[libcxx] Make it drastically simpler to link libc++.

Summary:
Currently on most platforms you have to manually link the c++ abi library used with libc++ whenever you use libc++. So your typical libc++ command like invocation might look like:

```
clang++ -stdlib=libc++ foo.cpp -lc++abi
```

Having to manually link `libc++abi.so` makes it harder for libc++ to be used generically. This patch fixes that by generating a linker script for `libc++.so` that correctly links the ABI library. On linux the linker script for libc++abi would look like:

```
# libc++.so
INPUT(libc++.so.1 -lc++abi)
```

With the linker script you can now use libc++ using only `-stdlib=libc++`. This is the technique that is used on FreeBSD in ordered to link cxxrt and I think it's the best approach to make our users lives simpler.

The CMake option used to enable this is `LIBCXX_ENABLE_ABI_LINKER_SCRIPT`. In future I would like to enable this by default on all platforms except for Darwin.

Reviewers: mclow.lists, danalbert, rsmith, jroelofs, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D12508

llvm-svn: 250319

1 files changed, 79 insertions, 0 deletions

diff --git a/libcxx/utils/gen_link_script/gen_link_script.py b/libcxx/utils/gen_link_script/gen_link_script.py
new file mode 100755
index 00000000000..5de18f9129c
--- /dev/null
+++ b/libcxx/utils/gen_link_script/gen_link_script.py
@@ -0,0 +1,79 @@
+#!/usr/bin/env python
+import os
+import sys
+
+def print_and_exit(msg):
+    sys.stderr.write(msg + '\n')
+    sys.exit(1)
+
+def usage_and_exit():
+    print_and_exit("Usage: ./gen_link_script.py [--help] [--dryrun] <path/to/libcxx.so> <abi_libname>")
+
+def help_and_exit():
+    help_msg = \
+"""Usage
+
+  gen_link_script.py [--help] [--dryrun] <path/to/libcxx.so> <abi_libname>
+
+  Generate a linker script that links libc++ to the proper ABI library.
+  The script replaces the specified libc++ symlink.
+  An example script for c++abi would look like "INPUT(libc++.so.1 -lc++abi)".
+
+Arguments
+  <path/to/libcxx.so> - The top level symlink to the versioned libc++ shared
+                        library. This file is replaced with a linker script.
+  <abi_libname>       - The name of the ABI library to use in the linker script.
+                        The name must be one of [c++abi, stdc++, supc++, cxxrt].
+
+Exit Status:
+  0 if OK,
+  1 if the action failed.
+"""
+    print_and_exit(help_msg)
+
+def parse_args():
+    args = list(sys.argv)
+    del args[0]
+    if len(args) == 0:
+        usage_and_exit()
+    if args[0] == '--help':
+        help_and_exit()
+    dryrun = '--dryrun' == args[0]
+    if dryrun:
+        del args[0]
+    if len(args) != 2:
+        usage_and_exit()
+    symlink_file = args[0]
+    abi_libname = args[1]
+    return dryrun, symlink_file, abi_libname
+
+def main():
+    dryrun, symlink_file, abi_libname = parse_args()
+
+    # Check that the given libc++.so file is a valid symlink.
+    if not os.path.islink(symlink_file):
+        print_and_exit("symlink file %s is not a symlink" % symlink_file)
+
+    # Read the symlink so we know what libc++ to link to in the linker script.
+    linked_libcxx = os.readlink(symlink_file)
+
+    # Check that the abi_libname is one of the supported values.
+    supported_abi_list = ['c++abi', 'stdc++', 'supc++', 'cxxrt']
+    if abi_libname not in supported_abi_list:
+        print_and_exit("abi name '%s' is not supported: Use one of %r" %
+                        (abi_libname, supported_abi_list))
+
+    # Generate the linker script contents and print the script and destination
+    # information.
+    contents = "INPUT(%s -l%s)" % (linked_libcxx, abi_libname)
+    print("GENERATING SCRIPT: '%s' as file %s" % (contents, symlink_file))
+
+    # Remove the existing libc++ symlink and replace it with the script.
+    if not dryrun:
+        os.unlink(symlink_file)
+        with open(symlink_file, 'w') as f:
+            f.write(contents + "\n")
+
+
+if __name__ == '__main__':
+    main()