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Try it

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

    i64.const 123456789012345
    i64.const 123456789070
    i64.const 98765432101
    i64.const 9876543210123
    i64.sub128
    call $log ;; log high 64 bits
    call $log ;; log low 64 bits
  )
  (start $main)
)

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

WAT syntax

i64.sub128

i64.sub128: The i64.sub128 instruction.

Operand stack

[low_left_input:i64 high_left_input:i64 low_right_input:i64 high_right_input:i64] -> [low_output:i64 high_output:i64]

low_left_input: An i64 representing the low 64 bits of the left-hand-side 128-bit integer.
high_left_input: An i64 representing the high 64 bits of the left-hand-side 128-bit integer.
low_right_input: An i64 representing the low 64 bits of the right-hand-side 128-bit integer.
high_right_input: An i64 representing the high 64 bits of the right-hand-side 128-bit integer.
low_output: An i64 representing the low 64 bits of the result.
high_output: An i64 representing the high 64 bits of the result.

Binary encoding

Instruction	Binary format	Example text => binary
`i64.sub128`	`0xfc 20:u32`	`i64.sub128` => `0xfc 0x14`

Description

The sub128 instruction subtracts one 128-bit integer, represented by two i64 values, from another to produce a 128-bit result represented by two i64 values.

This is one of the wide arithmetic operations, which are useful in situations where larger-than-64-bit numbers are being used in source languages with Wasm as a compile target. One such example is cryptographic algorithms.

These methods allow engines to generate more performant machine code for these operations than is possible with i64 arithmetic operations.