c++ - 64bit Applications and Inline Assembly

Question

I am using Visual C++ 2010 developing 32bit windows applications. There is something I really want to use inline assembly. But I just realized that visual C++ does not support inline assembly in 64bit applications. So porting to 64bit in the future is a big issue.

I have no idea how 64bit applications are different from 32bit applications. Is there a chance that 32bit applications will ALL have to be upgraded to 64bit in the future? I heard that 64bit CPUs have more registers. Since performance is not a concern for my applications, using these extra registers is not a concern to me. Are there any other reasons that a 32bit application needs to be upgraded to 64bit? Would a 64 bit application process things differently when compared with a 32bit application, apart from that the 64bit applications may use registers or instructions that are unique to 64bit CPUs?

My application needs to interact with other OS components e.g. drivers, which i know must be 64bit in 64bit windows. Would my 32bit application compatible with them?

Answer 1

Visual C++ does not support inline assembly for x64 (or ARM) processors, because generally using inline assembly is a bad idea.

1. Usually compilers produce better assembly than humans.
2. Even if you can produce better assembly than the compiler, using inline assembly generally defeats code optimizers of any type. Sure, your bit of hand optimized code might be faster, but the fact that code around it can't be optimized will generally lead to a slower overall program.
3. Compiler intrinsics are available from pretty much every major compiler that let you access advanced CPU features (e.g. SSE) in a manner that's consistent with the C and C++ languages, and does not defeat the optimizer.

I am wondering would there be a chance that 32bit applications will ALL have to be upgraded to 64bit in the future.

That depends on your target audience. If you're targeting servers, then yes, it's reasonable to allow users to not install the WOW64 subsystem because it's a server -- you know it'll probably not be running too much 32 bit code. I believe Windows Server 2008 R2 already allows this as an option if you install it as a "server core" instance.

Since performance is not a concern for my appli so using the extra 64bit registers is not a concern to me. Is there any other reasons that a 32bit appli has to be upgraded to 64bit in the future?

64 bit has nothing to do with registers. It has to do with size of addressable virtual memory.

Would a 64 bit app process different from a 32bit appl process apart from that the 64bit appli is using some registers/instructions that is unique to 64bit CPUs?

Most likely. 32 bit applications are constrained in that they can't map things more than ~2GB into memory at once. 64 bit applications don't have that problem. Even if they're not using more than 4GB of physical memory, being able to address more than 4GB of virtual memory is helpful for mapping files on disk into memory and similar.

My application needs to interact with other OS components e.g. drivers, which i know must be 64bit in 64bit windows. Would my 32bit application compatible with them?

That depends entirely on how you're communicating with those drivers. If it's through something like a "named file interface" then your app could stay as 32 bit. If you try to do something like shared memory (Yikes! Shared memory accessible from user mode with a driver?!?) then you're going to have to build your app as 64 bit.