nir: add 64-bit ufind_msb lowering support. (v2)

This adds the option to lower 64-bit ufind_msb opcodes.

v2: use split_x/y removes component loops (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2622,6 +2622,7 @@ typedef enum {
    nir_lower_shift64 = (1 << 11),
    nir_lower_imul_2x32_64 = (1 << 12),
    nir_lower_extract64 = (1 << 13),
+   nir_lower_ufind_msb64 = (1 << 14),
 } nir_lower_int64_options;
 
 typedef enum {

src/compiler/nir/nir_lower_int64.c

@@ -657,6 +657,19 @@ lower_extract(nir_builder *b, nir_op op, nir_ssa_def *x, nir_ssa_def *c)
       return lower_u2u64(b, extract32);
 }
 
+static nir_ssa_def *
+lower_ufind_msb64(nir_builder *b, nir_ssa_def *x)
+{
+
+   nir_ssa_def *x_lo = nir_unpack_64_2x32_split_x(b, x);
+   nir_ssa_def *x_hi = nir_unpack_64_2x32_split_y(b, x);
+   nir_ssa_def *lo_count = nir_ufind_msb(b, x_lo);
+   nir_ssa_def *hi_count = nir_ufind_msb(b, x_hi);
+   nir_ssa_def *valid_hi_bits = nir_ine(b, x_hi, nir_imm_int(b, 0));
+   nir_ssa_def *hi_res = nir_iadd(b, nir_imm_intN_t(b, 32, 32), hi_count);
+   return nir_bcsel(b, valid_hi_bits, hi_res, lo_count);
+}
+
 nir_lower_int64_options
 nir_lower_int64_op_to_options_mask(nir_op opcode)
 {
@@ -718,6 +731,8 @@ nir_lower_int64_op_to_options_mask(nir_op opcode)
    case nir_op_extract_u16:
    case nir_op_extract_i16:
       return nir_lower_extract64;
+   case nir_op_ufind_msb:
+      return nir_lower_ufind_msb64;
    default:
       return 0;
    }
@@ -819,6 +834,9 @@ lower_int64_alu_instr(nir_builder *b, nir_instr *instr, void *_state)
    case nir_op_extract_u16:
    case nir_op_extract_i16:
       return lower_extract(b, alu->op, src[0], src[1]);
+   case nir_op_ufind_msb:
+      return lower_ufind_msb64(b, src[0]);
+      break;
    default:
       unreachable("Invalid ALU opcode to lower");
    }
@@ -864,6 +882,11 @@ should_lower_int64_alu_instr(const nir_instr *instr, const void *_options)
       if (alu->src[0].src.ssa->bit_size != 64)
          return false;
       break;
+   case nir_op_ufind_msb:
+      assert(alu->src[0].src.is_ssa);
+      if (alu->src[0].src.ssa->bit_size != 64)
+         return false;
+      break;
    default:
       assert(alu->dest.dest.is_ssa);
       if (alu->dest.dest.ssa.bit_size != 64)