[flang] Check there's no dependency on C++ libs. NFC

Add a test to make sure the flang runtime doesn't pull in the C++
runtime libraries.

This is achieved by adding a C file that calls some functions from the
runtime (currently only CpuTime, but we should probably add anything
complicated enough, e.g. IO-related things). We force the C compiler to
use -std=c90 to make sure it's really in C mode (we don't really care
which version of the standard, this one is probably more widely
available). We only enable this test if CMAKE_C_COMPILER is set to
something (which is probably always true in practice).

This is a recommit of 7ddbf26, with 2 fixes:
* Replace C++ comments with C comments
* Only enable the test if libFortranRuntime.a exists (this might not be
the case if e.g. BUILD_SHARED_LIBS=On)

Differential Revision: https://reviews.llvm.org/D104290

flang/test/Runtime/no-cpp-dep.c

@@ -0,0 +1,23 @@
+/*
+This test makes sure that flang's runtime does not depend on the C++ runtime
+library. It tries to link this simple file against libFortranRuntime.a with
+a C compiler.
+
+REQUIRES: c-compiler
+
+RUN: %cc -std=c90 %s -I%runtimeincludes %libruntime -o /dev/null
+*/
+
+#include "entry-names.h"
+
+/*
+Manually add declarations for the runtime functions that we want to make sure
+we're testing. We can't include any headers directly since they likely contain
+C++ code that would explode here.
+*/
+double RTNAME(CpuTime)();
+
+int main() {
+  double x = RTNAME(CpuTime)();
+  return x;
+}

flang/test/lit.cfg.py

@@ -75,6 +75,21 @@ else:
    tools.append(ToolSubst('%flang_fc1', command=FindTool('f18'),
     unresolved='fatal'))
 
+# Define some variables to help us test that the flang runtime doesn't depend on
+# the C++ runtime libraries. For this we need a C compiler. If for some reason
+# we don't have one, we can just disable the test.
+if config.cc:
+    libruntime = os.path.join(config.flang_lib_dir, 'libFortranRuntime.a')
+    includes = os.path.join(config.flang_src_dir, 'runtime')
+
+    if os.path.isfile(libruntime) and os.path.isdir(includes):
+        config.available_features.add('c-compiler')
+        tools.append(ToolSubst('%cc', command=config.cc, unresolved='fatal'))
+        tools.append(ToolSubst('%libruntime', command=libruntime,
+            unresolved='fatal'))
+        tools.append(ToolSubst('%runtimeincludes', command=includes,
+            unresolved='fatal'))
+
 if config.flang_standalone_build:
     llvm_config.add_tool_substitutions(tools, [config.flang_llvm_tools_dir])
 else:

flang/test/lit.site.cfg.py.in

@@ -9,8 +9,10 @@ config.flang_src_dir = "@FLANG_SOURCE_DIR@"
 config.flang_tools_dir = "@FLANG_TOOLS_DIR@"
 config.flang_intrinsic_modules_dir = "@FLANG_INTRINSIC_MODULES_DIR@"
 config.flang_llvm_tools_dir = "@CMAKE_BINARY_DIR@/bin"
+config.flang_lib_dir = "@CMAKE_BINARY_DIR@/lib"
 config.python_executable = "@PYTHON_EXECUTABLE@"
 config.flang_standalone_build = @FLANG_STANDALONE_BUILD@
+config.cc = "@CMAKE_C_COMPILER@"
 
 # Control the regression test for flang-new driver
 import lit.util