os: make File.WriteString not allocate

[josharian]

This is os.File.WriteString's implementation:

// WriteString is like Write, but writes the contents of string s rather than
// a slice of bytes.
func (f *File) WriteString(s string) (n int, err error) {
	return f.Write([]byte(s))
}

This currently accounts for 10% 35% of allocated space in a service I'm maintaining.

We should be able to do an unsafe cast of s to b here, and avoid the allocation.

Do any kernels modify the bytes during Write? Any other reason why we wouldn't be able to do this?

cc @bradfitz

[josharian]

Problem: os can't import reflect, at least according to existing dependency rules. And I don't think #19367 will help here.

There are options: add go:linkname with package runtime support, allow os to import reflect, add StringHeader/SliceHeader to package reflectlite and teach the compiler about that. I imagine the first is the most palatable.

[odeke-em]

@josharian you are Josh and can make it work perhaps in the compiler by special recognition, if the symbol is a (*os.File).WriteString and its on an architecture that we've audited as won't be mutating bytes, rewrite it :)

[ianlancetaylor]

@josharian Permit me to introduce you to the exciting new internal/unsafeheader package.

[josharian]

Oooooooooooooooooh!

[gopherbot]

Change https://golang.org/cl/268020 mentions this issue: os: avoid allocation in File.WriteString