By using __has_extension(c_generic_selections), we can explicitly test
whether we're dealing with a version of Clang that supports _Generic().
That way we can use the improved <tgmath.h> code, even when not using
-std=c11. This massively reduces the compilation time when invoking
these functions.
I was thinking by myself, if the new code doesn't work with GCC 4.2, why
not simply turn it into an efficient version for C11 compilers? By
changing the code to use _Generic() directly in that case, I can build
the tgmath regression test in a matter of milliseconds with Clang,
instead of the 8 seconds it used to take.
So by the time C11 becomes the default, it will pick up the new code
automatically. And now I will refrain from making more changes to
<tgmath.h>.
Instead of using an exponential number of cases with respect to the
number of arguments, this version only uses a linear number.
Unfortunately, it works with Clang, GCC 4.6 and GCC 4.7, but not GCC
4.2. Therefore, leave it commented out.
The macro construction used now, is almost identical to the code
provided in C11 proposal N1404. This new version doesn't seem to
introduce any regressions according to the regression test in tools/,
but still seems to malfunction with Clang on certain aspects.
The new code does work successfully with GCC 4.2, 4.6 and 4.7. With 4.7,
it also works when __generic() is implemented on top of _Generic().
Discussed with: stefanf
for the <math.h> and <complex.h> functions that have float, double and long
double implementations. Such type-generic macros expand to an actual
function, depending on the types of the macro arguments, eg. if <tgmath.h>
is included, the invocation cos(1.0f) calls the function cosf().
