Fix LTO test on Cygwin

This partially reverts commit add502c6483bde9dc6a0ba80b3c79163304465a4.

In 'linkshared' test, annotate cppfunc() as imported, so an indirection
through an import stub is generated, avoiding a relocation size error
when building using gcc for Cygwin with LTO on.

Align with the example of how to write this portably in [1].

The 'c' language part of that test already gets this right.

[1] http://gcc.gnu.org/wiki/Visibility

4 files changed, 15 insertions, 8 deletions

diff --git a/run_unittests.py b/run_unittests.py
index f65eba3..ed8de78 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -3075,8 +3075,6 @@ class AllPlatformTests(BasePlatformTests):
 
     @skip_if_not_base_option('b_lto_threads')
     def test_lto_threads(self):
-        if is_cygwin():
-            raise unittest.SkipTest('LTO is broken on Cygwin.')
         testdir = os.path.join(self.common_test_dir, '6 linkshared')
 
         env = get_fake_env(testdir, self.builddir, self.prefix)
diff --git a/test cases/common/6 linkshared/cpplib.cpp b/test cases/common/6 linkshared/cpplib.cpp
index 395859d..247f820 100644
--- a/test cases/common/6 linkshared/cpplib.cpp
+++ b/test cases/common/6 linkshared/cpplib.cpp
@@ -1,8 +1,5 @@
-#if defined _WIN32
-    #define DLL_PUBLIC __declspec(dllexport)
-#else
-    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
-#endif
+#define BUILDING_DLL
+#include "cpplib.h"
 
 int DLL_PUBLIC cppfunc(void) {
     return 42;
diff --git a/test cases/common/6 linkshared/cpplib.h b/test cases/common/6 linkshared/cpplib.h
new file mode 100644
index 0000000..e2b0206
--- /dev/null
+++ b/test cases/common/6 linkshared/cpplib.h
@@ -0,0 +1,12 @@
+/* See http://gcc.gnu.org/wiki/Visibility#How_to_use_the_new_C.2B-.2B-_visibility_support */
+#if defined(_WIN32) || defined(__CYGWIN__)
+  #ifdef BUILDING_DLL
+    #define DLL_PUBLIC __declspec(dllexport)
+  #else
+    #define DLL_PUBLIC __declspec(dllimport)
+  #endif
+#else
+    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
+#endif
+
+int DLL_PUBLIC cppfunc(void);
diff --git a/test cases/common/6 linkshared/cppmain.cpp b/test cases/common/6 linkshared/cppmain.cpp
index 8e16485..29e9a44 100644
--- a/test cases/common/6 linkshared/cppmain.cpp
+++ b/test cases/common/6 linkshared/cppmain.cpp
@@ -1,4 +1,4 @@
-int cppfunc(void);
+#include "cpplib.h"
 
 int main(void) {
     return cppfunc() != 42;