Add auto-exporting of symbols from tools so that plugins work on Windows

[john.brawn]

Summary

The problem with plugins on Windows is that when building a plugin DLL it needs to explicitly link against something (an exe or DLL) if it uses symbols from that thing, and that thing must explicitly export those symbols. Also there's a limit of 65535 symbols that can be exported. This means that currently plugins only work on Windows when using BUILD_SHARED_LIBS, and that doesn't work with MSVC.

This patch adds an LLVM_EXPORT_SYMBOLS_FOR_PLUGINS option, which when enabled automatically exports from all LLVM tools the symbols that a plugin could want to use so that a plugin can link against a tool directly. Plugins can specify what tool they link against by using PLUGIN_TOOL argument to llvm_add_library. The option can also be enabled on Linux, though there all it should do is restrict the set of symbols that are exported as by default all symbols are exported.

This option is currently OFF by default, as while I've verified that it works with MSVC, linux gcc, and cygwin gcc, I haven't tried mingw gcc and I have no idea what will happen on OSX. Also unfortunately we can't turn on LLVM_ENABLE_PLUGINS when the option is ON as bugpoint-passes needs to be loaded by both bugpoint.exe and opt.exe which is incompatible with this approach. Also currently clang plugins don't work with this approach, which will be fixed in future patches.

[rnk]

This Python script is a wonderful, horrible hack. I submit it to the community as evidence that we should have real API export annotations in our public headers and build with -fvisibility=hidden on Unix. :)

utils/extract_symbols.py

164 ↗

(On Diff #52796)

You should probably test the tools in a determinstic order. We also might want to prefer llvm-readobj to dumpbin /symbols. dumpbin is pretty slow.

[john.brawn]

utils/extract_symbols.py

164 ↗

(On Diff #52796)

From some experiments dumpbin seems to be ~2x the speed of llvm-readobj. Looking at some of the largest libraries I get (building a release build, time in seconds running extract_symbols.py on the library):

Library	llvm-readobj	dumpbin
LLVMCodeGen	6.9	3.2
LLVMCore	4.6	2.1
LLVMAnalysis	4.6	2.5
LLVMScalarOpts	4.1	2.2

I do agree on using a deterministic order though.

[john.brawn]

Use a deterministic order for checking tools, instead of whatever dict.keys() happens to return.

[john.brawn]

Ping.

[rnk]

Added Chris, pinging other reviewers. Personally, I think we should go the other direction: build a DSO for LLVM, a DSO for Clang, and have opt plugins link against that. This, however, requires us to get serious about annotating our public API boundary.

[chandlerc]

In D18826#412821, @rnk wrote:

Added Chris, pinging other reviewers. Personally, I think we should go the other direction: build a DSO for LLVM, a DSO for Clang, and have opt plugins link against that. This, however, requires us to get serious about annotating our public API boundary.

I don't really disagree with you, but I don't know who is going to step up to do the (really huge) work here. Are you? If not, what do we do in the interim?

[john.brawn]

In D18826#414347, @chandlerc wrote:

In D18826#412821, @rnk wrote:

Added Chris, pinging other reviewers. Personally, I think we should go the other direction: build a DSO for LLVM, a DSO for Clang, and have opt plugins link against that. This, however, requires us to get serious about annotating our public API boundary.

I don't really disagree with you, but I don't know who is going to step up to do the (really huge) work here. Are you? If not, what do we do in the interim?

I had a look at doing this approach, but didn't as it does look like a huge amount of work (though looking at the amount of time I've sunk into getting this auto-export approach to work maybe it would have been better to just do the work instead of trying to be clever). There's also another problem, and that's global data symbols - if you build LLVM.dll using LLVM_LINK_LLVM_DYLIB and link opt.exe against it there are a number of global variables used across the dll-exe boundary that need to be arranged to be exported from the dll and explicitly imported by the exe (for function symbols you can get away with not explicity importing them, but that doesn't work for variables). Alternatively those uses could be changed to not use global variables e.g. have a function that returns the address of the variable and export that, but in some cases that looks impractical e.g. APFloat has several static fltSemantics members that are used all over the place, and passes have static PassID members used in inline constructors which would all need to be changed.

In unrelated news I've found some problems with the template handling in extract_symbols.py (it looks at the outermost name in the namespace but it should be looking at the innermost name) which makes it throw away some symbols that should be kept. I expect to have an updated patch sometime next week.

[john.brawn]

Updated extract_symbols.py to handle templates by looking at the innermost template outwards, which requires an unfortunate amount of demangling but is necessary as previously different member functions of the same template would be counted as the same thing which isn't what we want. Also fixes the handling of calling convention symbol decoration on 32-bit windows.

[john.brawn]

Ping.

[beanz]

I agree with @rnk's comments about philosophical design, but we shouldn't force this to wait on a giant header annotation pass.

