"It has nothing directly to do the with speed of the program" -> I don't think so, 64 bit do something for the speed of the program, but only with the apps wich are specifically developed for 64 bit. In fact, if you are working directly with 64 bit variables (on a 64 bit CPU) it's faster than working with 32 bit variables, only because the CPU can work with bigger numbers and for example for a database app it's far better for managing and using indexes.


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On ppc it is hard to tell what is faster. For example loading a 64-bit register may take up to 4 instructions. Small immediate values (up to 25-bit I believe) don't take anymore due to sign extension. Also if your data structures in memory are 64-bit now instead of 32-bit you pay a cache and memory bandwidth penalty.

On intel you actually often win with 64-bit. As long as you do not change the layout of your structures in memory to 64-bit then you do not pay the cache and memory bandwidth penalties, but you do get 16 GPRs (general purpose registers). In 32-bit code you have 8 GPRs (handwavingly since the 286 4 are very general purpose, the other 4 have some specialness to them). I actually saw up to 30% increase in performance recompiling some of my code as x86_64.

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