[flang][msvc] long double is not different from double.

[Meinersbur]

Summary

In Microsoft's ABI long double is an alias for double, which is allowed by the C++ ABI. Therefore it is also caught by the switch case for KIND(8). Therefore the kinds 10 and 16 are not implemented for this ABI.

The error from msvc was:

binary-floating-point.h(55): error C2607: static assertion failed
edit-output.h(68): note: see reference to function template instantiation 'Fortran::decimal::BinaryFloatingPointNumber<113>::BinaryFloatingPointNumber<A>(A)' being compiled
        with
        [
            A=long double
        ]

This patch is part of the series to make flang compilable with MS Visual Studio http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html.

[klausler]

Is there no access to 80-bit extended precision at all in MSVC?

Input and output conversion never actually has to do any floating-point arithmetic, so it would be possible to treat the 80-bit extended precision (and 128-bit quad precision) types in I/O as struct around a char[10 or 16] array instead of simply disallowing their use in I/O, which seems unfair since we support them in computation.

[Meinersbur]

Microsoft removed support for 80-bit floating points during the transition from 16 to 32 bit, so quite some time ago.

Formatting of extended and quadruple precision is already dependent on host compiler support. As you mentioned, the only reason for it seems to be to have a type of the required length. However, I am not going to rewrite flang's IO runtime now.

[Meinersbur]

• long double == double is generally true with msvc, not just with AMD64.

[klausler]

See D88752, which avoids the needless usage of the native floating types.

[Meinersbur]

Abandon in favor of D88752

flang/runtime/descriptor-io.h

case TypeCategory::Integer:
         return false;
       }
     case TypeCategory::Real:
       switch (kind) {
       case 4:
         return FormattedRealIO<24, float, DIR>(io, descriptor);
       case 8:
         return FormattedRealIO<53, double, DIR>(io, descriptor);
-#if __x86_64__
+#if _MSC_VER
+        // long double is the same as double with the Microsoft compiler.
+        // Treat as unimplemented.
+#elif __x86_64__
       case 10:
         return FormattedRealIO<64, long double, DIR>(io, descriptor);
 #else
       case 16:
         return FormattedRealIO<113, long double, DIR>(io, descriptor);
 #endif
       // TODO cases 2, 3
       default:
         io.GetIoErrorHandler().Crash(
             "DescriptorIO: Unimplemented REAL kind (%d) in descriptor", kind);
         return false;
       }
     case TypeCategory::Complex:
       switch (kind) {
       case 4:
         return FormattedComplexIO<24, float, DIR>(io, descriptor);
       case 8:
         return FormattedComplexIO<53, double, DIR>(io, descriptor);
-#if __x86_64__
+#if _MSC_VER
+        // long double is the same as double with the Microsoft compiler.
+        // Treat as unimplemented.
+#elif __x86_64__
       case 10:
         return FormattedComplexIO<64, long double, DIR>(io, descriptor);
 #else
       case 16:
         return FormattedComplexIO<113, long double, DIR>(io, descriptor);
 #endif
       // TODO cases 2, 3
       default: