c++ - Performance 32 bit vs. 64 bit arithmetic

Question

Are native 64 bit integer arithmetic instructions slower than their 32 bit counter parts (on x86_64 machine with 64 bit OS)?

Edit: On current CPUs such Intel Core2 Duo, i5/i7 etc.

Answer 1

It depends on the exact CPU and operation. On 64-bit Pentium IVs, for example, multiplication of 64-bit registers was quite a bit slower. Core 2 and later CPUs have been designed for 64-bit operation from the ground up.

Generally, even code written for a 64-bit platform uses 32-bit variables where values will fit in them. This isn't primarily because arithmetic is faster (on modern CPUs, it generally isn't) but because it uses less memory and memory bandwidth.

A structure containing a dozen integers will be half the size if those integers are 32-bit than if they are 64-bit. This means it will take half as many bytes to store, half as much space in the cache, and so on.

64-bit native registers and arithmetic are used where values may not fit into 32-bits. But the main performance benefits come from the extra general purpose registers available in the x86_64 instruction set. And of course, there are all the benefits that come from 64-bit pointers.

So the real answer is that it doesn't matter. Even if you use x86_64 mode, you can (and generally do) still use 32-bit arithmetic where it will do, and you get the benefits of larger pointers and more general purpose registers. When you use 64-bit native operations, it's because you need 64-bit operations, and you know they'll be faster than faking it with multiple 32-bit operations -- your only other choice. So the relative performance of 32-bit versus 64-bit registers should never be a deciding factor in any implementation decision.