[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

7 ↗

(On Diff #193837)

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

36 ↗

(On Diff #193837)

stray debugging leftover?

38 ↗

(On Diff #193837)

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

41 ↗

(On Diff #193837)

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

62 ↗

(On Diff #193837)

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

38 ↗

(On Diff #193837)

(unreplied)

62 ↗

(On Diff #193837)

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

7 ↗

(On Diff #193837)

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

38 ↗

(On Diff #193837)

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 ↗

(On Diff #193837)

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

38 ↗

(On Diff #193837)

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.

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")

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())