[llvm-commits] [llvm] r154497 - in /llvm/trunk: include/llvm/Metadata.h lib/VMCore/LLVMContextImpl.h lib/VMCore/Metadata.cpp

[Jakob Stoklund Olesen]

On Apr 11, 2012, at 7:06 AM, Benjamin Kramer wrote:

> Author: d0k
> Date: Wed Apr 11 09:06:54 2012
> New Revision: 154497
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=154497&view=rev
> Log:
> Cache the hash value of the operands in the MDNode.
> 
> 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.

Neat!

I would suggest one tweak:

All the nodes in a hash table chain are going to have identical low bits in the hash value. If you compute a 64-bit hash value and store the high 32 bits in the node while using the low bits to index the hash table, you can lower the probability of collisions even further.

/jakob