The CMake looks good. I have a few comments about the Python. Also might be nice to get someone like @silvas to take a look at the Python too. He has provided excellent review feedback on Python code in the past.

cmake/modules/AddLLVM.cmake

725 ↗

(On Diff #56271)

Nit: Since you have this in a python script it would be cleaner to have a -o option rather than a pipe here.

utils/extract_symbols.py

29 ↗

(On Diff #56271)

Rather than calling subprocess.Popen you might consider using subprocess.check_output. It would significantly reduce the amount of boiler plate code in this python script.

334 ↗

(On Diff #56271)

This is not a safe assumption. Apple ships nm but not objdump. In place of objdump we historically shipped otool. The most recent Xcode ships llvm-objdump, but you can't rely on that being present.

[silvas]

utils/extract_symbols.py

29 ↗

(On Diff #56271)

Yeah. That's my main comment here too.

[john.brawn]

utils/extract_symbols.py

29 ↗

(On Diff #56271)

Waiting for subprocess.check_output to have all of the output ready adds a significant delay. I did a quick experiment and with a debug build on Linux it increased time from ~25 seconds to ~45 seconds to run extract_symbols on all the libraries, and on Windows it increased from ~1 minutes to ~4 minutes. Using check_output in objdump_is_32bit_windows and readobj_is_32bit_windows is fine though, as the output is much smaller, so I'll do that.

334 ↗

(On Diff #56271)

Ack. I'm not expecting this to work on OSX, but there's no reason not to fix that at least.

[john.brawn]

Address review comments.

[john.brawn]

Ping.

[rnk]

Looks good to me. Based on other feedback, I think this is ready to land.

llvm/trunk/cmake/modules/AddLLVM.cmake

 #   LINK_COMPONENTS components...
 #     Same as the variable LLVM_LINK_COMPONENTS.
 #   LINK_LIBS lib_targets...
 #     Same semantics as target_link_libraries().
 #   ADDITIONAL_HEADERS
 #     May specify header files for IDE generators.
 #   SONAME
 #     Should set SONAME link flags and create symlinks
+#   PLUGIN_TOOL
+#     The tool (i.e. cmake target) that this plugin will link against
 #   )
 function(llvm_add_library name)
   cmake_parse_arguments(ARG
     "MODULE;SHARED;STATIC;OBJECT;DISABLE_LLVM_LINK_LLVM_DYLIB;SONAME"
-    "OUTPUT_NAME"
+    "OUTPUT_NAME;PLUGIN_TOOL"
     "ADDITIONAL_HEADERS;DEPENDS;LINK_COMPONENTS;LINK_LIBS;OBJLIBS"
     ${ARGN})
   list(APPEND LLVM_COMMON_DEPENDS ${ARG_DEPENDS})
   if(ARG_ADDITIONAL_HEADERS)
     # Pass through ADDITIONAL_HEADERS.
     set(ARG_ADDITIONAL_HEADERS ADDITIONAL_HEADERS ${ARG_ADDITIONAL_HEADERS})
   endif()
   if(ARG_OBJLIBS)
     set(ALL_FILES ${ARG_OBJLIBS})
   else()
     llvm_process_sources(ALL_FILES ${ARG_UNPARSED_ARGUMENTS} ${ARG_ADDITIONAL_HEADERS})
   endif()
 
   if(ARG_MODULE)
     if(ARG_SHARED OR ARG_STATIC)
       message(WARNING "MODULE with SHARED|STATIC doesn't make sense.")
     endif()
-    if(NOT LLVM_ENABLE_PLUGINS)
+    # Plugins that link against a tool are allowed even when plugins in general are not
+    if(NOT LLVM_ENABLE_PLUGINS AND NOT (ARG_PLUGIN_TOOL AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS))
       message(STATUS "${name} ignored -- Loadable modules not supported on this platform.")
       return()
     endif()
   else()
+    if(ARG_PLUGIN_TOOL)
+      message(WARNING "PLUGIN_TOOL without MODULE doesn't make sense.")
+    endif()
     if(BUILD_SHARED_LIBS AND NOT ARG_STATIC)
       set(ARG_SHARED TRUE)
     endif()
     if(NOT ARG_SHARED)
       set(ARG_STATIC TRUE)
     endif()
   endif()
 
if(NOT APPLE AND ARG_SONAME)
         COMPONENT ${name}
         ALWAYS_GENERATE)
       llvm_install_library_symlink(${output_name} ${library_name} SHARED
         COMPONENT ${name}
         ALWAYS_GENERATE)
     endif()
   endif()
 
