floor: Wasm numeric instruction

Try it

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

    f32.const -2.7 ;; load a number onto the stack
    f32.floor ;; round down
    call $log ;; log the result

  )
  (start $main)
)

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

Syntax

value_type.floor

value_type

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

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

floor

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

Type

[input] -> [output]

input

The input v128 value interpretation.

output

The output v128 value interpretation.

For a non-SIMD floor, these will be basic numeric values such as 14.3 or 3.0.

For a SIMD floor, these will be v128 value interpretations, for example f32x4 1.9 2.5 0.5 12.1. Each lane of the output pushed to the stack is the rounded down value of the corresponding lane in the input value.

Binary encoding

Examples

SIMD floor example

In this example, we demonstrate running floor 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.floor to round each lane down to the nearest integer. Finally we extract the value stored in lane 0 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.floor ;; Round each value down
    f64x2.extract_lane 0 ;; Extract a value from the result
    call $output
  )

  (start $main)
)

Result

The output is as follows:

3 is output because this is the result of rounding down lane 0 of the input value (3.9) to the nearest integer.

Specifications

Browser compatibility