Sign Select instructions

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -225,3 +225,7 @@ For example, `ImmLaneIdx16` is a byte with values in the range 0-15 (inclusive).
 | `f32x4.convert_i32x4_u`    |    `0xfb`| -                  |
 | `v128.load32_zero`         |    `0xfc`| -                  |
 | `v128.load64_zero`         |    `0xfd`| -                  |
+| `v8x16.signselect`         |  `0xfd7d`| -                  |
+| `v16x8.signselect`         |  `0xfd7e`| -                  |
+| `v32x4.signselect`         |  `0xfd7f`| -                  |
+| `v64x2.signselect`         |  `0xfd94`| -                  |

proposals/simd/ImplementationStatus.md

@@ -193,6 +193,10 @@
 | `f32x4.convert_i32x4_u`    |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `v128.load32_zero`         |                           | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `v128.load64_zero`         |                           | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
+| `v8x16.signselect`         |                           |                    |                    |                    |                    |
+| `v16x8.signselect`         |                           |                    |                    |                    |                    |
+| `v32x4.signselect`         |                           |                    |                    |                    |                    |
+| `v64x2.signselect`         |                           |                    |                    |                    |                    |
 
 [1] Tip of tree LLVM as of May 20, 2020
 

proposals/simd/SIMD.md

@@ -617,6 +617,17 @@ 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