runtime: consider caching previous map hash value #5147
Description
Michael Jones notes that the following pattern is common in Go code:
if v, ok := map[key]; ok { // or !ok
....
map[key] = newValue
}
If hashing that key type is expensive (say: large string/struct) and the map isn't empty
or very small, the above code does it twice.
If we kept a per-P cache of the last key & last computed hash value for that P, we
could eliminate some hash calls.
Probably worth measuring. (Probably not for Go 1.1)