Feature #13725: [PATCH] Hash#[]= deduplicates string keys if (and only if) fstring exists

• Status: Closed
• Assignee: -
• Target version: -

[normalperson (Eric Wong)]

Hash#[]= deduplicates string keys if (and only if) fstring exists

In typical applications, hash entries are read after being
written to. Blindly writing to hashes which are never read
makes little sense. So, for any hash which is read from, an
fstring entry for the key should already exist for the key.

We no longer blindly create fstrings if the code is blindly
setting random hash keys, preventing the performance regression
in the reverted r43870.

Regarding https://bugs.ruby-lang.org/issues/9188, this has a
minimum impact on the bm_so_k_nucleotide where hash keys are set
and not reused, performance is within 1-2% of existing cases.

• hash.c: #include gc.h for rb_objspace_garbage_object_p
  (hash_aset_str): do read-only check of fstring table and
  reuse fstring if it exists and is still alive (not garbage)

[normalperson (Eric Wong)]

I forget to mention, this might make the proposed [Feature #13721]
("net/imap: dedupe attr keys in Net::IMAP::FetchData")
obsolete: https://bugs.ruby-lang.org/issues/13721

Along with the necessity to make similar changes down the line.

• Status changed from Open to Closed

Applied in changeset trunk|r59304.

---

Hash#[]= deduplicates string keys if (and only if) fstring exists

In typical applications, hash entries are read after being
written to. Blindly writing to hashes which are never read
makes little sense. So, for any hash which is read from, an
fstring entry for the key should already exist for the key.

We no longer blindly create fstrings if the code is blindly
setting random hash keys, preventing the performance regression
in the reverted r43870.

Regarding https://bugs.ruby-lang.org/issues/9188, this has a
minimum impact on the bm_so_k_nucleotide where hash keys are set
and not reused, performance is within 1-2% of existing cases.

• hash.c: #include gc.h for rb_objspace_garbage_object_p
  (hash_aset_str): do read-only check of fstring table and
  reuse fstring if it exists and is still alive (not garbage)
  [ruby-core:81942] [Feature #13725]

[nobu (Nobuyoshi Nakada)]

• Related to Bug #13857: frozen string literal: can freeze same string into two unique frozen strings added