What is the most reliable way to find out CPU architecture when compiling C or C++ code? As far as I can tell, different compilers have their own set of non-standard preprocessor definitions (_M_X86 in MSVS, __i386__, __arm__ in GCC, etc).
Is there a standard way to detect the architecture I'm building for? If not, is there a source for a comprehensive list of such definitions for various compilers, such as a header with all the boilerplate #ifdefs?
There's nothing standard. Brian Hook documented a bunch of these in his "Portable Open Source Harness", and even tries to make them into something coherent and usable (ymmv regarding that). See the posh.h header on this site:
Note, the link above may require you to enter some bogus userid/password due to a DOS attack some time ago.
There's no inter-compiler standard, but each compiler tends to be quite consistent. You can build a header for yourself that's something like this:
#if MSVC
#ifdef _M_X86
#define ARCH_X86
#endif
#endif
#if GCC
#ifdef __i386__
#define ARCH_X86
#endif
#endif
There's not much point to a comprehensive list, because there are thousands of compilers but only 3-4 in widespread use (Microsoft C++, GCC, Intel CC, maybe TenDRA?). Just decide which compilers your application will support, list their #defines, and update your header as needed.
Here is some information about Pre-defined Architecture Macros and other types of pre-defined macros.
If you need a fine-grained detection of CPU features, the best approach is to ship also a CPUID program which outputs to stdout or some "cpu_config.h" file the set of features supported by the CPU. Then you integrate that program with your build process.