trunc: Wasm text instruction

Try it

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

    f32.const -2.7 ;; load a number onto the stack
    f32.trunc ;; discard everything after the decimal point
    call $log ;; log the result

  )
  (start $main)
)

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

Syntax

value_type.trunc

value_type

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

• f32
• f64
• v128 interpretations:
  • f32x4
  • f64x2

trunc

The trunc 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 trunc, these will be basic numeric values such as 14.3 or 3.0.

For a SIMD trunc, these will be v128 value interpretations, for example i32x4 100.5 6.0 7.8 83.1.

Binary encoding

Examples

SIMD trunc example

In this example, we demonstrate running trunc on a SIMD value and outputting one of the lane values of the result.

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 f64 parameter. We then declare a SIMD f64x2 value and use f64x2.trunc to remove the fractional part of each lane. Finally we extract the value stored in lane 1 of the output value using the extract_lane instruction, and output it to the DOM by calling the imported output() function.

(module
  ;; Import output function
  (import "obj" "output" (func $output (param f64)))

  (func $main
    ;; load a SIMD value onto the stack
    v128.const f64x2 3.9 2000.1

    f64x2.trunc ;; truncate each value
    f64x2.extract_lane 1 ;; Extract a value from the result
    call $output
  )

  (start $main)
)

Result

The output is as follows:

2000 is output because this is the result of removing the fractional part from lane 1 of the input value (2000.1).

See also

• trunc_f32_s
• trunc_f32_u
• trunc_f64_s
• trunc_f64_u