSB counts from the bottom and BitfieldExtract selects
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* individual bits. In either case, if run outside the range of
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* valid bits, we hit the undefined results case and we can return
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* anything we want.
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*/
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int_val = nir_iand(b, int_val,
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build_subgroup_mask(b, options->ballot_bit_size, options));
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}
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switch (intrin->intrinsic) {
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case nir_intrinsic_ballot_bitfield_extract:
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assert(intrin->src[1].is_ssa);
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