Merge System into Support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120298 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/IncludeFile.h

@@ -0,0 +1,79 @@
+//===- llvm/System/IncludeFile.h - Ensure Linking Of Library ---*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the FORCE_DEFINING_FILE_TO_BE_LINKED and DEFINE_FILE_FOR
+// macros.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SYSTEM_INCLUDEFILE_H
+#define LLVM_SYSTEM_INCLUDEFILE_H
+
+/// This macro is the public interface that IncludeFile.h exports. This gives
+/// us the option to implement the "link the definition" capability in any
+/// manner that we choose. All header files that depend on a specific .cpp
+/// file being linked at run time should use this macro instead of the
+/// IncludeFile class directly.
+///
+/// For example, foo.h would use:<br/>
+/// <tt>FORCE_DEFINING_FILE_TO_BE_LINKED(foo)</tt><br/>
+///
+/// And, foo.cp would use:<br/>
+/// <tt>DEFINING_FILE_FOR(foo)</tt><br/>
+#ifdef __GNUC__
+// If the `used' attribute is available, use it to create a variable
+// with an initializer that will force the linking of the defining file.
+#define FORCE_DEFINING_FILE_TO_BE_LINKED(name) \
+  namespace llvm { \
+    extern const char name ## LinkVar; \
+    __attribute__((used)) static const char *const name ## LinkObj = \
+      &name ## LinkVar; \
+  }
+#else
+// Otherwise use a constructor call.
+#define FORCE_DEFINING_FILE_TO_BE_LINKED(name) \
+  namespace llvm { \
+    extern const char name ## LinkVar; \
+    static const IncludeFile name ## LinkObj ( &name ## LinkVar ); \
+  }
+#endif
+
+/// This macro is the counterpart to FORCE_DEFINING_FILE_TO_BE_LINKED. It should
+/// be used in a .cpp file to define the name referenced in a header file that
+/// will cause linkage of the .cpp file. It should only be used at extern level.
+#define DEFINING_FILE_FOR(name) \
+  namespace llvm { const char name ## LinkVar = 0; }
+
+namespace llvm {
+
+/// This class is used in the implementation of FORCE_DEFINING_FILE_TO_BE_LINKED
+/// macro to make sure that the implementation of a header file is included
+/// into a tool that uses the header.  This is solely
+/// to overcome problems linking .a files and not getting the implementation
+/// of compilation units we need. This is commonly an issue with the various
+/// Passes but also occurs elsewhere in LLVM. We like to use .a files because
+/// they link faster and provide the smallest executables. However, sometimes
+/// those executables are too small, if the program doesn't reference something
+/// that might be needed, especially by a loaded share object. This little class
+/// helps to resolve that problem. The basic strategy is to use this class in
+/// a header file and pass the address of a variable to the constructor. If the
+/// variable is defined in the header file's corresponding .cpp file then all
+/// tools/libraries that \#include the header file will require the .cpp as
+/// well.
+/// For example:<br/>
+/// <tt>extern int LinkMyCodeStub;</tt><br/>
+/// <tt>static IncludeFile LinkMyModule(&LinkMyCodeStub);</tt><br/>
+/// @brief Class to ensure linking of corresponding object file.
+struct IncludeFile {
+  explicit IncludeFile(const void *);
+};
+
+}
+
+#endif