Sign Select instructions

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -249,3 +249,7 @@ For example, `ImmLaneIdx16` is a byte with values in the range 0-15 (inclusive).
 | `v128.store16_lane`         |     `TBD`| m:memarg, i:ImmLaneIdx8  |
 | `v128.store32_lane`         |     `TBD`| m:memarg, i:ImmLaneIdx4  |
 | `v128.store64_lane`         |     `TBD`| m:memarg, i:ImmLaneIdx2  |
+| `v8x16.signselect`          |     `TBD`| -                        |
+| `v16x8.signselect`          |     `TBD`| -                        |
+| `v32x4.signselect`          |     `TBD`| -                        |
+| `v64x2.signselect`          |     `TBD`| -                        |

proposals/simd/ImplementationStatus.md

@@ -217,6 +217,10 @@
 | `v128.store16_lane`         |                           |                    |                    |                    |                    |
 | `v128.store32_lane`         |                           |                    |                    |                    |                    |
 | `v128.store64_lane`         |                           |                    |                    |                    |                    |
+| `v8x16.signselect`          |                           |                    |                    |                    |                    |
+| `v16x8.signselect`          |                           |                    |                    |                    |                    |
+| `v32x4.signselect`          |                           |                    |                    |                    |                    |
+| `v64x2.signselect`          |                           |                    |                    |                    |                    |
 
 [1] Tip of tree LLVM as of May 20, 2020
 

proposals/simd/SIMD.md

@@ -662,6 +662,17 @@ 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