[Flang-RT] Environment introspection for quadmath.h (llvm#130411)

When compiling Flang-RT with Clang, query Clang for the GCC installation
it uses. If found, create `quadmath_wrapper.h` that points to the
`quadmath.h` of that GCC installation.

`quadmath.h` is only available when compiling with gcc, and Clang has no
equivalent even though gcc's version compiles fine with Clang (at least
up to and including gcc 13). It is still available into gcc's
installation resource dir (in constrast to a system-wide indirectory
such as `/usr/include` or `/usr/local/include`) and therefore not
available to any compiler other than the gcc of that installation.
quadmath may also be a different OS package than gcc itself, so it is
not necessarily presesent.

Clang actually already appropriates a GCC installation for its libraries
such that `libquadmath.a` is already found, but it does not do so for
the include paths. Because adding that directory to the header search
path may have wide-reaching consquences, we create only a wrapper header
that points to the real `quadmath.h` in the same GCC installation that
Clang uses.

Author: Meinersbur

flang-rt/CMakeLists.txt

@@ -294,14 +294,14 @@ elseif (FLANG_RT_GCC_RESOURCE_DIR)
 endif ()
 
 # Check if 128-bit float computations can be done via long double.
-check_cxx_source_compiles(
-  "#include <cfloat>
-   #if LDBL_MANT_DIG != 113
-   #error LDBL_MANT_DIG != 113
-   #endif
-   int main() { return 0; }
-  "
-  HAVE_LDBL_MANT_DIG_113)
+#check_cxx_source_compiles(
+#  "#include <cfloat>
+#   #if LDBL_MANT_DIG != 113
+#   #error LDBL_MANT_DIG != 113
+#   #endif
+#   int main() { return 0; }
+#  "
+#  HAVE_LDBL_MANT_DIG_113)
 
 
 #####################
@@ -325,7 +325,9 @@ set(CMAKE_CXX_STANDARD_REQUIRED YES)
 
 
 configure_file(cmake/config.h.cmake.in config.h)
-
+if (FLANG_RT_INCLUDE_QUADMATH_H)
+  configure_file("cmake/quadmath_wrapper.h.in" "${FLANG_RT_BINARY_DIR}/quadmath_wrapper.h")
+endif ()
 
 # The bootstrap build will create a phony target with the same as the top-level
 # directory ("flang-rt") and delegate it to the runtimes build dir.

flang-rt/cmake/clang_gcc_root.cpp

@@ -0,0 +1 @@
+int main() { return 0; }

flang-rt/cmake/quadmath_wrapper.h.in

@@ -0,0 +1,9 @@
+/*===-- cmake/quadmath_wrapper.h.in ---------------------=-----------*- 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
+ *
+ *===----------------------------------------------------------------------===*/
+
+#include ${FLANG_RT_INCLUDE_QUADMATH_H}

flang-rt/lib/quadmath/CMakeLists.txt

@@ -78,8 +78,7 @@ target_include_directories(FortranFloat128MathILib INTERFACE
 
 if (FLANG_RUNTIME_F128_MATH_LIB)
   if (${FLANG_RUNTIME_F128_MATH_LIB} STREQUAL "libquadmath")
-    check_include_file(quadmath.h FOUND_QUADMATH_HEADER)
-    if(FOUND_QUADMATH_HEADER)
+    if(FLANG_RT_INCLUDE_QUADMATH_H)
       add_compile_definitions(HAS_QUADMATHLIB)
     else()
       message(FATAL_ERROR

flang-rt/lib/quadmath/complex-math.h

@@ -13,7 +13,7 @@
 #include "flang/Runtime/entry-names.h"
 
 #if HAS_QUADMATHLIB
-#include "quadmath.h"
+#include "quadmath_wrapper.h"
 #define CAbs(x) cabsq(x)
 #define CAcos(x) cacosq(x)
 #define CAcosh(x) cacoshq(x)

flang-rt/lib/quadmath/math-entries.h

@@ -112,7 +112,7 @@ DEFINE_FALLBACK_F128(Yn)
 
 #if HAS_QUADMATHLIB
 // Define wrapper callers for libquadmath.
-#include "quadmath.h"
+#include "quadmath_wrapper.h"
 DEFINE_SIMPLE_ALIAS(Abs, fabsq)
 DEFINE_SIMPLE_ALIAS(Acos, acosq)
 DEFINE_SIMPLE_ALIAS(Acosh, acoshq)