[libcxx] Update gen_link_script.py to support different input and output

[phosek]

Summary

This enables the use of this script from other build systems like
GN which don't support post-build actions as well as for static
archives.

[phosek]

This was extracted out of D54726 since it turned out to be useful for D60253.

[thakis]

Nice! argparse makes this a lot simpler :)

libcxx/utils/gen_link_script.py

9

I think the LHS license is the new license – probably want to keep the new license?

49

stray debugging leftover?

51

The LHS didn't do this. Is this for Windows?

54

Probably don't want to do this if args.dryrun

62

We're losing this error, is this intentional?

[ldionne]

While we're at it, do you know why the LIBCXX_ENABLE_ABI_LINKER_SCRIPT isn't allowed on Apple platforms? It doesn't seem to me like this is something you'd want to necessarily prevent on Apple platforms.

[smeenai]

In D60309#1456569, @ldionne wrote:

While we're at it, do you know why the LIBCXX_ENABLE_ABI_LINKER_SCRIPT isn't allowed on Apple platforms? It doesn't seem to me like this is something you'd want to necessarily prevent on Apple platforms.

Linker scripts aren't a thing on Darwin, are they?

[thakis]

Looks good to me too except for the elif not os.path.exists(args.input): which I don't understand what it's for.

libcxx/utils/gen_link_script.py

51

(unreplied)

62

We're still losing this? I assume it's intentional, but might want to say why you're changing behavior here somewhere.

[ldionne]

In D60309#1456662, @smeenai wrote:

In D60309#1456569, @ldionne wrote:

While we're at it, do you know why the LIBCXX_ENABLE_ABI_LINKER_SCRIPT isn't allowed on Apple platforms? It doesn't seem to me like this is something you'd want to necessarily prevent on Apple platforms.

Linker scripts aren't a thing on Darwin, are they?

My point is just that you might want to create/test the creation of the linker script on Darwin. Also, you could potentially be building for a Unix on an Apple platform.

[ldionne]

In D60309#1456737, @ldionne wrote:

In D60309#1456662, @smeenai wrote:

In D60309#1456569, @ldionne wrote:

While we're at it, do you know why the LIBCXX_ENABLE_ABI_LINKER_SCRIPT isn't allowed on Apple platforms? It doesn't seem to me like this is something you'd want to necessarily prevent on Apple platforms.

Linker scripts aren't a thing on Darwin, are they?

My point is just that you might want to create/test the creation of the linker script on Darwin. Also, you could potentially be building for a Unix on an Apple platform.

I mean crosscompiling _from_ an Apple platform.

The restriction just appeared artificial to me, that's all.

[phosek]

libcxx/utils/gen_link_script.py

9

That's unintentional, I just copied the file from the original patch and forgot to update the header.

51

It's for the static case, where the input would be libc++static.a and output would be libc++.a, but libc++.a is what's actually being produced by the CMake, in that case the script is going to rename libc++.a to libc++static.a and generate libc++.a linker script. The --input and --output argument names are a bit misleading in that case, maybe something like --library and --link-script would be better?

62

This is intentional because in cases like the static library, there's no symlink.

[phosek]

In D60309#1456741, @ldionne wrote:

I mean crosscompiling _from_ an Apple platform.

The restriction just appeared artificial to me, that's all.

Cross-compiling works, we use it in our build when building Fuchsia runtimes on Apple platforms, so it's only when targeting Apple platform that's unsupported.

[phosek]

libcxx/utils/gen_link_script.py

51

I forgot to press reply.

[thakis]

Might want to mention in the commit message that this adds support for the static library chase. Is there no way to make cmake call the static lib c++static.a though? Having this script rename the build system's build output is a bit weird.

[phosek]

In D60309#1457204, @thakis wrote:

Might want to mention in the commit message that this adds support for the static library chase. Is there no way to make cmake call the static lib c++static.a though? Having this script rename the build system's build output is a bit weird.

I've cleaned up that part of the script since we don't really need the static library support yet.

libcxx/lib/CMakeLists.txt

foreach(lib ${LIBCXX_INTERFACE_LIBRARIES})
       else()
         list(APPEND LIBCXX_INTERFACE_LIBRARY_NAMES "${lib}")
       endif()
     endforeach()
     add_custom_command(TARGET cxx_shared POST_BUILD
       COMMAND
         ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/gen_link_script.py
       ARGS
-        "$<TARGET_LINKER_FILE:cxx_shared>"
+        --input "$<TARGET_SONAME_FILE:cxx_shared>"
+        --output "$<TARGET_LINKER_FILE:cxx_shared>"
         ${LIBCXX_INTERFACE_LIBRARY_NAMES}
       WORKING_DIRECTORY ${LIBCXX_BUILD_DIR}
     )
   endif()
 
   list(APPEND LIBCXX_BUILD_TARGETS "cxx_shared")
   if (LIBCXX_INSTALL_SHARED_LIBRARY)
     list(APPEND LIBCXX_INSTALL_TARGETS "cxx_shared")

libcxx/utils/gen_link_script.py

 #!/usr/bin/env python
 #===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===----------------------------------------------------------------------===##
 
-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> <public_libs>...")
-
-def help_and_exit():
-    help_msg = \
-"""Usage
-
-  gen_link_script.py [--help] [--dryrun] <path/to/libcxx.so> <public_libs>...
-
-  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.
-  <public_libs>       - List of library names to include in linker script.
-
-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]
-    public_libs = args[1:]
-    return dryrun, symlink_file, public_libs
+import argparse
+import os
+import sys
+
 
 def main():
-    dryrun, symlink_file, public_libs = parse_args()
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--dryrun", help="Don't write any output",
+                        action="store_true", default=False)
+    parser.add_argument("--rename", action="store_true", default=False,
+                        help="Rename the output as input so we can replace it")
+    parser.add_argument("--input", help="Path to libc++ library", required=True)
+    parser.add_argument("--output", help="Path to libc++ linker script", required=True)
+    parser.add_argument("libraries", nargs="+",
+                        help="List of libraries libc++ depends on")
+    args = parser.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)
+    # Prepare the list of public libraries to link.
+    public_libs = ['-l%s' % l for l in args.libraries]
 
-    # Read the symlink so we know what libc++ to link to in the linker script.
-    linked_libcxx = os.readlink(symlink_file)
+    # Generate the linker script contents.
+    contents = "INPUT(%s)" % ' '.join([args.input] + public_libs)
+    print("GENERATING SCRIPT: '%s' as file %s" % (contents, args.output))
 
-    # Prepare the list of public libraries to link.
-    public_libs = ['-l%s' % l for l in public_libs]
+    if args.dryrun:
+        return 0
+
+    # Remove the existing libc++ symlink if it exists.
+    if os.path.islink(args.output):
+        os.unlink(args.output)
 
-    # Generate the linker script contents and print the script and destination
-    # information.
-    contents = "INPUT(%s %s)" % (linked_libcxx, ' '.join(public_libs))
-    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")
+    # Replace it with the linker script.
+    with open(args.output, 'w') as f:
 
+    return 0
+
 
 if __name__ == '__main__':
-    main()
+    sys.exit(main())