-  if (DEFINED LLVM_LINK_COMPONENTS OR DEFINED ARG_LINK_COMPONENTS)
+  if(ARG_MODULE AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS AND ARG_PLUGIN_TOOL AND (WIN32 OR CYGWIN))
+    # On DLL platforms symbols are imported from the tool by linking against it.
+    set(llvm_libs ${ARG_PLUGIN_TOOL})
+  elseif (DEFINED LLVM_LINK_COMPONENTS OR DEFINED ARG_LINK_COMPONENTS)
     if (LLVM_LINK_LLVM_DYLIB AND NOT ARG_DISABLE_LLVM_LINK_LLVM_DYLIB)
       set(llvm_libs LLVM)
     else()
       llvm_map_components_to_libnames(llvm_libs
        ${ARG_LINK_COMPONENTS}
        ${LLVM_LINK_COMPONENTS}
        )
     endif()
macro(add_llvm_executable name)
   endif( LLVM_COMMON_DEPENDS )
 
   if(NOT ARG_IGNORE_EXTERNALIZE_DEBUGINFO)
     llvm_externalize_debuginfo(${name})
   endif()
 endmacro(add_llvm_executable name)
 
 function(export_executable_symbols target)
-  if (NOT MSVC) # MSVC's linker doesn't support exporting all symbols.
+  if (LLVM_EXPORTED_SYMBOL_FILE)
+    # The symbol file should contain the symbols we want the executable to
+    # export
+    set_target_properties(${target} PROPERTIES ENABLE_EXPORTS 1)
+  elseif (LLVM_EXPORT_SYMBOLS_FOR_PLUGINS)
+    # Extract the symbols to export from the static libraries that the
+    # executable links against.
+    set_target_properties(${target} PROPERTIES ENABLE_EXPORTS 1)
+    set(exported_symbol_file ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/${target}.symbols)
+    # We need to consider not just the direct link dependencies, but also the
+    # transitive link dependencies. Do this by starting with the set of direct
+    # dependencies, then the dependencies of those dependencies, and so on.
+    get_target_property(new_libs ${target} LINK_LIBRARIES)
+    set(link_libs ${new_libs})
+    while(NOT "${new_libs}" STREQUAL "")
+      foreach(lib ${new_libs})
+        get_target_property(lib_type ${lib} TYPE)
+        if("${lib_type}" STREQUAL "STATIC_LIBRARY")
+          list(APPEND static_libs ${lib})
+        else()
+          list(APPEND other_libs ${lib})
+        endif()
+        get_target_property(transitive_libs ${lib} INTERFACE_LINK_LIBRARIES)
+        foreach(transitive_lib ${transitive_libs})
+          list(FIND link_libs ${transitive_lib} idx)
+          if(TARGET ${transitive_lib} AND idx EQUAL -1)
+            list(APPEND newer_libs ${transitive_lib})
+            list(APPEND link_libs ${transitive_lib})
+          endif()
+        endforeach(transitive_lib)
+      endforeach(lib)
+      set(new_libs ${newer_libs})
+      set(newer_libs "")
+    endwhile()
+    if (MSVC)
+      set(mangling microsoft)
+    else()
+      set(mangling itanium)
+    endif()
+    add_custom_command(OUTPUT ${exported_symbol_file}
+                       COMMAND ${PYTHON_EXECUTABLE} ${LLVM_MAIN_SRC_DIR}/utils/extract_symbols.py --mangling=${mangling} ${static_libs} -o ${exported_symbol_file}
+                       WORKING_DIRECTORY ${LLVM_LIBRARY_OUTPUT_INTDIR}
+                       DEPENDS ${LLVM_MAIN_SRC_DIR}/utils/extract_symbols.py ${static_libs}
+                       VERBATIM
+                       COMMENT "Generating export list for ${target}")
+    add_llvm_symbol_exports( ${target} ${exported_symbol_file} )
+    # If something links against this executable then we want a
+    # transitive link against only the libraries whose symbols
+    # we aren't exporting.
+    set_target_properties(${target} PROPERTIES INTERFACE_LINK_LIBRARIES "${other_libs}")
+    # The default import library suffix that cmake uses for cygwin/mingw is
+    # ".dll.a", but for clang.exe that causes a collision with libclang.dll,
+    # where the import libraries of both get named libclang.dll.a. Use a suffix
+    # of ".exe.a" to avoid this.
+    if(CYGWIN OR MINGW)
+      set_target_properties(${target} PROPERTIES IMPORT_SUFFIX ".exe.a")
+    endif()
+  elseif(NOT (WIN32 OR CYGWIN))
+    # On Windows auto-exporting everything doesn't work because of the limit on
+    # the size of the exported symbol table, but on other platforms we can do
+    # it without any trouble.
     set_target_properties(${target} PROPERTIES ENABLE_EXPORTS 1)
     if (APPLE)
       set_property(TARGET ${target} APPEND_STRING PROPERTY
         LINK_FLAGS " -rdynamic")
     endif()
   endif()
 endfunction()
 

llvm/trunk/cmake/modules/HandleLLVMOptions.cmake

if (C_SUPPORTS_FNO_FUNCTION_SECTIONS)
       # Don't add -ffunction-section if it can be disabled with -fno-function-sections.
       # Doing so will break sanitizers.
       add_flag_if_supported("-ffunction-sections" FFUNCTION_SECTIONS)
     endif()
     add_flag_if_supported("-fdata-sections" FDATA_SECTIONS)
   endif()
 endif()
 
