The compiler already implements string(byteSlice) == "string literaal" without an allocation.

So I'm a little surprised that we don't get this for free right now. Maybe if switch were rewritten to a series of if statements then we'd get it for free?

/cc @mdempsky @josharian

Maybe if switch were rewritten to a series of if statements then we'd get it for free?

We would, but we'd lose the nice binary search properties. This should be an easy fix. I'll look soon.

The string(byteSlice) == "string literal" optimization doesn't extend to switch statements because we're actually doing:

tmp := string(byteSlice)
if tmp == "some_key" { ... }
else if tmp == "another" { ... }

(And actually more complicated, because we handle runs of constants with binary search.)

One easy optimization we could do here is that in swt.go, if s.exprname.Op is OARRAYBYTESTR and all of the case value expressions are side-effect free (at least no function calls or channel receives), we can rewrite the Op to ARRAYBYTESTRTMP.

It's important that we check for side-effects, because otherwise we'd miscompile expressions like:

x := []byte{'a'}
switch string(x) {
case func() string { x[0] = 'b'; return "b" }():
    panic("FAIL")
}

Change https://golang.org/cl/108035 mentions this issue: cmd/compile: avoid runtime call during switch string(byteslice)