Mesa (staging/20.0): nir/lower_subgroups: Mask off unused bits in ballot ops
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Mon Apr 27 18:08:05 UTC 2020
Module: Mesa
Branch: staging/20.0
Commit: 109caeeb436f01cd8bf50f6e4bcef37beee21dc0
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=109caeeb436f01cd8bf50f6e4bcef37beee21dc0
Author: Jason Ekstrand <jason at jlekstrand.net>
Date: Wed Apr 22 21:35:48 2020 -0500
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: d10de253097 "anv: Implement VK_EXT_subgroup_size_control"
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira at intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4694>
(cherry picked from commit fdf9b674ee3a17c98fd266750dec3475910542f6)
---
.pick_status.json | 2 +-
src/compiler/nir/nir_lower_subgroups.c | 26 ++++++++++++++++++++++++++
2 files changed, 27 insertions(+), 1 deletion(-)
diff --git a/.pick_status.json b/.pick_status.json
index b142b535575..1bffc7a352e 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -1147,7 +1147,7 @@
"description": "nir/lower_subgroups: Mask off unused bits in ballot ops",
"nominated": true,
"nomination_type": 1,
- "resolution": 0,
+ "resolution": 1,
"master_sha": null,
"because_sha": "d10de2530976ed3aba9c5d077e2edb141f71e7dd"
},
diff --git a/src/compiler/nir/nir_lower_subgroups.c b/src/compiler/nir/nir_lower_subgroups.c
index f5eebb85144..2a7361b7472 100644
--- a/src/compiler/nir/nir_lower_subgroups.c
+++ b/src/compiler/nir/nir_lower_subgroups.c
@@ -446,6 +446,32 @@ lower_subgroups_instr(nir_builder *b, nir_instr *instr, void *_options)
assert(intrin->src[0].is_ssa);
nir_ssa_def *int_val = ballot_type_to_uint(b, intrin->src[0].ssa,
options->ballot_bit_size);
+
+ if (intrin->intrinsic != nir_intrinsic_ballot_bitfield_extract &&
+ intrin->intrinsic != nir_intrinsic_ballot_find_lsb) {
+ /* For OpGroupNonUniformBallotFindMSB, the SPIR-V Spec says:
+ *
+ * "Find the most significant bit set to 1 in Value, considering
+ * only the bits in Value required to represent all bits of the
+ * group’s invocations. If none of the considered bits is set to
+ * 1, the result is undefined."
+ *
+ * It has similar text for the other three. This means that, in case
+ * the subgroup size is less than 32, we have to mask off the unused
+ * bits. If the subgroup size is fixed and greater than or equal to
+ * 32, the mask will be 0xffffffff and nir_opt_algebraic will delete
+ * the iand.
+ *
+ * We only have to worry about this for BitCount and FindMSB because
+ * FindLSB counts from the bottom and BitfieldExtract selects
+ * individual bits. In either case, if run outside the range of
+ * valid bits, we hit the undefined results case and we can return
+ * anything we want.
+ */
+ int_val = nir_iand(b, int_val,
+ build_subgroup_mask(b, options->ballot_bit_size, options));
+ }
+
switch (intrin->intrinsic) {
case nir_intrinsic_ballot_bitfield_extract:
assert(intrin->src[1].is_ssa);
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