bitselect: Wasm SIMD bitwise instruction

The bitselect SIMD bitwise instruction takes three v128 values as inputs — two inputs and a mask value — and returns a new v128 value with each byte calculated using the formula output = (input1 AND mask) OR (input2 AND NOT mask).

Try it

(module
  (import "console" "log" (func $log (param i32)))
  (func $main
    v128.const i8x16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
    v128.const i8x16 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15
    v128.const i8x16 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
    v128.bitselect

    i8x16.extract_lane_u 15
    call $log ;; log the result
  )
  (start $main)
)

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

In the above example, we've set all input value lanes to the same value for simplicity. Walking through how the output value (9) is calculated, using the formula output = (input1 AND mask) OR (input2 AND NOT mask):

1. The first input is 1, which is 0 0 0 0 0 0 0 1 in binary.
2. The second input is 15, which is 0 0 0 0 1 1 1 1 in binary.
3. The mask is 6, which is 0 0 0 0 0 1 1 0 in binary.
4. input1 AND mask is calculated as follows:

   input1           0 0 0 0 0 0 0 1
   mask             0 0 0 0 0 1 1 0
   input1 AND mask  0 0 0 0 0 0 0 0

5. input2 AND NOT mask is calculated as follows:

   input2               0 0 0 0 1 1 1 1
   NOT mask             1 1 1 1 1 0 0 1
   input2 AND NOT mask  0 0 0 0 1 0 0 1

6. We then OR the two results from steps 4. and 5.:

   result1  0 0 0 0 0 0 0 0
   result2  0 0 0 0 1 0 0 1
   OR       0 0 0 0 1 0 0 1

0 0 0 0 1 0 0 1 is the binary equivalent of 9.

Syntax

v128.bitselect

v128.bitselect

The v128.bitselect instruction.

Type

[input1, input2, mask] -> [output]

input1

The first input v128 value interpretation.

input2

The second input v128 value interpretation.

mask

The mask v128 value interpretation.

output

The output v128 value interpretation.

Binary encoding

Specifications

• WebAssembly Core Specification