Some timings, for fixed-sized hashtable
No storage reclamation in robdd
gcc 4.0.1
MacBook Pro 2.33GHz Intel Core 2 Duo, Memory: 2GB 667 MHz
Mac OS X 10.5.8
Collected Nov 13, 2010

#define BDD_LIMIT (1<<20)
#define BDD_HASHTABLE_SIZE (1<<18)
#define APPLY_HASHTABLE_SIZE (1<<16)

Order:
1. each col has a queen
2. col, row, up-diag, down-diag
Size = 402480
-O0 2.412u 0.466s
-O2 2.174u 0.464s
-O2 4.206u 0.505s with manual memory management (free)!

Order:
1. col, row, up-diag, down-diag
2. each col has a queen
Size = 969470
-O2 10.512u 1.259s

% valgrind ./nqueens
==3970== Memcheck, a memory error detector
==3970== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==3970== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==3970== Command: ./nqueens
==3970== 
Final bdd size: 402480 nodes
92 solutions
==3970== 
==3970== HEAP SUMMARY:
==3970==     in use at exit: 4,460 bytes in 9 blocks
==3970==   total heap usage: 14,342,942 allocs, 14,342,933 frees, 672,995,784 bytes allocated
==3970== 
==3970== LEAK SUMMARY:
==3970==    definitely lost: 0 bytes in 0 blocks
==3970==    indirectly lost: 0 bytes in 0 blocks
==3970==      possibly lost: 0 bytes in 0 blocks
==3970==    still reachable: 4,160 bytes in 2 blocks
==3970==         suppressed: 300 bytes in 7 blocks
==3970== Rerun with --leak-check=full to see details of leaked memory
==3970== 
==3970== For counts of detected and suppressed errors, rerun with: -v
==3970== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)