[PowerPC] Fix a test case failure when compiled with libcxx

[Conanap]

Summary

The test case is not ran unless libcxx is used, and a macro
may be undefined. This patch checks for the definition of the
macro before using it.

[• Quuxplusone]

LGTM mod comment.

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

95

Please De-Morgan-ize this condition, for readability. Make it:

#if defined(__PPC__) && __LONG_DOUBLE_128__ && !(defined(__LONG_DOUBLE_IEEE128__) && __LONG_DOUBLE_IEEE128__)

[Mordante]

Please update the description of this patch to address @ldionne's review comment https://reviews.llvm.org/D108071#2950754
Other than that and @Quuxplusone's comment LGTM.

[Conanap]

De Morgan's for readability

[Conanap]

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

95

updated, thanks!

[ldionne]

LGTM, I'll commit this (and reference http://llvm.org/PR51430 in the commit message), and cherry-pick it to release/13.x

libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

 
 //  Check two values "close to each other"
     T d1 = T(3.14);
     T d0 = std::nextafter(d1, T(2));
     T d2 = std::nextafter(d1, T(5));
     assert(d0 < d1);  // sanity checking
     assert(d1 < d2);  // sanity checking
 
-#if defined(__PPC__) && __LONG_DOUBLE_128__ && !__LONG_DOUBLE_IEEE128__
+#if defined(__PPC__) && __LONG_DOUBLE_128__ && !(defined(__LONG_DOUBLE_IEEE128__) && __LONG_DOUBLE_IEEE128__)
 //  For 128 bit long double implemented as 2 doubles on PowerPC,
 //  nextafterl() of libm gives imprecise results which fails the
 //  midpoint() tests below. So skip the test for this case.
     if constexpr (sizeof(T) != 16)
 #endif
     {
   //  Since there's nothing in between, the midpoint has to be one or the other
     T res;