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This set of benchmarks is actually very nice, since it compares pure 32bit code on a system without too many differences between 64bit and 32bit implementations.
On x86-64, however, recompiling to 64bit also gives the compiler access to additional registers, so it doesn't have to waste cycles re-arranging contents in the limited number of x86 registers. This offers a performance boost beyond simple recompilation and excluding the benefits of 64bits on operations which need it.
Therefore, readers considering Opterons and Athlon64s should be aware that the figures here do not represent the results they would discover from 64bit recompilation.