[libcxx] Make libc++.so a linker script by default on most platforms.

[EricWF]

Summary

This patch turns on LIBCXX_ENABLE_ABI_LINKER_SCRIPT by default whenever LLVM_HAVE_LINK_VERSION_SCRIPT is ON. This turns out to be whenever:

1. WIN32 is not defined.

2 UNIX is defined.

3. APPLE is not defined.

While LLVM_HAVE_LINK_VERSION_SCRIPT is meant to reflect exactly what we are asking I think it's close enough.

After committing this patch Linux users will no longer have to use "-lc++abi" explicitly!

[• cbergstrom]

‎And does this hardcode the abi and break cxxrt? It's not just me who uses that 

  Original Message  
From: Eric Fiselier
Sent: Thursday, October 15, 2015 03:26
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Subject: [PATCH] D13739: [libcxx] Make libc++.so a linker script by default on most platforms.

EricWF created this revision.
EricWF added reviewers: mclow.lists, jroelofs, danalbert, compnerd.
EricWF added subscribers: cfe-commits, cbergstrom, rengolin, emaste.

This patch turns on LIBCXX_ENABLE_ABI_LINKER_SCRIPT by default whenever LLVM_HAVE_LINK_VERSION_SCRIPT is ON. This turns out to be whenever:

1. WIN32 is not defined.

2 UNIX is defined.

3. APPLE is not defined.

While LLVM_HAVE_LINK_VERSION_SCRIPT is meant to reflect exactly what we are asking I think it's close enough.

After committing this patch Linux users will no longer have to use "-lc++abi" explicitly!

http://reviews.llvm.org/D13739

Files:
CMakeLists.txt
cmake/Modules/HandleOutOfTreeLLVM.cmake
docs/BuildingLibcxx.rst
docs/UsingLibcxx.rst

[EricWF]

In D13739#267286, @cbergstrom wrote:

‎And does this hardcode the abi and break cxxrt? It's not just me who uses that 

No it does not hard code the ABI. The linker script is generated based on the name of the ABI library selected at build time. For libcxxrt the linker script would be "INPUT(libc++.so.1 -lcxxrt)".
This shouldn't make it any harder to use libcxxrt.

[EricWF]

@cbergstrom After installing libcxxrt I ran the libc++ configuration with -DLIBCXX_CXX_ABI=libcxxrt and everything worked perfectly. It generated libc++.so as INPUT(libc++.so.1 -lcxxrt) and after running it against the test suite everything works great.

[jroelofs]

CMakeLists.txt

71

Should probably make this a cmake error, not just have it do something other than what you wanted.

76

s/On/on/ ?

78

DEFAULT_VALUE seems too generic a name for this... or is this a common CMake pattern that I'm unaware of?

cmake/Modules/HandleOutOfTreeLLVM.cmake

116

Where are LLVM_ON_WIN32 and LLVM_ON_UNIX used?

docs/BuildingLibcxx.rst

176

I find this sentence hard to parse. Maybe phrase it the same way you did in the comment:

"ON by default on UNIX platforms other than Apple unless 'LIBCXX_ENABLE_STATIC_ABI_LIBRARY' is ON"

docs/UsingLibcxx.rst

59

Looks like an inconsistent use of " vs ' for '-stdlib=libc++' and "-lc++abi". I don't have good RST-fu, but that seems fishy.

[EricWF]

Address review comments.

CMakeLists.txt

71

It already is :-). Theres some logic near line #179.

78

Nope, I was just being lazy with the name because it's not meant to be a global name but instead a scratch variable. I'll fix it anyway.

cmake/Modules/HandleOutOfTreeLLVM.cmake

116

Nowhere yet but they will be used to validate sanitizer flags and other UNIX specific behavior. The whole chunk is copy and pasted from HandleLLVMOptions.cmake

[jroelofs]

LGTM

[EricWF]

Thanks for the review. I'm going to wait a little to see if any objections crop up before committing.

[vkalintiris]

cmake/Modules/HandleOutOfTreeLLVM.cmake

123–133

I've created the review request D13025, that checks whether version scripts are supported in non-APPLE Unix platforms. As far as I know, LLD doesn't support version scripts at the moment. Would it make sense to check this in a similar fashion for out of tree LLVM?

[EricWF]

cmake/Modules/HandleOutOfTreeLLVM.cmake

123–133

Thank's for the heads up. I'm actually using this incorrectly to mean "linker script" and not specifically a version script. After grepping the lld sources it seems that it does support linker scripts with the "INPUT" command in them, but I haven't tested it.

Really I should just change my uses of "LLVM_HAVE_LINK_VERSION_SCRIPT" to "LLVM_UNIX AND NOT APPLE" which is really what I'm trying to say. After doing that I can probably just remove the "LLVM_HAVE_LINK_VERSION_SCRIPT" from libc++ entirely.

