Adding a _LIBCXX_HAS_NO_LONG_DOUBLE

Runtimes C++
1

Heya folks, new contributor here. I work at Google on the Asylo project for enclave development. I’m in the process of switching our toolchain over to clang from GCC, and I’ve found that I have some minor problems doing so with newlib as our libc implementation.

Mainly, long double math operations are not supported by newlib. In libstdc++, if long double functions weren’t defined in libc (#ifndef HAVE_HYPOTL, etc, …), it’d define them itself. That’s not the case in libc++, so I was wondering what y’all thought would be the best course of action.

Mainly, should libc++ act like libstdc++ and define its own long double implementations of math operations, or should it not add the functions to the std:: namespace if they’re not available?

If not adding them, then the following headers would guard given declarations within _LIBCPP_HAS_NO_LONG_DOUBLE/#endif:

#include :
using ::nexttoward;
using ::nexttowardf;

using ::acosl;
[…contiguously down to…]
using ::truncl;

inline _LIBCPP_INLINE_VISIBILITY long double hypot( long double x, long double y, long double z ) { return sqrt(xx + yy + z*z); }

#include :
using ::strtold;

#include :
[…] long double __do_strtod […]

#include <math.h>:
[…everything that mentions long double in its prototype…]

I can produce a patch quickly if this is an acceptable change to folks.

Regards,
-Dionna Glaze, PhD (she/her)

2

We added some support newlib with NaCl / PNaCl. IIRC we didn’t do anything for long double because it mapped to double on our target, which might be what you want to do in your case.

However, if you do want to support long double in some form, check out e.g. http://reviews.llvm.org/D7888. include/__config is probably where you want to look, and see how other platforms handle types like that.

3

Thanks, I’ve gone ahead and made https://reviews.llvm.org/D64105 since sizeof(long double) != sizeof(double) for our platform and I don’t have software implementations of long double math operations to contribute.