-if(CYGWIN OR MINGW)
-  # Prune --out-implib from executables. It doesn't make sense even
-  # with --export-all-symbols.
-  string(REGEX REPLACE "-Wl,--out-implib,[^ ]+ " " "
-    CMAKE_C_LINK_EXECUTABLE "${CMAKE_C_LINK_EXECUTABLE}")
-  string(REGEX REPLACE "-Wl,--out-implib,[^ ]+ " " "
-    CMAKE_CXX_LINK_EXECUTABLE "${CMAKE_CXX_LINK_EXECUTABLE}")
-endif()
-
 if(MSVC)
   # Remove flags here, for exceptions and RTTI.
   # Each target property or source property should be responsible to control
   # them.
   # CL.EXE complains to override flags like "/GR /GR-".
   string(REGEX REPLACE "(^| ) */EH[-cs]+ *( |$)" "\\1 \\2" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   string(REGEX REPLACE "(^| ) */GR-? *( |$)" "\\1 \\2" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()

 elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
   append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
                  CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 elseif(LLVM_ENABLE_LTO)
   append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
                  CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
+# This option makes utils/extract_symbols.py be used to determine the list of
+# symbols to export from LLVM tools. This is necessary when using MSVC if you
+# want to allow plugins, though note that the plugin has to explicitly link
+# against (exactly one) tool so we can't unilaterally turn on
+# LLVM_ENABLE_PLUGINS when it's enabled.
+option(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS "Export symbols from LLVM tools so that plugins can import them" OFF)
+if(BUILD_SHARED_LIBS AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS)
+  message(FATAL_ERROR "BUILD_SHARED_LIBS not compatible with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS")
+endif()
+if(LLVM_LINK_LLVM_DYLIB AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS)
+  message(FATAL_ERROR "LLVM_LINK_LLVM_DYLIB not compatible with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS")
+endif()
+
 # Plugin support
 # FIXME: Make this configurable.
 if(WIN32 OR CYGWIN)
   if(BUILD_SHARED_LIBS)
     set(LLVM_ENABLE_PLUGINS ON)
   else()
     set(LLVM_ENABLE_PLUGINS OFF)
   endif()
 else()
   set(LLVM_ENABLE_PLUGINS ON)
 endif()

llvm/trunk/cmake/modules/LLVMConfig.cmake.in

 
 set(LLVM_ENABLE_ZLIB @LLVM_ENABLE_ZLIB@)
 
 set(LLVM_NATIVE_ARCH @LLVM_NATIVE_ARCH@)
 
 set(LLVM_ENABLE_PIC @LLVM_ENABLE_PIC@)
 
 set(LLVM_ENABLE_PLUGINS @LLVM_ENABLE_PLUGINS@)
+set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS @LLVM_EXPORT_SYMBOLS_FOR_PLUGINS@)
 set(LLVM_PLUGIN_EXT @LLVM_PLUGIN_EXT@)
 
 set(LLVM_ON_UNIX @LLVM_ON_UNIX@)
 set(LLVM_ON_WIN32 @LLVM_ON_WIN32@)
 
 set(LLVM_LIBDIR_SUFFIX @LLVM_LIBDIR_SUFFIX@)
 
 set(LLVM_INCLUDE_DIRS "@LLVM_CONFIG_INCLUDE_DIRS@")

llvm/trunk/lib/Transforms/Hello/CMakeLists.txt

if(WIN32 OR CYGWIN)
   set(LLVM_LINK_COMPONENTS Core Support)
 endif()
 
 add_llvm_loadable_module( LLVMHello
   Hello.cpp
 
   DEPENDS
   intrinsics_gen
+  PLUGIN_TOOL
+  opt
   )

