[llvm-dev] RFC: Revisiting LLD-as-a-library design

[Michael Spencer via llvm-dev]

Adding Erik (not subscribed) who has previously had issues with LLD not
being a library to provide some additional use cases.

- Michael Spencer


On Thu, Jun 10, 2021 at 12:15 PM Reid Kleckner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hey all,
>
> Long ago, the LLD project contributors decided that they weren't going to
> design LLD as a library, which stands in opposition to the way that the
> rest of LLVM strives to be a reusable library. Part of the reasoning was
> that, at the time, LLD wasn't done yet, and the top priority was to finish
> making LLD a fast, useful, usable product. If sacrificing reusability
> helped LLD achieve its project goals, the contributors at the time felt
> that was the right tradeoff, and that carried the day.
>
> However, it is now ${YEAR} 2021, and I think we ought to reconsider this
> design decision. LLD was a great success: it works, it is fast, it is
> simple, many users have adopted it, it has many ports
> (COFF/ELF/mingw/wasm/new MachO). Today, we have actual users who want to
> run the linker as a library, and they aren't satisfied with the option of
> launching a child process. Some users are interested in process reuse as a
> performance optimization, some are including the linker in the frontend.
> Who knows. I try not to pre-judge any of these efforts, I think we should
> do what we can to enable experimentation.
>
> So, concretely, what could change? The main points of reusability are:
> - Fatal errors and warnings exit the process without returning control to
> the caller
> - Conflicts over global variables between threads
>
> Error recovery is the big imposition here. To avoid a giant rewrite of all
> error handling code in LLD, I think we should *avoid* returning failure via
> the llvm::Error class or std::error_code. We should instead use an approach
> more like clang, where diagnostics are delivered to a diagnostic consumer
> on the side. The success of the link is determined by whether any errors
> were reported. Functions may return a simple success boolean in cases where
> higher level functions need to exit early. This has worked reasonably well
> for clang. The main failure mode here is that we miss an error check, and
> crash or report useless follow-on errors after an error that would normally
> have been fatal.
>
> Another motivation for all of this is increasing the use of parallelism in
> LLD. Emitting errors in parallel from threads and then exiting the process
> is risky business. A new diagnostic context or consumer could make this
> more reliable. MLIR has this issue as well, and I believe they use this
> pattern. They use some kind of thread shard index to order the diagnostics,
> LLD could do the same.
>
> Finally, we'd work to eliminate globals. I think this is mainly a small
> matter of programming (SMOP) and doesn't need much discussion, although the
> `make` template presents interesting challenges.
>
> Thoughts? Tomatoes? Flowers? I apologize for the lack of context links to
> the original discussions. It takes more time than I have to dig those up.
>
> Reid
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