[flang][runtime] Use __float128 where possible & needed in runtime

On targets with __float128 available and distinct from long double,
use it to support more kind=16 entry points.  This affects mostly
x86-64 targets.  This means that more runtime entry points are
defined for lowering to call.

Delete Common/long-double.h and its LONG_DOUBLE macro in favor of
testing the standard macro LDBL_MANT_DIG.

Differential Revision: https://reviews.llvm.org/D127025

Author: klausler

flang/include/flang/Common/long-double.h

@@ -13,8 +13,11 @@
 
 #include "flang/Common/Fortran.h"
 #include "flang/Common/uint128.h"
+#include "flang/Runtime/float128.h"
+#include <cfloat>
 #include <complex>
 #include <cstdint>
+#include <type_traits>
 
 namespace Fortran::runtime {
 
@@ -24,6 +27,12 @@ template <TypeCategory CAT, int KIND> struct CppTypeForHelper {};
 template <TypeCategory CAT, int KIND>
 using CppTypeFor = typename CppTypeForHelper<CAT, KIND>::type;
 
+template <TypeCategory CAT, int KIND, bool SFINAE = false>
+constexpr bool HasCppTypeFor{false};
+template <TypeCategory CAT, int KIND>
+constexpr bool HasCppTypeFor<CAT, KIND, true>{
+    !std::is_void_v<typename CppTypeForHelper<CAT, KIND>::type>};
+
 template <int KIND> struct CppTypeForHelper<TypeCategory::Integer, KIND> {
   using type = common::HostSignedIntType<8 * KIND>;
 };
@@ -35,12 +44,21 @@ template <> struct CppTypeForHelper<TypeCategory::Real, 4> {
 template <> struct CppTypeForHelper<TypeCategory::Real, 8> {
   using type = double;
 };
+#if LDBL_MANT_DIG == 64
 template <> struct CppTypeForHelper<TypeCategory::Real, 10> {
   using type = long double;
 };
+#endif
+#if LDBL_MANT_DIG == 113
+using CppFloat128Type = long double;
+#elif HAS_FLOAT128
+using CppFloat128Type = __float128;
+#endif
+#if LDBL_MANT_DIG == 113 || HAS_FLOAT128
 template <> struct CppTypeForHelper<TypeCategory::Real, 16> {
-  using type = long double;
+  using type = CppFloat128Type;
 };
+#endif
 
 template <int KIND> struct CppTypeForHelper<TypeCategory::Complex, KIND> {
   using type = std::complex<CppTypeFor<TypeCategory::Real, KIND>>;

flang/include/flang/Runtime/cpp-type.h

@@ -0,0 +1,32 @@
+/*===-- flang/Runtime/float128.h ----------------------------------*- C -*-===
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ *===----------------------------------------------------------------------===*/
+
+/* This header is usable in both C and C++ code.
+ * Isolates build compiler checks to determine the presence of an IEEE-754
+ * quad-precision type named __float128 type that isn't __ibm128
+ * (double/double). We don't care whether the type has underlying hardware
+ * support or is emulated.
+ *
+ * 128-bit arithmetic may be available via "long double"; this can
+ * be determined by LDBL_MANT_DIG == 113.  A machine may have both 128-bit
+ * long double and __float128; prefer long double by testing for it first.
+ */
+
+#ifndef FORTRAN_RUNTIME_FLOAT128_H_
+#define FORTRAN_RUNTIME_FLOAT128_H_
+
+#undef HAS_FLOAT128
+#if __x86_64__
+#if __GNUC__ >= 7 || __clang_major >= 7
+#define HAS_FLOAT128 1
+#endif
+#elif defined __PPC__ && __GNUC__ >= 8
+#define HAS_FLOAT128 1
+#endif
+
+#endif /* FORTRAN_RUNTIME_FLOAT128_H_ */