[llvm-dev] RFC: ELF Autolinking

[Rui Ueyama via llvm-dev]

This proposal seems much better than the generic .linker-options scheme
that potentially allows arbitrary linker options to be embedded to an
object file. The proposed scheme is basically the same mechanism as the
"comment lib" feature implemented on Microsoft linker, which I found mildly
useful and at least not harmful.

As a use case, what I heard of was that in the game industry where many
developers are using Visual Studio as an IDE and familiar with Windows'
semantics of linking, people find it annoying that to build the same
program on Unix, they had to add bunch of -lfoo to the linker command line
while they are automatically handled on Windows. I can understand that --
if you have to add `-lm` 99.9% of the time when #include <math.h> for
example, that's not too odd to think why this is not processed
automatically.

But the above story was from the game industry. Just like Ben, I'd like to
hear from other people if they really want this feature.

Details inline:

On Thu, Mar 14, 2019 at 6:08 AM bd1976 llvm via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>

I thought that the scope of this mechanism is essentially to add `-lfoo`
automatically to the command line if you include a header that requires
`libfoo`. From that perspective, the item 3 seems odd. Why do you need that?

4. Duplicate autolinked inputs are ignored.
>

I'd say duplicate autolinked inputs are processed normally, but because of
the same reason why  the second parameter in `-lfoo -lfoo` is basically
no-op, duplicated autolinked inputs are naturally 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.
>

Again, this seems like a little beyond the scope of what I expect (and it
looks like you want to allow an .o file using this scheme).


> 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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