extend32_s: Wasm numeric instruction

Try it

(module
  (import "console" "log" (func $log (param i64)))
  (func $main

    i64.const 0xffffffff
    i64.extend32_s
    call $log

  )
  (start $main)
)

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

In this example, i64.const 0xffffffff pushes the value 0xffffffff (4294967295) onto the stack. This value represents -1 when read as a signed 32-bit number; i64.extend32_s sign-extends it to 0xffffffffffffffff — which is -1 represented as a full 64-bit signed integer.

WAT syntax

i64.extend32_s <input> <output>

i64.extend32_s

The i64.extend32_s instruction.

Immediates

None.

Operand stack

[input: i64] -> [output: i64]

input

The input i64 integer.

output

The output i64 integer.

Binary encoding

Description

Sign extension is useful because Wasm integers are a fixed width (32- or 64-bit), but you often want to work with smaller values — like an i8, i16, or i32 — stored inside them. If you zero-pad a negative small value to fill the rest of the bits, you get the wrong number: the bit pattern that means -1 represented as a 32-bit value, for example, no longer represents -1 once you've zero-padded it to 64 bits.

The extend32_s instruction fixes this by taking the low 32 bits of the value, treating them as a signed 32-bit integer, and copying the sign bit (bit 31) up through all the remaining bits of the i64.

This is useful whenever you've loaded or produced an 32-bit value (say, from a four-byte value in memory) and need to use it correctly in arithmetic at the full 64-bit width.

Specifications

Browser compatibility

See also

• extend8_s
• extend16_s