[libc++] Work around gcc/Power9 bug in `include/thread`

[jdenny]

Summary

This fixes PR39696, which breaks the libcxx build with gcc (I tested
7.5.0) on Power9. This fix was suggested at

https://bugs.llvm.org/show_bug.cgi?id=39696#c38

but never applied. It just reverts 0583d9ea8d5e, which reverses
components of the original fix in 3bf63cf3b366, which is correct.

Fixes https://llvm.org/PR39696

[ldionne]

Can you confirm that basically 0583d9ea8d5edc60d84971ced29db042b59302ba should not have been committed, since the original fix 3bf63cf3b366d3a57cf5cbad4112a6abf6c0c3b1 was correct in the first place. And all this does is revert 0583d9ea8d5edc60d84971ced29db042b59302ba.

I did read the bug report discussion and that is my understanding, I just want to double check because the bug report discussion is tortuous. If I got that right, LGTM.

[jdenny]

In D81438#2082403, @ldionne wrote:

Can you confirm that basically 0583d9ea8d5edc60d84971ced29db042b59302ba should not have been committed, since the original fix 3bf63cf3b366d3a57cf5cbad4112a6abf6c0c3b1 was correct in the first place. And all this does is revert 0583d9ea8d5edc60d84971ced29db042b59302ba.

Yes, that's my understanding (and I just checked that git revert produces the same result). I've updated the summary to clarify this.

I did read the bug report discussion and that is my understanding, I just want to double check because the bug report discussion is tortuous. If I got that right, LGTM.

Thanks for the quick review.

[ldionne]

Thanks for the confirmation. Ship it, and please update the bug report :-)

libcxx/include/thread

 void
 sleep_for(const chrono::duration<_Rep, _Period>& __d)
 {
     using namespace chrono;
     if (__d > duration<_Rep, _Period>::zero())
     {
 #if defined(_LIBCPP_COMPILER_GCC) && (__powerpc__ || __POWERPC__)
     //  GCC's long double const folding is incomplete for IBM128 long doubles.
-        _LIBCPP_CONSTEXPR duration<long double> _Max = nanoseconds::max();
-#else
         _LIBCPP_CONSTEXPR duration<long double> _Max = duration<long double>(ULLONG_MAX/1000000000ULL) ;
+#else
+        _LIBCPP_CONSTEXPR duration<long double> _Max = nanoseconds::max();
 #endif
         nanoseconds __ns;
         if (__d < _Max)
         {
             __ns = duration_cast<nanoseconds>(__d);
             if (__ns < __d)
                 ++__ns;
         }