[llvm-dev] RFC: ELF Autolinking

[bd1976 llvm via llvm-dev]

At Sony we offer autolinking as a feature in our ELF toolchain. We would
like to see full support for this feature upstream as there is anecdotal
evidence that it would find use beyond Sony.

In general autolinking (https://en.wikipedia.org/wiki/Auto-linking) allows
developers to specify inputs to the linker in their source code. LLVM and
Clang already have support for autolinking on ELF via embedding strings,
which specify linker behavior, into a .linker-options section in
relocatable object files, see:

RFC - http://lists.llvm.org/pipermail/llvm-dev/2018-January/120101.html
LLVM -
https://llvm.org/docs/Extensions.html#linker-options-section-linker-options,
https://reviews.llvm.org/D40849
Clang -
https://clang.llvm.org/docs/LanguageExtensions.html#specifying-linker-options-on-elf-targets,
https://reviews.llvm.org/D42758

However, although support was added to Clang and LLVM, no support has been
implemented in LLD; and, I get the sense, from reading the reviews, that
there wasn't agreement on the implementation when the changes landed. The
original motivation seems to have been to remove the "autolink-extract"
mechanism used by Swift to workaround the lack of autolinking support for
ELF. However, looking at the Swift source code, Swift still seems to be
using the "autolink-extract" method.

So my first question: Are there any users of the current implementation for
ELF?

Assuming that no one is using the current code, I would like to suggest a
different mechanism for autolinking.

For ELF we need limited autolinking support. Specifically, we only need
support for "comment lib" pragmas (
https://docs.microsoft.com/en-us/cpp/preprocessor/comment-c-cpp?view=vs-2017)
in C/C++ e.g. #pragma comment(lib, "foo"). My suggestion that we keep the
implementation as lean as possible.

Principles to guide the implementation:
- Developers should be able to easily understand autolinking behavior.
- Developers should be able to override autolinking from the linker command
line.
- Inputs specified via pragmas should be handled in a general way to allow
the same source code to work in different environments.

I would like to propose that we focus on autolinking exclusively and that
we divorce the implementation from the idea of "linker options" which, by
nature, would tie source code to the vagaries of particular linkers. I
don't see much value in supporting other linker operations so I suggest
that the binary representation be a mergable string section (SHF_MERGE,
SHF_STRINGS), called .autolink, with custom type SHT_LLVM_AUTOLINK
(0x6fff4c04), and SHF_EXCLUDE set (to avoid the contents appearing in the
output). The compiler can form this section by concatenating the arguments
of the "comment lib" pragmas in the order they are encountered. Partial
(-r, -Ur) links can be handled by concatenating .autolink sections with the
normal mergeable string section rules. The current .linker-options can
remain (or be removed); but, "comment lib" pragmas for ELF should be
lowered to .autolink not to .linker-options. This makes sense as there is
no linker option that "comment lib" pragmas map directly to. As an example,
#pragma comment(lib, "foo") would result in:

.section ".autolink","eMS", at llvm_autolink,1
        .asciz "foo"

For LTO, equivalent information to the contents of a the .autolink section
will be written to the IRSymtab so that it is available to the linker for
symbol resolution.

The linker will process the .autolink strings in the following way:

1. Inputs from the .autolink sections of a relocatable object file are
added when the linker decides to include that file (which could itself be
in a library) in the link. Autolinked inputs behave as if they were
appended to the command line as a group after all other options. As a
consequence the set of autolinked libraries are searched last to resolve
symbols.
2. It is an error if a file cannot be found for a given string.
3. Any command line options in effect at the end of the command line
parsing apply to autolinked inputs, e.g. --whole-archive.
4. Duplicate autolinked inputs are ignored.
5. The linker tries to add a library or relocatable object file from each
of the strings in a .autolink section by; first, handling the string as if
it was specified on the commandline; second, by looking for the string in
each of the library search paths in turn; third, by looking for a
lib<string>.a or lib<string>.so (depending on the current mode of the
linker) in each of the library search paths.
6. A new command line option --no-llvm-autolink will tell LLD to ignore the
.autolink sections.

Rationale for the above points:

1. Adding the autolinked inputs last makes the process simple to understand
from a developers perspective. All linkers are able to implement this
scheme.
2. Error-ing for libraries that are not found seems like better behavior
than failing the link during symbol resolution.
3. It seems useful for the user to be able to apply command line options
which will affect all of the autolinked input files. There is a potential
problem of surprise for developers, who might not realize that these
options would apply to the "invisible" autolinked input files; however,
despite the potential for surprise, this is easy for developers to reason
about and gives developers the control that they may require.
4. Unlike on the command line it is probably easy to include the same input
file twice via pragmas and might be a pain to fix; think of Third-party
libraries supplied as binaries.
5. This algorithm takes into account all of the different ways that ELF
linkers find input files. The different search methods are tried by the
linker in most obvious to least obvious order.
6. I considered adding finer grained control over which .autolink inputs
were ignored (e.g. MSVC has /nodefaultlib:<library>); however, I concluded
that this is not necessary: if finer control is required developers can
recreate the same effect autolinking would have had using command line
options.

Thoughts?
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