Rust Inline Assembly Inputs and outputs
Example
#![feature(asm)]
#[cfg(any(target_arch="x86", target_arch="x86_64"))]
fn subtract(first: i32, second: i32) {
unsafe {
// Output values must either be unassigned (let result;) or mutable.
let result: i32;
// Each value that you pass in will be in a certain register, which
// can be accessed with $0, $1, $2...
//
// The registers are assigned from left to right, so $0 is the
// register containing 'result', $1 is the register containing
// 'first' and $2 is the register containing 'second'.
//
// Rust uses AT&T syntax by default, so the format is:
// SUB source, destination
// which is equivalent to:
// destination -= source;
//
// Because we want to subtract the first from the second,
// we use the 0 constraint on 'first' to use the same
// register as the output.
// Therefore, we're doing:
// SUB second, first
// and getting the value of 'first'
asm!("SUB $2, $0" : "=r"(result) : "0"(first), "r"(second));
println!("{}", result);
}
}
LLVM's constraint codes can be found here, but this may vary depending on the version of LLVM used by your rustc compiler.
