Why does the Linux kernel #define a symbol as itself?

Question

In the Linux kernel, there can be found a line of code that looks redundant to me:

#define __arch_swahb32 __arch_swahb32

What is the purpose of an idiom like this?

Answer 1

Consider the following code:

#ifdef foo
    foo();
#endif

If you want a snippet like the above to call function foo, you need to define foo. However, if you just

#define foo

then the function foo name will be replaced with an empty token, and the first snippet is preprocessed to just ();. If, however, you

#define foo foo

then the first snippet will preprocess to foo(); as it should.