llvm/trunk/utils/extract_symbols.py

+#!/usr/bin/env python
+
+"""A tool for extracting a list of symbols to export
+
+When exporting symbols from a dll or exe we either need to mark the symbols in
+the source code as __declspec(dllexport) or supply a list of symbols to the
+linker. This program automates the latter by inspecting the symbol tables of a
+list of link inputs and deciding which of those symbols need to be exported.
+
+We can't just export all the defined symbols, as there's a limit of 65535
+exported symbols and in clang we go way over that, particularly in a debug
+build. Therefore a large part of the work is pruning symbols either which can't
+be imported, or which we think are things that have definitions in public header
+files (i.e. template instantiations) and we would get defined in the thing
+importing these symbols anyway.
+"""
+
+import sys
+import re
+import os
+import subprocess
+import multiprocessing
+import argparse
+
+# Define functions which extract a list of symbols from a library using several
+# different tools. We use subprocess.Popen and yield a symbol at a time instead
+# of using subprocess.check_output and returning a list as, especially on
+# Windows, waiting for the entire output to be ready can take a significant
+# amount of time.
+
+def dumpbin_get_symbols(lib):
+    process = subprocess.Popen(['dumpbin','/symbols',lib], bufsize=1,
+                               stdout=subprocess.PIPE, stdin=subprocess.PIPE)
+    process.stdin.close()
+    for line in process.stdout:
+        # Look for external symbols that are defined in some section
+        match = re.match("^.+SECT.+External\s+\|\s+(\S+).*$", line)
+        if match:
+            yield match.group(1)
+    process.wait()
+
+def nm_get_symbols(lib):
+    process = subprocess.Popen(['nm',lib], bufsize=1,
+                               stdout=subprocess.PIPE, stdin=subprocess.PIPE)
+    process.stdin.close()
+    for line in process.stdout:
+        # Look for external symbols that are defined in some section
+        match = re.match("^\S+\s+[BDGRSTVW]\s+(\S+)$", line)
+        if match:
+            yield match.group(1)
+    process.wait()
+
+def readobj_get_symbols(lib):
+    process = subprocess.Popen(['llvm-readobj','-symbols',lib], bufsize=1,
+                               stdout=subprocess.PIPE, stdin=subprocess.PIPE)
+    process.stdin.close()
+    for line in process.stdout:
+        # When looking through the output of llvm-readobj we expect to see Name,
+        # Section, then StorageClass, so record Name and Section when we see
+        # them and decide if this is a defined external symbol when we see
+        # StorageClass.
+        match = re.search('Name: (\S+)', line)
+        if match:
+            name = match.group(1)
+        match = re.search('Section: (\S+)', line)
+        if match:
+            section = match.group(1)
+        match = re.search('StorageClass: (\S+)', line)
+        if match:
+            storageclass = match.group(1)
+            if section != 'IMAGE_SYM_ABSOLUTE' and \
+               section != 'IMAGE_SYM_UNDEFINED' and \
+               storageclass == 'External':
+                yield name
+    process.wait()
+
+# Define functions which determine if the target is 32-bit Windows (as that's
+# where calling convention name decoration happens).
+
+def dumpbin_is_32bit_windows(lib):
+    # dumpbin /headers can output a huge amount of data (>100MB in a debug
+    # build) so we read only up to the 'machine' line then close the output.
+    process = subprocess.Popen(['dumpbin','/headers',lib], bufsize=1,
+                               stdout=subprocess.PIPE, stdin=subprocess.PIPE)
+    process.stdin.close()
+    retval = False
+    for line in process.stdout:
+        match = re.match('.+machine \((\S+)\)', line)
+        if match:
+            retval = (match.group(1) == 'x86')
+            break
+    process.stdout.close()
+    process.wait()
+    return retval
+
+def objdump_is_32bit_windows(lib):
+    output = subprocess.check_output(['objdump','-f',lib])
+    for line in output:
+        match = re.match('.+file format (\S+)', line)
+        if match:
+            return (match.group(1) == 'pe-i386')
+    return False
+
+def readobj_is_32bit_windows(lib):
+    output = subprocess.check_output(['llvm-readobj','-file-headers',lib])
+    for line in output:
+        match = re.match('Format: (\S+)', line)
+        if match:
+            return (match.group(1) == 'COFF-i386')
+    return False
+
+# MSVC mangles names to ?<identifier_mangling>@<type_mangling>. By examining the
+# identifier/type mangling we can decide which symbols could possibly be
+# required and which we can discard.
+def should_keep_microsoft_symbol(symbol, calling_convention_decoration):
+    # Keep unmangled (i.e. extern "C") names
+    if not '?' in symbol:
+        if calling_convention_decoration:
+            # Remove calling convention decoration from names
+            match = re.match('[_@]([^@]+)', symbol)
+            if match:
+                return match.group(1)
+        return symbol
+    # Function template instantiations start with ?$, discard them as it's
+    # assumed that the definition is public
+    elif symbol.startswith('??$'):
