https://llvm.org/bugs/show_bug.cgi?id=24643
Bug ID: 24643
Summary: FoldingSetNodeID::AddPointer is unnecessarily slow
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Support Libraries
Assignee: unassignedb...@nondot.org
Reporter: ben.cr...@codeaurora.org
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
FoldingSetNodeID houses the hash and equality material for FoldingSet's
intrusive hash map data structure. The raw material is stored as a
SmallVector<unsigned, 32>. The current implementation of AddPointer does a
ranged append, like so...
Bits.append(reinterpret_cast<unsigned *>(&Ptr),
reinterpret_cast<unsigned *>(&Ptr+1));
This is done in order to support 32-bit and 64-bit pointers. Unfortunately,
SmallVector::append with 1 element is slower than SmallVector::push_back with 1
element. Append with 2 elements is also slower than two push_backs.
On MSVC, the append call eventually turns into a memmove + some bookkeeping.
With push_back, it turns into a memcpy + some bookkeeping.
When using the Clang static analyzer, AddPointer ends up being one of the top
hits for exclusive / self time.
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