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1 //===- llvm/Support/IncludeFile.h - Ensure Linking Of Library ---*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the FORCE_DEFINING_FILE_TO_BE_LINKED and DEFINE_FILE_FOR
11 // macros.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_SYSTEM_INCLUDEFILE_H
16 #define LLVM_SYSTEM_INCLUDEFILE_H
17 
18 /// This macro is the public interface that IncludeFile.h exports. This gives
19 /// us the option to implement the "link the definition" capability in any
20 /// manner that we choose. All header files that depend on a specific .cpp
21 /// file being linked at run time should use this macro instead of the
22 /// IncludeFile class directly.
23 ///
24 /// For example, foo.h would use:<br/>
25 /// <tt>FORCE_DEFINING_FILE_TO_BE_LINKED(foo)</tt><br/>
26 ///
27 /// And, foo.cp would use:<br/>
28 /// <tt>DEFINING_FILE_FOR(foo)</tt><br/>
29 #ifdef __GNUC__
30 // If the `used' attribute is available, use it to create a variable
31 // with an initializer that will force the linking of the defining file.
32 #define FORCE_DEFINING_FILE_TO_BE_LINKED(name) \
33   namespace llvm { \
34     extern const char name ## LinkVar; \
35     __attribute__((used)) static const char *const name ## LinkObj = \
36       &name ## LinkVar; \
37   }
38 #else
39 // Otherwise use a constructor call.
40 #define FORCE_DEFINING_FILE_TO_BE_LINKED(name) \
41   namespace llvm { \
42     extern const char name ## LinkVar; \
43     static const IncludeFile name ## LinkObj ( &name ## LinkVar ); \
44   }
45 #endif
46 
47 /// This macro is the counterpart to FORCE_DEFINING_FILE_TO_BE_LINKED. It should
48 /// be used in a .cpp file to define the name referenced in a header file that
49 /// will cause linkage of the .cpp file. It should only be used at extern level.
50 #define DEFINING_FILE_FOR(name) \
51   namespace llvm { const char name ## LinkVar = 0; }
52 
53 namespace llvm {
54 
55 /// This class is used in the implementation of FORCE_DEFINING_FILE_TO_BE_LINKED
56 /// macro to make sure that the implementation of a header file is included
57 /// into a tool that uses the header.  This is solely
58 /// to overcome problems linking .a files and not getting the implementation
59 /// of compilation units we need. This is commonly an issue with the various
60 /// Passes but also occurs elsewhere in LLVM. We like to use .a files because
61 /// they link faster and provide the smallest executables. However, sometimes
62 /// those executables are too small, if the program doesn't reference something
63 /// that might be needed, especially by a loaded share object. This little class
64 /// helps to resolve that problem. The basic strategy is to use this class in
65 /// a header file and pass the address of a variable to the constructor. If the
66 /// variable is defined in the header file's corresponding .cpp file then all
67 /// tools/libraries that \#include the header file will require the .cpp as
68 /// well.
69 /// For example:<br/>
70 /// <tt>extern int LinkMyCodeStub;</tt><br/>
71 /// <tt>static IncludeFile LinkMyModule(&LinkMyCodeStub);</tt><br/>
72 /// @brief Class to ensure linking of corresponding object file.
73 struct IncludeFile {
74   explicit IncludeFile(const void *);
75 };
76 
77 }
78 
79 #endif
80