+        return None
+    # Deleting destructors start with ?_G or ?_E and can be discarded because
+    # link.exe gives you a warning telling you they can't be exported if you
+    # don't
+    elif symbol.startswith('??_G') or symbol.startswith('??_E'):
+        return None
+    # Constructors (?0) and destructors (?1) of templates (?$) are assumed to be
+    # defined in headers and not required to be kept
+    elif symbol.startswith('??0?$') or symbol.startswith('??1?$'):
+        return None
+    # An anonymous namespace is mangled as ?A(maybe hex number)@. Any symbol
+    # that mentions an anonymous namespace can be discarded, as the anonymous
+    # namespace doesn't exist outside of that translation unit.
+    elif re.search('\?A(0x\w+)?@', symbol):
+        return None
+    # Keep mangled llvm:: and clang:: function symbols. How we detect these is a
+    # bit of a mess and imprecise, but that avoids having to completely demangle
+    # the symbol name. The outermost namespace is at the end of the identifier
+    # mangling, and the identifier mangling is followed by the type mangling, so
+    # we look for (llvm|clang)@@ followed by something that looks like a
+    # function type mangling. To spot a function type we use (this is derived
+    # from clang/lib/AST/MicrosoftMangle.cpp):
+    # <function-type> ::= <function-class> <this-cvr-qualifiers>
+    #                     <calling-convention> <return-type>
+    #                     <argument-list> <throw-spec>
+    # <function-class> ::= [A-Z]
+    # <this-cvr-qualifiers> ::= [A-Z0-9_]*
+    # <calling-convention> ::= [A-JQ]
+    # <return-type> ::= .+
+    # <argument-list> ::= X   (void)
+    #                 ::= .+@ (list of types)
+    #                 ::= .*Z (list of types, varargs)
+    # <throw-spec> ::= exceptions are not allowed
+    elif re.search('(llvm|clang)@@[A-Z][A-Z0-9_]*[A-JQ].+(X|.+@|.*Z)$', symbol):
+        return symbol
+    return None
+
+# Itanium manglings are of the form _Z<identifier_mangling><type_mangling>. We
+# demangle the identifier mangling to identify symbols that can be safely
+# discarded.
+def should_keep_itanium_symbol(symbol, calling_convention_decoration):
+    # Start by removing any calling convention decoration (which we expect to
+    # see on all symbols, even mangled C++ symbols)
+    if calling_convention_decoration and symbol.startswith('_'):
+        symbol = symbol[1:]
+    # Keep unmangled names
+    if not symbol.startswith('_') and not symbol.startswith('.'):
+        return symbol
+    # Discard manglings that aren't nested names
+    match = re.match('_Z(T[VTIS])?(N.+)', symbol)
+    if not match:
+        return None
+    # Demangle the name. If the name is too complex then we don't need to keep
+    # it, but it the demangling fails then keep the symbol just in case.
+    try:
+        names, _ = parse_itanium_nested_name(match.group(2))
+    except TooComplexName:
+        return None
+    if not names:
+        return symbol
+    # Constructors and destructors of templates classes are assumed to be
+    # defined in headers and not required to be kept
+    if re.match('[CD][123]', names[-1][0]) and names[-2][1]:
+        return None
+    # Discard function template instantiations as it's assumed that the
+    # definition is public
+    elif names[-1][1]:
+        return None
+    # Keep llvm:: and clang:: names
+    elif names[0][0] == '4llvm' or names[0][0] == '5clang':
+        return symbol
+    # Discard everything else
+    else:
+        return None
+
+# Certain kinds of complex manglings we assume cannot be part of a public
+# interface, and we handle them by raising an exception.
+class TooComplexName(Exception):
+    pass
+
+# Parse an itanium mangled name from the start of a string and return a
+# (name, rest of string) pair.
+def parse_itanium_name(arg):
+    # Check for a normal name
+    match = re.match('(\d+)(.+)', arg)
+    if match:
+        n = int(match.group(1))
+        name = match.group(1)+match.group(2)[:n]
+        rest = match.group(2)[n:]
+        return name, rest
+    # Check for constructor/destructor names
+    match = re.match('([CD][123])(.+)', arg)
+    if match:
+        return match.group(1), match.group(2)
+    # Assume that a sequence of characters that doesn't end a nesting is an
+    # operator (this is very imprecise, but appears to be good enough)
+    match = re.match('([^E]+)(.+)', arg)
+    if match:
+        return match.group(1), match.group(2)
+    # Anything else: we can't handle it
+    return None, arg
+
+# Parse an itanium mangled template argument list from the start of a string
+# and throw it away, returning the rest of the string.
+def skip_itanium_template(arg):
+    # A template argument list starts with I
+    assert arg.startswith('I'), arg
+    tmp = arg[1:]
+    while tmp:
+        # Check for names
+        match = re.match('(\d+)(.+)', tmp)
+        if match:
+            n = int(match.group(1))
+            tmp =  match.group(2)[n:]
+            continue
+        # Check for substitutions
+        match = re.match('S[A-Z0-9]*_(.+)', tmp)
+        if match:
+            tmp = match.group(1)
+        # Start of a template
+        elif tmp.startswith('I'):
+            tmp = skip_itanium_template(tmp)
