How should application perform in 64-bit vs. 32-bit intel architectures?

Question

I want to know the relative performances of a normal C++ application in the following scenarios:

1. Built as 32-bit app, run on Intel 64-bit processor (x64-64)
2. Built as 32-bit app, run on Intel 32-bit processor (x86)
3. Built as 64-bit app.

Also, what factors should I consider when modifying / developing the application to make it to run faster on 64-bit processors?

Answer 1

Short answer: you probably won't notice much of a difference.

Longer answer: 64-bit x86 has more general purpose registers, which gives the compiler more of an opportunity to optimize local variables into registers for faster access. the compiler can also assume more modern features, eg. not having to optimize code for a 386, and can assume your CPU has stuff like SSE instead of the old x87 FPU for floating point math. but pointers will be twice as wide, which is worse for the cache.

Answer 2

CPU-intensive programs might be noticeably faster on 64-bit. The processor has 16 instead of 8 general purpose registers available which are also twice as wide (64 instead of 32 bits).

Also the number of registers for SSE instructions is doubled from 8 to 16 which helps for multimedia-applications or other applications which do a lot of floating-point computations.

For details see x86-64 on Wikipedia.

One thing that has not been mentioned yet is that 64-bit versions of operating systems such as Windows and Linux use a different calling convention for function calls on 64-bit systems; instead of passing arguments on the stack, arguments are (preferrably) passed in registers, which is in principle faster. So software will be faster because there is less function call overhead.