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annotate tools/clang/docs/MSVCCompatibility.rst @ 95:afa8332a0e37 LLVM3.8

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LLVM 3.8
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Tue, 13 Oct 2015 17:48:58 +0900
parents 60c9769439b8
children 1172e4bd9c6f
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3 <style type="text/css">
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7 </style>
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9 .. role:: none
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10 .. role:: partial
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11 .. role:: good
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13 ==================
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14 MSVC compatibility
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15 ==================
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16
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17 When Clang compiles C++ code for Windows, it attempts to be compatible with
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18 MSVC. There are multiple dimensions to compatibility.
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19
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20 First, Clang attempts to be ABI-compatible, meaning that Clang-compiled code
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21 should be able to link against MSVC-compiled code successfully. However, C++
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22 ABIs are particularly large and complicated, and Clang's support for MSVC's C++
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23 ABI is a work in progress. If you don't require MSVC ABI compatibility or don't
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24 want to use Microsoft's C and C++ runtimes, the mingw32 toolchain might be a
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25 better fit for your project.
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26
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27 Second, Clang implements many MSVC language extensions, such as
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28 ``__declspec(dllexport)`` and a handful of pragmas. These are typically
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29 controlled by ``-fms-extensions``.
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30
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31 Third, MSVC accepts some C++ code that Clang will typically diagnose as
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32 invalid. When these constructs are present in widely included system headers,
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33 Clang attempts to recover and continue compiling the user's program. Most
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34 parsing and semantic compatibility tweaks are controlled by
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35 ``-fms-compatibility`` and ``-fdelayed-template-parsing``, and they are a work
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36 in progress.
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37
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38 Finally, there is :ref:`clang-cl`, a driver program for clang that attempts to
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39 be compatible with MSVC's cl.exe.
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40
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41 ABI features
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42 ============
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43
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44 The status of major ABI-impacting C++ features:
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45
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46 * Record layout: :good:`Complete`. We've tested this with a fuzzer and have
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47 fixed all known bugs.
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48
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49 * Class inheritance: :good:`Mostly complete`. This covers all of the standard
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50 OO features you would expect: virtual method inheritance, multiple
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51 inheritance, and virtual inheritance. Every so often we uncover a bug where
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52 our tables are incompatible, but this is pretty well in hand. This feature
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53 has also been fuzz tested.
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54
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55 * Name mangling: :good:`Ongoing`. Every new C++ feature generally needs its own
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56 mangling. For example, member pointer template arguments have an interesting
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57 and distinct mangling. Fortunately, incorrect manglings usually do not result
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58 in runtime errors. Non-inline functions with incorrect manglings usually
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59 result in link errors, which are relatively easy to diagnose. Incorrect
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60 manglings for inline functions and templates result in multiple copies in the
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61 final image. The C++ standard requires that those addresses be equal, but few
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62 programs rely on this.
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63
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64 * Member pointers: :good:`Mostly complete`. Standard C++ member pointers are
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65 fully implemented and should be ABI compatible. Both `#pragma
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66 pointers_to_members`_ and the `/vm`_ flags are supported. However, MSVC
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67 supports an extension to allow creating a `pointer to a member of a virtual
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68 base class`_. Clang does not yet support this.
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69
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70 .. _#pragma pointers_to_members:
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71 http://msdn.microsoft.com/en-us/library/83cch5a6.aspx
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72 .. _/vm: http://msdn.microsoft.com/en-us/library/yad46a6z.aspx
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73 .. _pointer to a member of a virtual base class: http://llvm.org/PR15713
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74
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75 * Debug info: :partial:`Minimal`. Clang emits both CodeView line tables
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76 (similar to what MSVC emits when given the ``/Z7`` flag) and DWARF debug
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77 information into the object file.
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78 Microsoft's link.exe will transform the CodeView line tables into a PDB,
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79 enabling stack traces in all modern Windows debuggers. Clang does not emit
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80 any CodeView-compatible type info or description of variable layout.
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81 Binaries linked with either binutils' ld or LLVM's lld should be usable with
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82 GDB however sophisticated C++ expressions are likely to fail.
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83
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84 * RTTI: :good:`Complete`. Generation of RTTI data structures has been
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85 finished, along with support for the ``/GR`` flag.
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86
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87 * Exceptions and SEH: :partial:`Partial`.
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88 C++ exceptions (``try`` / ``catch`` / ``throw``) and
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89 structured exceptions (``__try`` / ``__except`` / ``__finally``) mostly
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90 work on x64. 32-bit exception handling support is being worked on. LLVM does
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91 not model asynchronous exceptions, so it is currently impossible to catch an
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92 asynchronous exception generated in the same frame as the catching ``__try``.
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93 C++ exception specifications are ignored, but this is `consistent with Visual
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94 C++`_.
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95
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96 .. _consistent with Visual C++:
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97 https://msdn.microsoft.com/en-us/library/wfa0edys.aspx
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98
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99 * Thread-safe initialization of local statics: :good:`Complete`. MSVC 2015
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100 added support for thread-safe initialization of such variables by taking an
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101 ABI break.
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102 We are ABI compatible with both the MSVC 2013 and 2015 ABI for static local
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103 variables.
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104
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105 * Lambdas: :good:`Mostly complete`. Clang is compatible with Microsoft's
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106 implementation of lambdas except for providing overloads for conversion to
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107 function pointer for different calling conventions. However, Microsoft's
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108 extension is non-conforming.
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109
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110 Template instantiation and name lookup
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111 ======================================
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112
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113 MSVC allows many invalid constructs in class templates that Clang has
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114 historically rejected. In order to parse widely distributed headers for
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115 libraries such as the Active Template Library (ATL) and Windows Runtime Library
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116 (WRL), some template rules have been relaxed or extended in Clang on Windows.
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117
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118 The first major semantic difference is that MSVC appears to defer all parsing
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119 an analysis of inline method bodies in class templates until instantiation
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120 time. By default on Windows, Clang attempts to follow suit. This behavior is
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121 controlled by the ``-fdelayed-template-parsing`` flag. While Clang delays
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122 parsing of method bodies, it still parses the bodies *before* template argument
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123 substitution, which is not what MSVC does. The following compatibility tweaks
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124 are necessary to parse the template in those cases.
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125
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126 MSVC allows some name lookup into dependent base classes. Even on other
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127 platforms, this has been a `frequently asked question`_ for Clang users. A
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128 dependent base class is a base class that depends on the value of a template
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129 parameter. Clang cannot see any of the names inside dependent bases while it
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130 is parsing your template, so the user is sometimes required to use the
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131 ``typename`` keyword to assist the parser. On Windows, Clang attempts to
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132 follow the normal lookup rules, but if lookup fails, it will assume that the
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133 user intended to find the name in a dependent base. While parsing the
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134 following program, Clang will recover as if the user had written the
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135 commented-out code:
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136
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137 .. _frequently asked question:
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138 http://clang.llvm.org/compatibility.html#dep_lookup
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139
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140 .. code-block:: c++
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141
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142 template <typename T>
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143 struct Foo : T {
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144 void f() {
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145 /*typename*/ T::UnknownType x = /*this->*/unknownMember;
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146 }
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147 };
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148
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149 After recovery, Clang warns the user that this code is non-standard and issues
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150 a hint suggesting how to fix the problem.
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151
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152 As of this writing, Clang is able to compile a simple ATL hello world
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153 application. There are still issues parsing WRL headers for modern Windows 8
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154 apps, but they should be addressed soon.