shr_s: Wasm numeric instruction

The shr_s numeric instruction, short for shift-right signed, is used for performing a bitwise right-shift on signed integers, similar to the >>> operator in other languages.

Try it

(module

  (func (export "shift_right") (param $num i32) (param $by i32) (result i32)
    ;; load the number to shift and the by how many spots
    local.get $num
    local.get $by

    ;; shift and return the result
    i32.shr_s
  )

)

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

    const res = shift_right(0b00000000_00000000_00000000_00000111, 1);
    console.log(numToBin(res));
    // Expected output: "00000000_00000000_00000000_00000011"
  },
);

function numToBin(num) {
  return (num >>> 0)
    .toString(2)
    .padStart(32, "0")
    .match(/.{1,8}/g)
    .join("_");
}

Syntax

value_type.shr_s

value_type

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

• i32
• i64
• v128 interpretations:
  • i8x16
  • i16x8
  • i32x4
  • i64x2

shr_s

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

Type

[input, shift_value] -> [output]

input

The input value.

shift_value

The value that you want to shift the value by.

output

The output value.

For a non-SIMD shr_s, the input and output will be basic numeric values such as 3 or 12.

For a SIMD shr_s, the input and output will be v128 value interpretations, for example i32x4 2 30 86 120. Each lane of the output pushed to the stack contains the corresponding lane in the input value shifted to the right by the specified shift_value.

Binary encoding

SIMD right shift

In this example, we demonstrate performing a right-shift on a SIMD value and outputting one of the lane values.

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 i16x8 value, then right-shift it by 2 using i16x8.shr_s. Finally we extract the value stored in lane 6 of the output SIMD 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 two SIMD values onto the stack
    v128.const i16x8 9 10 11 12 89 90 91 92
    i32.const 2

    i16x8.shr_s ;; shift-right by 2
    i16x8.extract_lane_s 6 ;; Extract a value from the result

    call $output
  )

  (start $main)
)

Result

The output is as follows:

The result is 22, because the value stored in lane 6 of the input value is 91. Once shifted right by two positions, the output value's lane 6 will contain the value 22.

Specifications

• WebAssembly Core Specification

See also

• shl
• shr_u