max: Wasm numeric instruction

The max numeric instruction is used for getting the higher of two floating point numbers.

Try it

(module
  (import "console" "log" (func $log (param f32)))
  (func $main
    ;; load 10 and 2 onto the stack
    f32.const 10
    f32.const 2

    f32.max ;; calculate the higher number
    call $log ;; log the result
  )
  (start $main)
)
const url = "{%wasm-url%}";
await WebAssembly.instantiateStreaming(fetch(url), { console });

Syntax

value_type.max
value_type

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

max

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

Type

[input1, input2] -> [output]
input1

The first input value.

input2

The second input value.

output

The output value.

For a non-SIMD max, the inputs will be basic numeric values such as 3.0 or 3.5, and the output will be the larger of input1 and input2.

For a SIMD max, the inputs will be v128 value interpretations, for example f32x4 2.0 30 86.9 120. Each lane of the output pushed to the stack is the larger of the corresponding lanes in the input values.

Binary encoding

InstructionBinary formatExample text => binary
f32.max0x97f32.max => 0x97
f64.max0xa5f64.max => 0xa5
f32x4.max0xfd 233:u32f32x4.max => 0xfd 0xe9 0x01
f64x2.max0xfd 245:u32f64x2.max => 0xfd 0xf5 0x01

Examples

undefined

SIMD max example

In this example, we demonstrate using max to return the greater value of the same lane index from two SIMD 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.

js
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 f32 parameter. We then declare two SIMD f32x4 values, then use f32x4.max to return a new f32x4 value that contains the higher lane value out of the two inputs in each case. Finally we extract the value stored in lane 3 of the output value using the extract_lane instruction, and output it to the DOM by calling the imported output() function.

wat
(module
  ;; Import output function
  (import "obj" "output" (func $output (param f32)))

  (func $main
    ;; load two SIMD values onto the stack
    v128.const f32x4 20 0 1015 1000
    v128.const f32x4 4 38 15 108

    ;; Return a new f32x4 containing the highest lane value in each case
    f32x4.max
    f32x4.extract_lane 3 ;; Extract a value from the result

    call $output
  )

  (start $main)
)

Result

The output is as follows:

The result is 1000. This is because the value stored in lane 3 of the first input value is 1000, and the value stored in lane 3 of the second input value is 108. Since 1000 is greater than 108, the new f32x4 value outputted by the f32x4.max instruction has 1000 set in lane 3, which we then extract and output to the DOM.

Specifications

See also