Rounding Average instructions

Author: Maratyszcza

proposals/simd/BinarySIMD.md

@@ -189,3 +189,5 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i64x2.load32x2_s`         |    `0xd6`| m:memarg           |
 | `i64x2.load32x2_u`         |    `0xd7`| m:memarg           |
 | `v128.andnot`              |    `0xd8`| -                  |
+| `i8x16.avgr_u`             |    `0xd9`|                    |
+| `i16x8.avgr_u`             |    `0xda`|                    |

proposals/simd/ImplementationStatus.md

@@ -87,6 +87,7 @@
 | `i8x16.sub`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i8x16.sub_saturate_s`     |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i8x16.sub_saturate_u`     |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
+| `i8x16.avgr_u`             |                           |                       |                    |                    |
 | `i16x8.neg`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.any_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.all_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
@@ -100,6 +101,7 @@
 | `i16x8.sub_saturate_s`     |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.sub_saturate_u`     |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i16x8.mul`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
+| `i16x8.avgr_u`             |                           |                       |                    |                    |
 | `i32x4.neg`                |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i32x4.any_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |
 | `i32x4.all_true`           |               `-msimd128` |    :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |

proposals/simd/SIMD.md

@@ -457,6 +457,20 @@ def S.sub_saturate_u(a, b):
     return S.lanewise_binary(subsat, S.AsUnsigned(a), S.AsUnsigned(b))
 ```
 
+### Integer rounding average
+* `i8x16.avgr_u(a: v128, b: v128) -> v128`
+* `i16x8.avgr_u(a: v128, b: v128) -> v128`
+
+Lane-wise rounding average:
+
+```python
+def S.RoundingAverage(x, y):
+    return (x + y + 1) // 2
+
+def S.avgr_u(a, b):
+    return S.lanewise_binary(S.RoundingAverage, S.AsUnsigned(a), S.AsUnsigned(b))
+```
+
 ## Bit shifts
 
 ### Left shift by scalar