neg: Wasm text instruction

Try it

(module
  (import "console" "log" (func $log (param f32)))
  (func $main

    f32.const 10 ;; load a number onto the stack
    f32.neg ;; negate the values
    call $log ;; log the result

  )
  (start $main)
)

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

Syntax

value_type.neg

value_type

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

• f32
• f64
• i8x16
• i16x8
• i32x4
• i64x2
• f32x4
• f64x2

neg

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

Type

[input] -> [output]

input2

The input value.

output

The output value.

For a non-SIMD neg, the values will be basic numeric values such as 3 or -3.5.

For a SIMD neg, the values will be v128 value interpretations, for example f32x4 8 -27.3 30 -0.5.

Binary encoding

Examples

SIMD negation

In this example, we demonstrate using neg to negate a SIMD value.

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.neg to negate all its lane values. 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 -1 0 3 8 15 -2 -32 -11 -2 1 5 8 -3 -5 0 -6

    i8x16.neg ;; Negate all the lanes
    i8x16.extract_lane_s 15 ;; Extract a value from the result
    call $output
  )

  (start $main)
)

Result

The output is as follows:

6 is output because it is the negation of the value in lane 15 of the input value.