bitmask: Wasm SIMD bitwise instruction

The bitmask SIMD bitwise instruction inspects the most significant bit (MSB) of each lane of a v128 value interpretation. This is the sign bit if the lane is treated as signed. The instruction's output value is equal to all of those bits collected into a single i32, with lane 0's MSB in bit 0 of the result, lane 1's MSB in bit 1, and so on.

Try it

(module
  (import "console" "log" (func $log (param i32)))
  (func $main
    v128.const i8x16 10 23 45 6 90 1 12 120 0 78 89 13 240 51 62 0
    i8x16.bitmask

    call $log ;; log the result
  )
  (start $main)
)

WebAssembly.instantiateStreaming(fetch("{%wasm-url%}"), { console });

In the above example, only lane 12 of the i8x16 value has a value of greater than or equal to 128 (240, to be exact), so its MSB is 1. All other byte's MSBs are set to 0.

The output i32 is therefore equal to:

0000 0001 0000 0000 0000

(only bit 12 is set to 1).

The example's output value is 4096, which is the decimal equivalent of the above binary pattern.

Syntax

value_type.bitmask

value_type

The type of value the instruction is being run on. The following types support bitmask:

• i8x16
• i16x8
• i32x4
• i64x2

bitmask

The bitmask instruction. Must always be included after the value_type and a period (.).

Type

[input] -> [output]

input

The input v128 value interpretation.

output

The output i32 value.

Binary encoding

Specifications

• WebAssembly Core Specification