nir/lower_double_ops: lower round_even()

At least i965 hardware does not have native support for round_even() on doubles.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir.h

@@ -2420,7 +2420,8 @@ typedef enum {
    nir_lower_dtrunc = (1 << 3),
    nir_lower_dfloor = (1 << 4),
    nir_lower_dceil = (1 << 5),
-   nir_lower_dfract = (1 << 6)
+   nir_lower_dfract = (1 << 6),
+   nir_lower_dround_even = (1 << 7)
 } nir_lower_doubles_options;
 
 void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);

src/compiler/nir/nir_lower_double_ops.c

@@ -389,6 +389,55 @@ lower_fract(nir_builder *b, nir_ssa_def *src)
    return nir_fsub(b, src, nir_ffloor(b, src));
 }
 
+static nir_ssa_def *
+lower_round_even(nir_builder *b, nir_ssa_def *src)
+{
+   /* If fract(src) == 0.5, then we will have to decide the rounding direction.
+    * We will do this by computing the mod(abs(src), 2) and testing if it
+    * is < 1 or not.
+    *
+    * We compute mod(abs(src), 2) as:
+    * abs(src) - 2.0 * floor(abs(src) / 2.0)
+    */
+   nir_ssa_def *two = nir_imm_double(b, 2.0);
+   nir_ssa_def *abs_src = nir_fabs(b, src);
+   nir_ssa_def *mod =
+      nir_fsub(b,
+               abs_src,
+               nir_fmul(b,
+                        two,
+                        nir_ffloor(b,
+                                   nir_fmul(b,
+                                            abs_src,
+                                            nir_imm_double(b, 0.5)))));
+
+   /*
+    * If fract(src) != 0.5, then we round as floor(src + 0.5)
+    *
+    * If fract(src) == 0.5, then we have to check the modulo:
+    *
+    *   if it is < 1 we need a trunc operation so we get:
+    *      0.5 -> 0,   -0.5 -> -0
+    *      2.5 -> 2,   -2.5 -> -2
+    *
+    *   otherwise we need to check if src >= 0, in which case we need to round
+    *   upwards, or not, in which case we need to round downwards so we get:
+    *      1.5 -> 2,   -1.5 -> -2
+    *      3.5 -> 4,   -3.5 -> -4
+    */
+   nir_ssa_def *fract = nir_ffract(b, src);
+   return nir_bcsel(b,
+                    nir_fne(b, fract, nir_imm_double(b, 0.5)),
+                    nir_ffloor(b, nir_fadd(b, src, nir_imm_double(b, 0.5))),
+                    nir_bcsel(b,
+                              nir_flt(b, mod, nir_imm_double(b, 1.0)),
+                              nir_ftrunc(b, src),
+                              nir_bcsel(b,
+                                        nir_fge(b, src, nir_imm_double(b, 0.0)),
+                                        nir_fadd(b, src, nir_imm_double(b, 0.5)),
+                                        nir_fsub(b, src, nir_imm_double(b, 0.5)))));
+}
+
 static void
 lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options options)
 {
@@ -432,6 +481,11 @@ lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options options)
          return;
       break;
 
+   case nir_op_fround_even:
+      if (!(options & nir_lower_dround_even))
+         return;
+      break;
+
    default:
       return;
    }
@@ -467,6 +521,10 @@ lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options options)
    case nir_op_ffract:
       result = lower_fract(&bld, src);
       break;
+   case nir_op_fround_even:
+      result = lower_round_even(&bld, src);
+      break;
+
    default:
       unreachable("unhandled opcode");
    }