Re: [tip:x86/mce] x86/bitops: Move BIT_64() for a wider use

[Linus Torvalds]

On Wed, May 23, 2012 at 9:53 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> How about the following completely untested chunk:

No, you can't do that. All standard C operations will return *one*
type. That very much includes the ternary ?: operator.

The *only* ways I know of to get two types are

 - C preprocessor stuff, ie

     #define BIT(x)  __BIT_##x

   and then just enumerate all the 64 cases. This is portable, but it gets old.

 - using __builtin_choose_expr(), which actually allows the two
expressions to have different types, but requires a very strict
compile-time constant (ie you cannot rely on the optimizer making it a
constant - because it needs to choose the expression before the
optimizer runs)

There might be some other magic gcc extension, of course.

                 Linus
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