nir/subgroups: Add lowering for vote_ieq/vote_feq to a ballot

This is based heavily on 97f10934ed, "ac/nir: Add vote_ieq/vote_feq
lowering pass." from Bas Nieuwenhuizen.  This version is a bit more
general since it's in common code.  It also properly handles NaN due to
not flipping the comparison for floats.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/nir/nir.h

@@ -2557,6 +2557,7 @@ typedef struct nir_lower_subgroups_options {
    uint8_t ballot_bit_size;
    bool lower_to_scalar:1;
    bool lower_vote_trivial:1;
+   bool lower_vote_eq_to_ballot:1;
    bool lower_subgroup_masks:1;
    bool lower_shuffle:1;
    bool lower_quad:1;

src/compiler/nir/nir_lower_subgroups.c

@@ -141,6 +141,51 @@ lower_vote_eq_to_scalar(nir_builder *b, nir_intrinsic_instr *intrin)
    return result;
 }
 
+static nir_ssa_def *
+lower_vote_eq_to_ballot(nir_builder *b, nir_intrinsic_instr *intrin,
+                        const nir_lower_subgroups_options *options)
+{
+   assert(intrin->src[0].is_ssa);
+   nir_ssa_def *value = intrin->src[0].ssa;
+
+   /* We have to implicitly lower to scalar */
+   nir_ssa_def *all_eq = NULL;
+   for (unsigned i = 0; i < intrin->num_components; i++) {
+      nir_intrinsic_instr *rfi =
+         nir_intrinsic_instr_create(b->shader,
+                                    nir_intrinsic_read_first_invocation);
+      nir_ssa_dest_init(&rfi->instr, &rfi->dest,
+                        1, value->bit_size, NULL);
+      rfi->num_components = 1;
+      rfi->src[0] = nir_src_for_ssa(nir_channel(b, value, i));
+      nir_builder_instr_insert(b, &rfi->instr);
+
+      nir_ssa_def *is_eq;
+      if (intrin->intrinsic == nir_intrinsic_vote_feq) {
+         is_eq = nir_feq(b, &rfi->dest.ssa, nir_channel(b, value, i));
+      } else {
+         is_eq = nir_ieq(b, &rfi->dest.ssa, nir_channel(b, value, i));
+      }
+
+      if (all_eq == NULL) {
+         all_eq = is_eq;
+      } else {
+         all_eq = nir_iand(b, all_eq, is_eq);
+      }
+   }
+
+   nir_intrinsic_instr *ballot =
+      nir_intrinsic_instr_create(b->shader, nir_intrinsic_ballot);
+   nir_ssa_dest_init(&ballot->instr, &ballot->dest,
+                     1, options->ballot_bit_size, NULL);
+   ballot->num_components = 1;
+   ballot->src[0] = nir_src_for_ssa(nir_inot(b, all_eq));
+   nir_builder_instr_insert(b, &ballot->instr);
+
+   return nir_ieq(b, &ballot->dest.ssa,
+                  nir_imm_intN_t(b, 0, options->ballot_bit_size));
+}
+
 static nir_ssa_def *
 lower_shuffle(nir_builder *b, nir_intrinsic_instr *intrin,
               bool lower_to_scalar)
@@ -219,6 +264,9 @@ lower_subgroups_intrin(nir_builder *b, nir_intrinsic_instr *intrin,
       if (options->lower_vote_trivial)
          return nir_imm_int(b, NIR_TRUE);
 
+      if (options->lower_vote_eq_to_ballot)
+         return lower_vote_eq_to_ballot(b, intrin, options);
+
       if (options->lower_to_scalar && intrin->num_components > 1)
          return lower_vote_eq_to_scalar(b, intrin);
       break;