What version of Go are you using (go version)?

I tested with Go 1.15 and "tip" from https://go.godbolt.org.

Does this issue reproduce with the latest release?

I have not tested Go 1.16, but it most likely will due to "tip" (Go 1.17 in the making) shows the same thing.

What operating system and processor architecture are you using (go env)?

The amd64 architecture over on https://go.godbolt.org.

What did you do?

Math rules (order of operation) says that a * b + a * c == a * (b + c) which also applies for a * b + a * c + a * d == a * (b + c + d) and so on . This means that the same calculation can be done with less multiplications and more additions. It might not be a huge difference, but I would assume that the compiler would be able to optimize away the left part to the right part to save some multiplications, but that seems to not be the case.

This math simplification seems like it is being made correctly for int https://go.godbolt.org/z/aEK63sPvj (even when doing the same thing but with four variables https://go.godbolt.org/z/es1561rqa), but not for float32 and float64 https://go.godbolt.org/z/q31ErddaP.

What did you expect to see?

I had expected that the output would have been simplified to the following (ignore function name).

func MultiplySimple(a, b, c float32) float32 {
    return a * (b + c)
}

text    "".MultiplySimple(SB), NOSPLIT|ABIInternal, $0-16
        funcdata        $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        funcdata        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        funcdata        $5, "".MultiplySimple.arginfo1(SB)
        addss   X2, X1
        mulss   X1, X0
        ret

What did you see instead?

I saw the two multiplications and one add, instead of two adds and one multiplication.

func MultiplyExpensive(a, b, c float32) float32 {
    return a * b + a * c
}

I got the following assembly output:

text    "".MultiplyExpensive(SB), NOSPLIT|ABIInternal, $0-16
        funcdata        $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        funcdata        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        funcdata        $5, "".MultiplyExpensive.arginfo1(SB)
        mulss   X0, X1
        mulss   X2, X0
        addss   X1, X0
        ret