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FoldingSet is implemented as a chained hash table. When there is a hash collision during insertion, which is common as we fill the table until a load factor of 2.0 is hit, we walk the chained elements, comparing every operand with the new element's operands. This can be very expensive if the MDNode has many operands. We sacrifice a word of space in MDNode to cache the full hash value, reducing compares on collision to a minimum. MDNode grows from 28 to 32 bytes + operands on x86. On x86_64 the new bits fit nicely into existing padding, not growing the struct at all. The actual speedup depends a lot on the test case and is typically between 1% and 2% for C++ code with clang -c -O0 -g. llvm-svn: 154497
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