Sign Select instructions

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -189,3 +189,7 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i64x2.load32x2_s`         |    `0xd6`| m:memarg           |
 | `i64x2.load32x2_u`         |    `0xd7`| m:memarg           |
 | `v128.andnot`              |    `0xd8`| -                  |
+| `v8x16.signselect`         |    `0xd9`|                    |
+| `v16x8.signselect`         |    `0xda`|                    |
+| `v32x4.signselect`         |    `0xdb`|                    |
+| `v64x2.signselect`         |    `0xdc`|                    |

proposals/simd/ImplementationStatus.md

@@ -75,6 +75,10 @@
 | `v128.or`                  |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `v128.xor`                 |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `v128.bitselect`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
+| `v8x16.signselect`         |                           |                       |                    |                    |
+| `v16x8.signselect`         |                           |                       |                    |                    |
+| `v32x4.signselect`         |                           |                       |                    |                    |
+| `v64x2.signselect`         |                           |                       |                    |                    |
 | `i8x16.neg`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i8x16.any_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i8x16.all_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |

proposals/simd/SIMD.md

@@ -539,6 +539,17 @@ Note that the normal WebAssembly `select` instruction also works with vector
 types. It selects between two whole vectors controlled by a single scalar value,
 rather than selecting bits controlled by a control mask vector.
 
+### Sign select
+* `v8x16.signselect(v1: v128, v2: v128, c: v128) -> v128`
+* `v16x8.signselect(v1: v128, v2: v128, c: v128) -> v128`
+* `v32x4.signselect(v1: v128, v2: v128, c: v128) -> v128`
+* `v64x2.signselect(v1: v128, v2: v128, c: v128) -> v128`
+
+Use the sign bits in the control mask `c` to select the corresponding element
+from `v1` when 1 (negative sign) and `v2` when 0 (positive sign).
+
+Note that these instructions work for both signed integer and floating-point
+control masks.
 
 ## Boolean horizontal reductions