+        # Start of a nested name
+        elif tmp.startswith('N'):
+            _, tmp = parse_itanium_nested_name(tmp)
+        # Start of an expression: assume that it's too complicated
+        elif tmp.startswith('L') or tmp.startswith('X'):
+            raise TooComplexName
+        # End of the template
+        elif tmp.startswith('E'):
+            return tmp[1:]
+        # Something else: probably a type, skip it
+        else:
+            tmp = tmp[1:]
+    return None
+
+# Parse an itanium mangled nested name and transform it into a list of pairs of
+# (name, is_template), returning (list, rest of string).
+def parse_itanium_nested_name(arg):
+    # A nested name starts with N
+    assert arg.startswith('N'), arg
+    ret = []
+
+    # Skip past the N, and possibly a substitution
+    match = re.match('NS[A-Z0-9]*_(.+)', arg)
+    if match:
+        tmp = match.group(1)
+    else:
+        tmp = arg[1:]
+
+    # Skip past CV-qualifiers and ref qualifiers
+    match = re.match('[rVKRO]*(.+)', tmp);
+    if match:
+        tmp = match.group(1)
+
+    # Repeatedly parse names from the string until we reach the end of the
+    # nested name
+    while tmp:
+        # An E ends the nested name
+        if tmp.startswith('E'):
+            return ret, tmp[1:]
+        # Parse a name
+        name_part, tmp = parse_itanium_name(tmp)
+        if not name_part:
+            # If we failed then we don't know how to demangle this
+            return None, None
+        is_template = False
+        # If this name is a template record that, then skip the template
+        # arguments
+        if tmp.startswith('I'):
+            tmp = skip_itanium_template(tmp)
+            is_template = True
+        # Add the name to the list
+        ret.append((name_part, is_template))
+
+    # If we get here then something went wrong
+    return None, None
+
+def extract_symbols(arg):
+    get_symbols, should_keep_symbol, calling_convention_decoration, lib = arg
+    symbols = dict()
+    for symbol in get_symbols(lib):
+        symbol = should_keep_symbol(symbol, calling_convention_decoration)
+        if symbol:
+            symbols[symbol] = 1 + symbols.setdefault(symbol,0)
+    return symbols
+
+if __name__ == '__main__':
+    tool_exes = ['dumpbin','nm','objdump','llvm-readobj']
+    parser = argparse.ArgumentParser(
+        description='Extract symbols to export from libraries')
+    parser.add_argument('--mangling', choices=['itanium','microsoft'],
+                        required=True, help='expected symbol mangling scheme')
+    parser.add_argument('--tools', choices=tool_exes, nargs='*',
+                        help='tools to use to extract symbols and determine the'
+                        ' target')
+    parser.add_argument('libs', metavar='lib', type=str, nargs='+',
+                        help='libraries to extract symbols from')
+    parser.add_argument('-o', metavar='file', type=str, help='output to file')
+    args = parser.parse_args()
+
+    # Determine the function to use to get the list of symbols from the inputs,
+    # and the function to use to determine if the target is 32-bit windows.
+    tools = { 'dumpbin' : (dumpbin_get_symbols, dumpbin_is_32bit_windows),
+              'nm' : (nm_get_symbols, None),
+              'objdump' : (None, objdump_is_32bit_windows),
+              'llvm-readobj' : (readobj_get_symbols, readobj_is_32bit_windows) }
+    get_symbols = None
+    is_32bit_windows = None
+    # If we have a tools argument then use that for the list of tools to check
+    if args.tools:
+        tool_exes = args.tools
+    # Find a tool to use by trying each in turn until we find one that exists
+    # (subprocess.call will throw OSError when the program does not exist)
+    get_symbols = None
+    for exe in tool_exes:
+        try:
+            # Close std streams as we don't want any output and we don't
+            # want the process to wait for something on stdin.
+            p = subprocess.Popen([exe], stdout=subprocess.PIPE,
+                                 stderr=subprocess.PIPE,
+                                 stdin=subprocess.PIPE)
+            p.stdout.close()
+            p.stderr.close()
+            p.stdin.close()
+            p.wait()
+            # Keep going until we have a tool to use for both get_symbols and
+            # is_32bit_windows
+            if not get_symbols:
+                get_symbols = tools[exe][0]
+            if not is_32bit_windows:
+                is_32bit_windows = tools[exe][1]
+            if get_symbols and is_32bit_windows:
+                break
+        except OSError:
+            continue
+    if not get_symbols:
+        print >>sys.stderr, "Couldn't find a program to read symbols with"
+        exit(1)
+    if not is_32bit_windows:
+        print >>sys.stderr, "Couldn't find a program to determing the target"
+        exit(1)
+
+    # How we determine which symbols to keep and which to discard depends on
+    # the mangling scheme
+    if args.mangling == 'microsoft':
+        should_keep_symbol = should_keep_microsoft_symbol
+    else:
+        should_keep_symbol = should_keep_itanium_symbol
+
+    # Get the list of libraries to extract symbols from
+    libs = list()
