[PATCH] D16360: unordered_map: Avoid unnecessary mallocs when no insert occurs

[Duncan P. N. Exon Smith via cfe-commits]

dexonsmith updated this revision to Diff 50869.
dexonsmith added a comment.

Eric and I had a quick chat on IRC.

- The asymmetric usage of __extract_key and __can_extract_key was awkward, as Eric pointed out already.
- However, a symmetric __extract_key would have caused Clang (and other compilers) to IRGen extra copies of operator().
- Eric suggested using a custom tag type (instead of true_type/false_type) to avoid the problem entirely.

I implemented that in this updated patch, and it's way cleaner.  I'm using:

- __extract_key_fail_tag: cannot extract key.
- __extract_key_self_tag: use the argument itself as the key.
- __extract_key_first_tag: use ".first" on the argument to grab the key.


http://reviews.llvm.org/D16360

Files:
  include/__hash_table
  test/libcxx/containers/unord/unord.map/insert_dup_alloc.pass.cpp
  test/libcxx/containers/unord/unord.set/insert_dup_alloc.pass.cpp

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