32 bit/64 bit Compiling and Application Speed

Question

I just got a new 64 bit computer and I am still trying to understand the differences between 32 bit and 64 bit. I understand that applications built using 64-bit dependencies can only run on a 64 bit, but applications built with 32-bit dependencies can run on both 32 and 64 bit systems.

However, is there any other differences? I know some programs have two different windows versions you can download, one for 64 bit and one for 32 bit. Why do they provide the two different types? Is there a speed increase for compiling a program with 64 bit dependencies for a program to run on a 64 bit system?

Answer 1

Besides from speed some programs do not have a choice. They need to match the bitness of the host operating system exactly.

A good example is TortoiseSVN which installs a shell extension. Shell extensions are DLLs loaded into other processes. So TortoiseSVN must provide a 64 bit DLL on x64 system if it wants to provide a shell extension.

Drivers are a second example of this.

When it comes to speed there is a difference of course. It depends very much on what the program does. If a program does not require high performance providing an x64 executable does not help and is a waste of time for both developers just as for users.

Here is what causes a performance difference for the case 32 bit on 64 bit OS:

1. Higher kernel-call cost because of the mode switch
2. More registers, higher code size: For calculation intensive programs this works vastly in favor of native 64 bit apps. For other types of app, or for business style web applications the bigger code and pointer sizes might cause a net loss (so 64 bit slower than 32 bit!)
3. Access to more than 3GB of memory. This obviously comes into play rarely as of 2012. Think of databases, Photoshop, ...

Answer 2

The speed increase depends on what the application does, as usr said. But fundamentally a 64-bit CPU is able to process twice as much data as a 32-bit one in the same amount of time; but the machine instructions to do so are specific to the 64-bit processor, so existing native programs have to be recompiled (at the very least) to show the benefit.

In the case of x86_64, memory pointers are also now 64 bits wide instead of 32, meaning much more memory can be addressed by programs. However, the code takes up roughly twice as much space in disk and on memory; if you're on a 64-bit system that's starved for memory, swapping can potentially be a lot worse.

Answer 3

Performance will roughly the same for most languages.

The main reason to go for 64 bit is the addressable memory. A32 bit process has only access to 2 GB of data, but a 64 bit process can access 18 quintillion Bytes (that is way more than the amount of RAM you can put in your machine).