Add .bitmask instruction family (#201)

* Add .bitmask instruction family

i8x16.bitmask and i32x4.bitmask directly map to SSE movemask
instructions; i16x8.bitmask can be synthesized using packs+movemask.

These instructions are important to be able to do lane-wise processing
after a vector comparison - for example, these can be used together with
ctz to find the index of the first lane with the matching values after a
comparison instruction.

* Update opcode tables with bitmask

Author: zeux

proposals/simd/BinarySIMD.md

@@ -114,6 +114,7 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i8x16.neg`                |    `0x61`| -                  |
 | `i8x16.any_true`           |    `0x62`| -                  |
 | `i8x16.all_true`           |    `0x63`| -                  |
+| `i8x16.bitmask`            |    `0x64`| -                  |
 | `i8x16.narrow_i16x8_s`     |    `0x65`| -                  |
 | `i8x16.narrow_i16x8_u`     |    `0x66`| -                  |
 | `i8x16.shl`                |    `0x6b`| -                  |
@@ -134,6 +135,7 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i16x8.neg`                |    `0x81`| -                  |
 | `i16x8.any_true`           |    `0x82`| -                  |
 | `i16x8.all_true`           |    `0x83`| -                  |
+| `i16x8.bitmask`            |    `0x84`| -                  |
 | `i16x8.narrow_i32x4_s`     |    `0x85`| -                  |
 | `i16x8.narrow_i32x4_u`     |    `0x86`| -                  |
 | `i16x8.widen_low_i8x16_s`  |    `0x87`| -                  |
@@ -159,6 +161,7 @@ The `v8x16.shuffle` instruction has 16 bytes after `simdop`.
 | `i32x4.neg`                |    `0xa1`| -                  |
 | `i32x4.any_true`           |    `0xa2`| -                  |
 | `i32x4.all_true`           |    `0xa3`| -                  |
+| `i32x4.bitmask`            |    `0xa4`| -                  |
 | `i32x4.widen_low_i16x8_s`  |    `0xa7`| -                  |
 | `i32x4.widen_high_i16x8_s` |    `0xa8`| -                  |
 | `i32x4.widen_low_i16x8_u`  |    `0xa9`| -                  |

proposals/simd/ImplementationStatus.md

@@ -87,6 +87,7 @@
 | `i8x16.neg`                |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i8x16.any_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i8x16.all_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
+| `i8x16.bitmask`            | `-munimplemented-simd128` | :heavy_check_mark: |                    |                    |                    |
 | `i8x16.narrow_i16x8_s`     |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i8x16.narrow_i16x8_u`     |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i8x16.shl`                |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
@@ -107,6 +108,7 @@
 | `i16x8.neg`                |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i16x8.any_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i16x8.all_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
+| `i16x8.bitmask`            | `-munimplemented-simd128` | :heavy_check_mark: |                    |                    |                    |
 | `i16x8.narrow_i32x4_s`     |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i16x8.narrow_i32x4_u`     |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i16x8.widen_low_i8x16_s`  |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
@@ -132,6 +134,7 @@
 | `i32x4.neg`                |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i32x4.any_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i32x4.all_true`           |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
+| `i32x4.bitmask`            | `-munimplemented-simd128` | :heavy_check_mark: |                    |                    |                    |
 | `i32x4.widen_low_i16x8_s`  |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i32x4.widen_high_i16x8_s` |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |
 | `i32x4.widen_low_i16x8_u`  |               `-msimd128` | :heavy_check_mark: |                    |                    | :heavy_check_mark: |

proposals/simd/NewOpcodes.md

@@ -82,7 +82,7 @@
 | i8x16.neg            | 0x61   | i16x8.neg                | 0x81   | i32x4.neg                | 0xa1   | i64x2.neg   | 0xc1   |
 | i8x16.any_true       | 0x62   | i16x8.any_true           | 0x82   | i32x4.any_true           | 0xa2   | ----        | 0xc2   |
 | i8x16.all_true       | 0x63   | i16x8.all_true           | 0x83   | i32x4.all_true           | 0xa3   | ----        | 0xc3   |
-| ---- bitmask ----    | 0x64   | ---- bitmask ----        | 0x84   | ---- bitmask ----        | 0xa4   | ----        | 0xc4   |
+| i8x16.bitmask        | 0x64   | i16x8.bitmask            | 0x84   | i32x4.bitmask            | 0xa4   | ----        | 0xc4   |
 | i8x16.narrow_i16x8_s | 0x65   | i16x8.narrow_i32x4_s     | 0x85   | ---- narrow ----         | 0xa5   | ----        | 0xc5   |
 | i8x16.narrow_i16x8_u | 0x66   | i16x8.narrow_i32x4_u     | 0x86   | ---- narrow ----         | 0xa6   | ----        | 0xc6   |
 | ---- widen ----      | 0x67   | i16x8.widen_low_i8x16_s  | 0x87   | i32x4.widen_low_i16x8_s  | 0xa7   | ----        | 0xc7   |

proposals/simd/SIMD.md

@@ -648,6 +648,24 @@ def S.all_true(a):
     return 1
 ```
 
+## Bitmask extraction
+
+* `i8x16.bitmask(a: v128) -> i32`
+* `i16x8.bitmask(a: v128) -> i32`
+* `i32x4.bitmask(a: v128) -> i32`
+
+These operations extract the high bit for each lane in `a` and produce a scalar
+mask with all bits concatenated.
+
+```python
+def S.bitmask(a):
+    result = 0
+    for i in range(S.Lanes):
+        if a[i] < 0:
+            result = result | (1 << i)
+    return result
+```
+
 ## Comparisons
 
 The comparison operations all compare two vectors lane-wise, and produce a mask