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

pawel k. via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 11 23:00:03 PDT 2021


Hello all,
Do I understand correctly lld is binary linker? Does it take arch/os
specific or llvm-specific object files as input or both?

Im very young devel llvm- and clang-wise albeit if libraryfication of lld
doesnt involve too heavy algorithmical and architectural changes and
someone can intro me on working with git (which branch to start from on
which branch to work whether to create separate branch and how), i could
try to prepare working prototype of this solution. Please note though im
still learning.

For macho code, we could use samples in testsuite where I could make sure
nothing breaks after my changes. We could use some such samples too for
lto. In perfect world, we could use some testset too for each arch/os combo.

As on avoiding merge conflicts with current devel including macho code, id
suggest finnishing work there and stabilizing lld first, then when no new
code is changed heavily or introduced anymore, i could try to focus on
architecture rework safely.

As on unification with jit or lto as was mentioned, i dont know enough
about this task yet.

As on avoiding/fixing memory leaks between compilation units etc, i think i
could try to fix those.

Please note though I still dont have clear view as on how to make lld
process more parallel so I may be not yet usable for this project.

Best regards,
Pawel

sob., 12.06.2021, 05:20 u┼╝ytkownik Michael Spencer via llvm-dev <
llvm-dev at lists.llvm.org> napisał:

> 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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>>
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