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

Petr Hosek via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 11 10:58:49 PDT 2021

That can be controlled, see

On Fri, Jun 11, 2021 at 6:18 AM Jean-Daniel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> One other point to consider for library usage is memory management.
> I don’t know the state of LLD  today, but I think it was purposefully
> leaking memory as a tradeoff for performance at some point in the past (why
> deallocating everything before exiting main as the system will take care of
> this).
> This presentation even says:
> What makes LLD so fast? - Previous FOSDEM
> Editionshttps://archive.fosdem.org › llvm_lld › slides › W…
> <https://archive.fosdem.org/2019/schedule/event/llvm_lld/attachments/slides/3423/export/events/attachments/llvm_lld/slides/3423/WhatMakesLLDSoFastPresenterNotes.>
> “ • Large amounts of memory allocated up front and never released. »
> Le 11 juin 2021 à 12:56, Peter Smith via llvm-dev <llvm-dev at lists.llvm.org>
> a écrit :
> 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> 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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