+    for lib in args.libs:
+        # When invoked by cmake the arguments are the cmake target names of the
+        # libraries, so we need to add .lib/.a to the end and maybe lib to the
+        # start to get the filename. Also allow objects.
+        suffixes = ['.lib','.a','.obj','.o']
+        if not any([lib.endswith(s) for s in suffixes]):
+            for s in suffixes:
+                if os.path.exists(lib+s):
+                    lib = lib+s
+                    break
+                if os.path.exists('lib'+lib+s):
+                    lib = 'lib'+lib+s
+                    break
+        if not any([lib.endswith(s) for s in suffixes]):
+            print >>sys.stderr, "Don't know what to do with argument "+lib
+            exit(1)
+        libs.append(lib)
+
+    # Check if calling convention decoration is used by inspecting the first
+    # library in the list
+    calling_convention_decoration = is_32bit_windows(libs[0])
+
+    # Extract symbols from libraries in parallel. This is a huge time saver when
+    # doing a debug build, as there are hundreds of thousands of symbols in each
+    # library.
+    pool = multiprocessing.Pool()
+    try:
+        # Only one argument can be passed to the mapping function, and we can't
+        # use a lambda or local function definition as that doesn't work on
+        # windows, so create a list of tuples which duplicates the arguments
+        # that are the same in all calls.
+        vals = [(get_symbols, should_keep_symbol, calling_convention_decoration, x) for x in libs]
+        # Do an async map then wait for the result to make sure that
+        # KeyboardInterrupt gets caught correctly (see
+        # http://bugs.python.org/issue8296)
+        result = pool.map_async(extract_symbols, vals)
+        pool.close()
+        libs_symbols = result.get(3600)
+    except KeyboardInterrupt:
+        # On Ctrl-C terminate everything and exit
+        pool.terminate()
+        pool.join()
+        exit(1)
+
+    # Merge everything into a single dict
+    symbols = dict()
+    for this_lib_symbols in libs_symbols:
+        for k,v in this_lib_symbols.items():
+            symbols[k] = v + symbols.setdefault(k,0)
+
+    # Count instances of member functions of template classes, and map the
+    # symbol name to the function+class. We do this under the assumption that if
+    # a member function of a template class is instantiated many times it's
+    # probably declared in a public header file.
+    template_function_count = dict()
+    template_function_mapping = dict()
+    template_function_count[""] = 0
+    for k in symbols:
+        name = None
+        if args.mangling == 'microsoft':
+            # Member functions of templates start with
+            # ?<fn_name>@?$<class_name>@, so we map to <fn_name>@?$<class_name>.
+            # As manglings go from the innermost scope to the outermost scope
+            # this means:
+            #  * When we have a function member of a subclass of a template
+            #    class then <fn_name> will actually contain the mangling of
+            #    both the subclass and the function member. This is fine.
+            #  * When we have a function member of a template subclass of a
+            #    (possibly template) class then it's the innermost template
+            #    subclass that becomes <class_name>. This should be OK so long
+            #    as we don't have multiple classes with a template subclass of
+            #    the same name.
+            match = re.search("^\?(\??\w+\@\?\$\w+)\@", k)
+            if match:
+                name = match.group(1)
+        else:
+            # Find member functions of templates by demangling the name and
+            # checking if the second-to-last name in the list is a template.
+            match = re.match('_Z(T[VTIS])?(N.+)', k)
+            if match:
+                try:
+                    names, _ = parse_itanium_nested_name(match.group(2))
+                    if names and names[-2][1]:
+                        name = ''.join([x for x,_ in names])
+                except TooComplexName:
+                    # Manglings that are too complex should already have been
+                    # filtered out, but if we happen to somehow see one here
+                    # just leave it as-is.
+                    pass
+        if name:
+            old_count = template_function_count.setdefault(name,0)
+            template_function_count[name] = old_count + 1
+            template_function_mapping[k] = name
+        else:
+            template_function_mapping[k] = ""
+
+    # Print symbols which both:
+    #  * Appear in exactly one input, as symbols defined in multiple
+    #    objects/libraries are assumed to have public definitions.
+    #  * Aren't instances of member functions of templates which have been
+    #    instantiated 100 times or more, which are assumed to have public
+    #    definitions. (100 is an arbitrary guess here.)
+    if args.o:
+        outfile = open(args.o,'w')
+    else:
+        outfile = sys.stdout
+    for k,v in symbols.items():
+        template_count = template_function_count[template_function_mapping[k]]
+        if v == 1 and template_count < 100:
+            print >>outfile, k