bitselect: Wasm SIMD bitwise instruction

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

Browser compatibility

See also

• SIMD bitwise instructions