From: Eric Wong <normalperson@...>
Date: 2014-01-03T08:51:27+00:00
Subject: [ruby-core:59506] Re: [ruby-trunk - Feature #8998] string keys for hash literals should use fstrings

> * Should string freeze-duping in Hash#[]= always dedup, regardless of
> whether the string has already been frozen or the Hash has
> compare_by_identity == true? Or else what combination of those
> conditions should lead to deduping?

MRI doesn't replace/alter the key at all if compare_by_identity is true.
Thus keys remain mutable strings in the case of compare_by_identity.

Also, in my original proposed patch, it only deduped when a new frozen
string was required, so there's almost zero chance of incompatibility.
Pre-frozen strings are used as-is when used as keys in all cases.

Btw, in addition to the current optimization and in lieu of my original
patch, I also have a proposed patch in
https://bugs.ruby-lang.org/issues/9188 which also optimizes aref along
with aset only when literal strings are used.  This means we won't bust
the fstring cache with dynamically-generated strings, but string
literals in the source are not duplicated.

I'm not sure what the real-world performance impact is, but I suspect
Rails apps will benefit from that.