popcnt: Wasm numeric instruction

The popcnt numeric instruction, short for population count, is used to count the amount of 1s in a number's binary representation.

Try it

(module

  (func (export "count1s") (param $num i32) (result i32)
    ;; load the number onto the stack
    local.get $num

    ;; count the amount of 1s and return the result
    i32.popcnt
  )

)

const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console }).then(
  (result) => {
    const count1s = result.instance.exports.count1s;

    console.log(count1s(0b10000010));
    // Expected output: 2
  },
);

Syntax

value_type.popcnt

value_type

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

• i32
• i64
• i8x16 v128 interpretation

popcnt

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

Type

[input] -> [output]

input

The input value.

output

The output value.

For a non-SIMD popcnt, these will be basic numeric values such as 3.

For a SIMD popcnt, these will be v128 value interpretations, for example i8x16 0 1 34 12 2 2 2 8 19 20 3 -1 -45 0 0 30. Each lane of the output pushed to the stack contains the number of 1s in the binary equivalent of the corresponding lane in the input value.

Binary encoding

Examples

SIMD popcnt usage

In this example, we demonstrate using popcnt to count the amount of 1s in the binary representation of a SIMD value's lane contents.

JavaScript

In our script, we grab a reference to a <p> element that we will output our result to, then define an object for import into Wasm containing a single function that writes a value to the output <p>. We then compile and instantiate our Wasm module using the WebAssembly.instantiateStreaming() method, importing the object in the process.

<p></p>

const outputElem = document.querySelector("p");

const obj = {
  output(val) {
    outputElem.textContent += val;
  },
};

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

Wasm

In our Wasm module, we first import the JavaScript output() function, making sure to declare that it has an i32 parameter. We then declare a SIMD i8x16 value and use i8x16.popcnt to count the binary 1s in all its lanes. Finally we extract the value stored in lane 15 of the output value using the extract_lane_s instruction, and output it to the DOM by calling the imported output() function.

(module
  ;; Import output function
  (import "obj" "output" (func $output (param i32)))

  (func $main
    ;; load a SIMD value onto the stack
    v128.const i8x16 0 1 34 12 2 2 2 8 19 20 3 -1 -45 0 0 30

    i8x16.popcnt ;; count 1s in all the lanes and output as a new i8x16 value
    i8x16.extract_lane_s 15 ;; Extract a value from the result
    call $output
  )

  (start $main)
)

Result

The output is as follows:

4 is output because the value in lane 15 of the input value is 30. 30 in binary is 00011110, which has 4 1s in it.

Specifications

• WebAssembly Core Specification