[llvm-dev] [LLD] Adding WebAssembly support to lld

[Sam Clegg via llvm-dev]

On Fri, Jun 30, 2017 at 7:10 PM, Rui Ueyama <ruiu at google.com> wrote:
> Hi Sam,
>
> First, I want to know the symbol resolution semantics. I can imagine that
> that is set in stone yet, but just that you guys are still discussing what
> would be the best semantics or file format for the linkable wasm object
> file. I think by knowing more about the format and semantics, we can give
> you guys valuable feedback, as we've been actively working on the linker for
> a few years now. (And we know a lot of issues in existing object file
> format, so I don't want you guys to copy these failures.)

I've been aiming the match the semantics of native linkers in or order
to minimize porting efforts and allow existing software and build
systems to target WebAssembly.

Specifically, I'm trying to match what the ELF port of lld does in
terms of loading archives, objects and symbols including the
resolution of weak symbols.  Indeed, you can see that I borrow a large
amount of the SymbolTable code.  Like the other lld ports it does not
use the left-to-right-only strategy of symbol resolution.

> As Sean pointed out, this looks very different from ELF or COFF in object
> construction. Does this mean the linker has to reconstruct everything? The
> ELF and COFF linkers are multi-threaded, as each thread can work on
> different sections simultaneously when writing to an output file. I wonder
> if it's still doable in wasm.

It should be doable for the data and code sections.  For the other
section types (of which there are at least 8), we will most likely be
forced to reconstruct them fully and I'm not sure if that will be
parallelizable in the same say.  I'm hoping that doing code and data
and relocations in parallel will still be a big win.

>
> Also, I wonder if there's a way to parallelize symbol resolution. Since
> there's no linkable wasm programs, we can take a radical approach.
>
> Have you ever considered making the file format more efficiently than ELF or
> COFF so that they are linked really fast? For example, in order to avoid a
> lot of (possibly very long due to name mangling) symbols, you could store
> SHA hashes or something so that linkers are able to handle symbols as an
> array of fixed-size elements.
>
> That is just an example. There are a lot of possible improvements we can
> make for a completely new file format.

At this point I've mostly been focused on producing a working linker
that matches the semantics of existing native linkers.   My short goal
is to provide something that can replace the current bitcode linking
solution.   Perhaps I'm aiming too low :)

>
> On Fri, Jun 30, 2017 at 5:19 PM, Sean Silva via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>>
>> Can you link to docs about the wasm object format? (both relocatable and
>> executable)
>>
>> Also, traditional object file linkers are primarily concerned with
>> concatenating binary blobs with small amount of patching of said binary
>> blobs based on computed virtual (memory) addresses. Or perhaps to put it
>> another way, what traditional object file linkers do is construct program
>> images meant to be mapped directly into memory.
>>
>> My understanding is that wasm is pretty different from this (though
>> "linker frontend" things like the symbol resolution process is presumably
>> similar). Looking at Writer::run in your patch it seems like wasm is indeed
>> very different. E.g. the linker is aware of things like "types" and knowing
>> internal structure of functions (e.g. write_sig knows about how many
>> parameters a function has)
>>
>> Can you elaborate on semantically what the linker is actually doing for
>> wasm?
>>
>> -- Sean Silva
>>
>> On Fri, Jun 30, 2017 at 4:46 PM, Sam Clegg via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>>
>>> Hi llvmers,
>>>
>>> As you may know, work has been progressing on the experimental
>>> WebAssembly backend in llvm.  However,  there is currently not a good
>>> linking story.  Most the of existing linking strategies (i.e. those in
>>> the emscripten toolchain) involve bitcode linking and whole program
>>> compilation at link time.
>>>
>>> To improve this situation I've been working on adding a wasm backend
>>> for lld.   My current work is here: https://reviews.llvm.org/D34851
>>>
>>> Although this port is not ready for production use (its missing
>>> several key features such as comdat support and full support for weak
>>> aliases) its already getting a some testing on the wasm waterfall:
>>> https://wasm-stat.us/builders/linux
>>>
>>> I'm hopeful that my patch may now be at an MVP stage that could be
>>> considered for merging into upstream lld.  Thoughts?  LLD maintainers,
>>> would you support the addition of a new backend?
>>>
>>> cheers,
>>> sam
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
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>