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

[Bill Wendling]

On Apr 11, 2012, at 7:06 AM, Benjamin Kramer <benny.kra at googlemail.com> 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.
> 
> Modified:
>    llvm/trunk/include/llvm/Metadata.h
>    llvm/trunk/lib/VMCore/LLVMContextImpl.h
>    llvm/trunk/lib/VMCore/Metadata.cpp
> 
> Modified: llvm/trunk/include/llvm/Metadata.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Metadata.h?rev=154497&r1=154496&r2=154497&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Metadata.h (original)
> +++ llvm/trunk/include/llvm/Metadata.h Wed Apr 11 09:06:54 2012
> @@ -75,6 +75,10 @@
>   void operator=(const MDNode &);        // DO NOT IMPLEMENT
>   friend class MDNodeOperand;
>   friend class LLVMContextImpl;
> +  friend struct FoldingSetTrait<MDNode>;
> +
> +  /// NumOperands - If the MDNode is uniqued cache the hash to speed up lookup.
        ^^^^^^^^^^^^ - Should be Hash. :)

> +  unsigned Hash;
> 
>   /// NumOperands - This many 'MDNodeOperand' items are co-allocated onto the
>   /// end of this MDNode.
> 

-bw