https://llvm.org/bugs/show_bug.cgi?id=31153
Bug ID: 31153
Summary: unordered_set's operator= or insert() is slower
without a reserve before it
Product: libc++
Version: unspecified
Hardware: PC
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: arpha...@gmail.com
CC: llvm-bugs@lists.llvm.org, mclow.li...@gmail.com
Classification: Unclassified
Created attachment 17643
--> https://llvm.org/bugs/attachment.cgi?id=17643&action=edit
A sample file that reproduces this problem
The use of unordered_set's "operator =" or call to "insert" is faster if you
call "reserve" first (using the size of the copied or inserted set). I get
about 10 - 15% improvements with "reserve".
I've attached a sample file that can be used to reproduce this problem. I've
tested it using "clang++ unordered_set_reserve.cpp -O3 -stdlib=libc++
-std=c++11" with ToT clang (r287852) and libcxx (r287829) on macOS (2015
MacBook Pro with 2.8 GHz Quad Core i7).
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