[Bug 39631] add optimisation helpers

https://bugs.freedesktop.org/show_bug.cgi?id=39631

--- Comment #6 from Stephan Bergmann <sbergman@redhat.com> ---
also note that GCC __builtin_expect (with its first parameter being of type
long) is plain, simple, and generally does not work as one would naively hope:

> class C {
> private:
>     struct S;
>     typedef void (S::* B)();
> public:
>     operator B();
> };
> int f(C c) {
>     if (c) {
>         if (__builtin_expect(c, 1)) {
>             return 1;
>         } else {
>             return 2;
>         }
>     } else {
>         return 3;
>     }
> }

will cause a compilation error "cannot convert ‘C’ to ‘long int’ for argument
‘1’ to ‘long int __builtin_expect(long int, long int)’"

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