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

[Peter Smith via llvm-dev]

One of the earliest discussions about the LLD as a library design, at least after it had matured enough to be practical was this rather long thread https://lists.llvm.org/pipermail/llvm-dev/2016-December/107981.html

I don’t have any objections about making LLD more useable as a library.

What I would say is that we should come up with a good idea of what the functionality needed from the library is. For example I can see one use case with a relatively coarse interface that is similar to running LLD from the command line, with object files passed in memory buffers. I can see that working as an extension of the existing design. A more ambitious use case would permit more fine grain control of the pipeline, or construct LLD data structures directly rather than using object files could require quite a bit more work. I think people are thinking along the lines of the former, but it is worth making sure.

I think one of the reasons the library use case faltered was that there was no-one with a use case that was able to spend enough time to make it happen. The existing maintainers had enough work to do to catch up with Gold and BFD. Do we have someone willing to do the work?

Peter

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of James Henderson via llvm-dev
Sent: 11 June 2021 08:42
To: Reid Kleckner <rnk at google.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; jezng at fb.com
Subject: Re: [llvm-dev] RFC: Revisiting LLD-as-a-library design

No objections here (although I don't have a specific use-case currently).

Regarding the error handling, I support some sort of callback approach to report the errors (https://www.youtube.com/watch?v=YSEY4pg1YB0). This doesn't solve the problem of what to do after a fatal error has been reported. In the debug line parsing code which inspired that talk, we had a concept of unrecoverable and recoverable errors, whereby the parser would either stop parsing if it found something it couldn't recover from, by bailing out of the function, or it would set some assumed values and continue parsing. This may work for some cases in LLD, but the fatal cases need to stop the linking completely, so we'll need some way to bail out of the LLD call stack in those cases still somehow - personally, I think we should use llvm::Error for that up to the point of public interface with the library, to avoid the failure being unchecked. The error callbacks could then return Error to allow a client to force LLD to stop, even if the error would otherwise be non-fatal.

James

On Thu, 10 Jun 2021 at 19:15, Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org<mailto: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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