Nope. Recompiling the code in 64 bits should get you 0-15% slowdown. That is normal due to the larger variables. Extra registers on AMD64 give some of the lost performance back, sometimes more than what was lost when going to 64 bits. System libraries is the OS concern, and it seems like they can add a little to the slowdown. However, all this is well worth the extra address space!
It's no problem to use 32bit int everywhere. And if you define "int" variables then these should be 32bit. Recompiled apps will still use 32bit then. BTW using them doesn't require a REX prefix byte for the first 8 regs. Below I listed some URLs where you can verify the effect of 8 additional regs both for Int and FP code.
Most pointers and address immediate values are 64bit then - that has the disadvantages that it needs more bytes in the cache and it could push some code bytes out of a decoder window (resulting in less decoded instructions during that cycle).
I'd like to see what GCC and PGI will produce for 64bit mode when their produced code matches the performance of the Intel compilers on the same platform (32bit). And we don't have results for any .NET compilers.
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