Question

I am developing a C app using Code::Blocks, and need to target multiple platforms (32 bit, 64 bit). My development box is 64 bit, so I tried to check the i386 (-march=i386) option for the compiler in the project settings. Now when I compile I get:

mainc:1: error: CPU you selected does not support x86-64 instruction set

1. I don’t understand this message! By specifying i386 shouldn’t it be just generating x86 instructions (not -64)? Beside, my CPU is X86-64 so why say my cpu doesn’t support this?

2. Am I doing this the wrong way? Is there a RIGHT way to target different architectures?

Answer 1

You must distinguish several things here:

• There are 32 and 64 bit “platforms” that run on the same processor. The same X86-64 processor can operate in either x86 or AMD64 mode. This determines among other things the number of available registers, their sizes, stack layout, different calling convention, and some different opcodes (also note that on many operating systems, 32 bit code will run in some kind of compatibility layer in a 64bit OS without problems)
• There are compilers for either one of these platforms, and there are compilers which can produce either of both (but not both at the same time).
• The compiler can be a different platform from what the program is built for (so-called cross-compiler, very common thing e.g. when developing for embedded systems, but you can also cross-compile for example for Windows-32 from Linux-64 or the other way around, or compile for Win64 on a Win32 computer).
• In addition to the platform, there is a model-specific difference in generated code (-march switch). This decides what processor generation is assumed, which determines what instructions can be used in generating code. Not every combination of platform and architecture is valid (this is your very problem: assuming a 25 year old 386 processor means that it cannot possibly run in AMD64 mode).
• Optionally, you can even schedule for something different than you generate code for. You can for example generate code that uses instructions for up to Pentium III, but schedule for Core i7. This will run (sub-optimally, but it will run) on pretty much every processor, and run optimally on the newest line.
• Finally, there are many switches such as the -msse family which further enable/disable instructions.

You did not specify “Windows” or “Linux” or anything else, so it is hard to give an exact answer to what your problem is, but let’s assume you might be using for example MingW-w64. This compiler lets you create both 32 and 64 bit code, depending on what you tell it to do. Obviously, if you tell it to generate code in 64 mode (the default!), then i386 is not a good choice. If you give it -m32 as a commandline option, it will generate 32bit code, and it will work.

It may help finding a more concrete answer if you tell exactly what compiler you use. Code::Blocks comes (optionally) with the TDM 4.5.1 build on Windows, for example, but uses whatever is there under other operating systems.