I recently needed a hash map/set implementation for a C++ project. No problem I thought. I went over and grabbed Google’s Sparsehash implementation and I was off and running. Things came to a screeching halt when I tried to compile my code with the Intel C++ Compiler. Usually this gives me a 30-40% speed increase for CPU-bound code, but only if it can actually compile. After a couple hours of searching and trying various other hash map implementations I realized I was going to have to fix this myself.

The problem lies in Sparsehash’s reliance on the std::tr1 namespace. This adds lots of neat things like smart pointers and regular expressions, but unfortunately g++’s implementation is not compatible with ICPC even when using the compiler flag -std=c++0x.

To remove the reliance we’re going to need to make a couple of changes and unfortunately, these changes will require you to always define your own hash function. First we’ll need to modify /usr/include/google/sparsehash/sparseconfig.h adding #ifndef __ICC around the bad parts so it looks like this:

/* Namespace for Google classes */
#define GOOGLE_NAMESPACE ::google
 
#ifndef __ICC
/* the location of the header defining hash functions */
#define HASH_FUN_H <tr1/functional>
 

/* the namespace of the hash<> function */
#define HASH_NAMESPACE std::tr1
#endif

and then further down

#ifndef __ICC
/* The system-provided hash function including the namespace. */
#define SPARSEHASH_HASH HASH_NAMESPACE::hash
#endif

Then in all the of the following files: sparse_hash_set, sparse_hash_map, dense_hash_set, and dense_hash_map we’ll make the following changes.

#include HASH_FUN_H // defined in config.h

to

#ifdef HASH_FUN_H
#include HASH_FUN_H // defined in config.h
#endif

and

class HashFcn = SPARSEHASH_HASH, // defined in sparseconfig.h

to

#ifdef SPARSEHASH_HASH
class HashFcn = SPARSEHASH_HASH, // defined in sparseconfig.h
#else
class HashFcn,
#endif

And there you go! Google’s efficient hash map/set implementation for the Intel Compiler. Fortunately the keys for all my hash_maps were integers so I didn’t really need a hash function, but Intel provides much faster ones in their IPP Crypto package anyways. Enjoy the performance boost!