CMakeLists.txt

 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 
 # ABI Library options ---------------------------------------------------------
 set(LIBCXX_CXX_ABI "${LIBCXX_CXX_ABI}" CACHE STRING
     "Specify C++ ABI library to use." FORCE)
 set(CXXABIS none libcxxabi libcxxrt libstdc++ libsupc++)
 set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 
+# Use a static copy of the ABI library when linking libc++. This option
+# cannot be used with LIBCXX_ENABLE_ABI_LINKER_SCRIPT.
 option(LIBCXX_ENABLE_STATIC_ABI_LIBRARY "Statically link the ABI library" OFF)
 
 # Generate and install a linker script inplace of libc++.so. The linker script
-# will link libc++ to the correct ABI library.
+# will link libc++ to the correct ABI library. This option is on by default
+# On UNIX platforms other than Apple unless 'LIBCXX_ENABLE_STATIC_ABI_LIBRARY'
+# is on.
+set(ENABLE_LINKER_SCRIPT_DEFAULT_VALUE OFF)
+if (LLVM_HAVE_LINK_VERSION_SCRIPT AND NOT LIBCXX_ENABLE_STATIC_ABI_LIBRARY)
+    set(ENABLE_LINKER_SCRIPT_DEFAULT_VALUE ON)
+endif()
+
 option(LIBCXX_ENABLE_ABI_LINKER_SCRIPT
-      "Use and install a linker script for the given ABI library" OFF)
+      "Use and install a linker script for the given ABI library"
+      ${ENABLE_LINKER_SCRIPT_DEFAULT_VALUE})
 
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})
 set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")

cmake/Modules/HandleOutOfTreeLLVM.cmake

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   # Required doc configuration
   if (LLVM_ENABLE_SPHINX)
     message(STATUS "Sphinx enabled.")
     find_package(Sphinx REQUIRED)
   else()
     message(STATUS "Sphinx disabled.")
   endif()
 
+  # FIXME - This is cribbed from HandleLLVMOptions.cmake.
+  if(WIN32)
+    set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
+    if(CYGWIN)
+      set(LLVM_ON_WIN32 0)
+      set(LLVM_ON_UNIX 1)
+    else(CYGWIN)
+      set(LLVM_ON_WIN32 1)
+      set(LLVM_ON_UNIX 0)
+    endif(CYGWIN)
+  else(WIN32)
+    if(UNIX)
+      set(LLVM_ON_WIN32 0)
+      set(LLVM_ON_UNIX 1)
+      if(APPLE)
+        set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
+      else(APPLE)
+        set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
+      endif(APPLE)
+    else(UNIX)
+      MESSAGE(SEND_ERROR "Unable to determine platform")
+    endif(UNIX)
+  endif(WIN32)
+
   # Add LLVM Functions --------------------------------------------------------
   include(AddLLVM OPTIONAL)
 endif()

docs/BuildingLibcxx.rst

 
 .. option:: LIBCXX_ENABLE_STATIC_ABI_LIBRARY:BOOL
 
   **Default**: ``OFF``
 
   If this option is enabled, libc++ will try and link the selected ABI library
   statically.
 
+.. option:: LIBCXX_ENABLE_ABI_LINKER_SCRIPT:BOOL
+
+  **Default**: ``ON`` by default on UNIX platforms other than Apple unless
+  'LIBCXX_ENABLE_STATIC_ABI_LIBRARY' is ON. Otherwise the default value is ``OFF``.
+
+  This option generate and installs a linker script as ``libc++.so`` which
+  links the correct ABI library.
+
 .. option:: LIBCXXABI_USE_LLVM_UNWINDER:BOOL
 
   **Default**: ``OFF``
 
   Build and use the LLVM unwinder. Note: This option can only be used when
   libc++abi is the C++ ABI library used.
 
 

docs/UsingLibcxx.rst

.. code-block:: bash
   $ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
   $ ./a.out # Searches for libc++ along LD_LIBRARY_PATH
 
 
 
 Using libc++ on Linux
 =====================
 
-On Linux libc++ typically links to a shared version of libc++abi. Unfortunately
-you can't simply run clang with "-stdlib=libc++" as clang is not set up to
-link for this configuration. To get around this you'll have to manually
-link libc++abi yourself. For example:
+On Linux libc++ can typically be used with only '-stdlib=libc++'. However
+some libc++ installations require the user manually link libc++abi themselves.
+If you are running into linker errors when using libc++ try adding '-lc++abi'
+to the link line.  For example:
 
 .. code-block:: bash
 
   $ clang++ -stdlib=libc++ test.cpp -lc++ -lc++abi -lm -lc -lgcc_s -lgcc
 
 Alternately, you could just add libc++abi to your libraries list, which in
 most situations will give the same result: