optimization: division by constant as multiplication by inverse

Couldn’t division by constants be optimized by replacing the operation by a multiplication of its inverse (i.e x / 2 === x * (1/2) === x * 0.5)

input

// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// ==/ClosureCompiler==

function hello(x) {
 alert( x / 5);
}
const x = prompt('x')
hello(x);

output

alert(prompt("x")/5);

better optimization

alert(prompt("x")*0.2);

see example on “closure-compiler.appspot.com”

[gerardolima]

hey, @tjgq, generally I’d agree with you, but there are more js engines around than I could benchmark (*). I expect closure-compiler project would have such test suits, though, that could shed some hard evidence on these numbers.

(*) some time ago, I did such benchmark (here) and, at the time on my computer, chrome yielded 14% better performance and IE 10%.

[tjgq]

Why do you believe this to be an optimization? For an integer divisor, multiplication by the reciprocal will always result in increased code size (in your example, *0.2 is longer than /5).

If the intent is to optimize for execution speed rather than code size, this would need to be justified by a benchmark. It’s very likely that JavaScript engines already perform the optimization you describe.