[Flang] Detect endianness in the preprocessor

Summary:
Currently we use `TestBigEndian` in CMake to determine endianness. This
doesn't work on all platforms and is deprecated since CMake 3.20.
Instead of using CMake, we can just use the GNU/Clang preprocessor
definitions.

The only difficulty is MSVC, mostly because they don't support the same
macros. But, as far as I'm aware, MSVC / Windows targets are always
little endian, and if not we can just override it for that specific
target in the future.

Author: jhuber6

flang/cmake/modules/FlangCommon.cmake

@@ -24,30 +24,16 @@ if (FLANG_RUNTIME_F128_MATH_LIB)
   add_compile_definitions(FLANG_RUNTIME_F128_MATH_LIB="${FLANG_RUNTIME_F128_MATH_LIB}")
 endif()
 
-# The NVPTX target can't emit a binary due to the PTXAS dependency, just
-# hard-code this.
-if ("${LLVM_RUNTIMES_TARGET}" MATCHES "^nvptx")
-  add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
-else ()
-  # Check if 128-bit float computations can be done via long double
-  # Note that '-nostdinc++' might be implied when this code kicks in
-  # (see 'runtimes/CMakeLists.txt'), so we cannot use 'cfloat' C++ header
-  # file in the test below.
-  # Compile it as C.
-  check_c_source_compiles(
-    "#include <float.h>
-     #if LDBL_MANT_DIG != 113
-     #error LDBL_MANT_DIG != 113
-     #endif
-     int main() { return 0; }
-    "
-    HAVE_LDBL_MANT_DIG_113)
-
-  include(TestBigEndian)
-  test_big_endian(IS_BIGENDIAN)
-  if (IS_BIGENDIAN)
-    add_compile_definitions(FLANG_BIG_ENDIAN=1)
-  else ()
-    add_compile_definitions(FLANG_LITTLE_ENDIAN=1)
-  endif ()
-endif ()
+# Check if 128-bit float computations can be done via long double
+# Note that '-nostdinc++' might be implied when this code kicks in
+# (see 'runtimes/CMakeLists.txt'), so we cannot use 'cfloat' C++ header
+# file in the test below.
+# Compile it as C.
+check_c_source_compiles(
+  "#include <float.h>
+   #if LDBL_MANT_DIG != 113
+   #error LDBL_MANT_DIG != 113
+   #endif
+   int main() { return 0; }
+  "
+  HAVE_LDBL_MANT_DIG_113)

flang/include/flang/Common/api-attrs.h

@@ -189,4 +189,19 @@
 #define RT_OPTNONE_ATTR
 #endif
 
+/* Detect system endianness if it was not explicitly set. */
+#if !defined(FLANG_LITTLE_ENDIAN) && !defined(FLANG_BIG_ENDIAN)
+
+/* We always assume Windows is little endian, otherwise use the GCC compatible
+ * flags. */
+#if defined(_MSC_VER) || defined(_WIN32)
+#define FLANG_LITTLE_ENDIAN 1
+#elif defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
+#define FLANG_LITTLE_ENDIAN 1
+#elif defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
+#define FLANG_BIG_ENDIAN 1
+#endif
+
+#endif /* !defined(FLANG_LITTLE_ENDIAN) && !defined(FLANG_BIG_ENDIAN) */
+
 #endif /* !FORTRAN_RUNTIME_API_ATTRS_H_ */