nir: Add lowering for nir_op_bit_count.

This is basically the same as the GLSL lowering path.

v2: Fix typo in the link

Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

2 files changed, 38 insertions, 0 deletions

diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index 06154aa9900..bb477742dc6 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -1913,6 +1913,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_bitfield_insert_to_shifts;
    /** Lowers bitfield_reverse to shifts. */
    bool lower_bitfield_reverse;
+   /** Lowers bit_count to shifts. */
+   bool lower_bit_count;
    /** Lowers bfm to shifts and subtracts. */
    bool lower_bfm;
    /** Lowers ifind_msb to compare and ufind_msb */
diff --git a/src/compiler/nir/nir_lower_alu.c b/src/compiler/nir/nir_lower_alu.c
index ff977f01696..8d1a1d37467 100644
--- a/src/compiler/nir/nir_lower_alu.c
+++ b/src/compiler/nir/nir_lower_alu.c
@@ -94,6 +94,42 @@ lower_alu_instr(nir_alu_instr *instr, nir_builder *b)
       }
       break;
 
+   case nir_op_bit_count:
+      if (b->shader->options->lower_bit_count) {
+         /* For more details, see:
+          *
+          * http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+          */
+         nir_ssa_def *c1 = nir_imm_int(b, 1);
+         nir_ssa_def *c2 = nir_imm_int(b, 2);
+         nir_ssa_def *c4 = nir_imm_int(b, 4);
+         nir_ssa_def *c24 = nir_imm_int(b, 24);
+         nir_ssa_def *c33333333 = nir_imm_int(b, 0x33333333);
+         nir_ssa_def *c55555555 = nir_imm_int(b, 0x55555555);
+         nir_ssa_def *c0f0f0f0f = nir_imm_int(b, 0x0f0f0f0f);
+         nir_ssa_def *c01010101 = nir_imm_int(b, 0x01010101);
+
+         lowered = nir_ssa_for_alu_src(b, instr, 0);
+
+         lowered = nir_isub(b, lowered,
+                            nir_iand(b, nir_ushr(b, lowered, c1), c55555555));
+
+         lowered = nir_iadd(b,
+                            nir_iand(b, lowered, c33333333),
+                            nir_iand(b, nir_ushr(b, lowered, c2), c33333333));
+
+         lowered = nir_ushr(b,
+                            nir_imul(b,
+                                     nir_iand(b,
+                                              nir_iadd(b,
+                                                       lowered,
+                                                       nir_ushr(b, lowered, c4)),
+                                              c0f0f0f0f),
+                                     c01010101),
+                            c24);
+      }
+      break;
+
    case nir_op_imul_high:
    case nir_op_umul_high:
       if (b->shader->options->lower_mul_high) {