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List:       llvm-dev
Subject:    Re: [LLVMdev] Is lld the linker we need for our project ?
From:       Michael Spencer <bigcheesegs () gmail ! com>
Date:       2014-03-14 10:13:44
Message-ID: CACXTSinPtPfNa99aU6ua_yqwNkgrv-PNX-vvHLd2N5cJ+DtqFA () mail ! gmail ! com
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On Fri, Mar 14, 2014 at 1:59 AM, Rodolphe Suescun <rsu.medal@dolphin.fr> wrote:
> Hi,
>
> Thanks a lot for your answer. It seems lld is still the best
> solution, even if it does not work "right out of the box" for
> us today.
>
> We already have a solution for the "objcopy" part (added the
> required output format to llvm-objdump).

Interesting solution. Would it be much work to implement an
llvm-objcopy that only supports -O binary? I plan to fully implement
objcopy one of these days, but having just that as a start would be
very useful.

>
> The ScriptLayout class seems to be empty for now (on the master
> branch at least), but we do not need linker scripts today.
> All that is required for now is to be able to assign a fixed
> address to a few atoms (the ones that will hold the reset &
> interrupt vectors) and place code/data sections in code/data
> memory (so we can simulate generated code and fix and optimize
> our LLVM target).
> I guess that can be done by adapting the DefaultLayout code in
> our own Layout class, but any hint or documentation about how
> to do this in a clean manner is welcome.

If you're willing to hard code your target into lld that works fine.
It's what we are currently doing for the default linker script for
glibc/linux. There's not really any documentation about how to do
this. I'll try and figure out the basics tomorrow, as it's quite late.
I do know that there will be some issues with getting the base segment
to not start at the ELF file header. Shankar should also be familiar
with this.

>
> I have another question: are there any plans concerning debug
> information ? In v3.4, the documentation says lld does not
> support DWARF info, although there are debug-related sections
> in lld's output.

lld currently passes through DWARF sections correctly and executables
are debuggable. However It does not merge debug info or construct any
acceleration tables. So debugging works fine, you just get giant debug
sections.

>
>
> Thanks a lot in advance,
> Rod

- Michael Spencer

>
>
> Le 11/03/2014 20:39, Michael Spencer a écrit :
>
>> It sounds like you need linker scripts and objcopy -O binary.
>>
>> lld currently has very limited linker script support. Support for
>> layout using linker scripts would need to be completed. It is
>> currently stubbed out in DefaultLayout.h:class ScriptLayout. I've
>> cc'ed Shankar who knows more about how that is intended to work.
>>
>> Both Shankar and I intend to implement linker scripts eventually, but
>> it's not a high priority for me right now.
>>
>> So to answer your question. If you need a linker right now lld isn't
>> really going to work for you, but your use case is definitely in scope
>> for lld.
>>
>> - Michael Spencer
>>
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