[llvm-dev] Extending llvm-objcopy to support COFF

Tue Mar 13 00:06:20 PDT 2018

Hey everyone,

Sorry to jump in on this so late. My two cents is that it should remain GNU
objoppy compatible most likely. It was always vaguely a desire to have
command line compatibility but it has turned out over time that this is
actually a crucial feature and should be one of the top priorities. You
can't just go into a giant build system and swap out all the uses of GNU
objcopy with their llvm-objcopy equivalents. This is the whole reason I'm
adding the llvm-strip interface. We know of multiple instances where GNU
strip is used in large systems and it's non-trivial to replace every last
individual usage of GNU strip with the corresponding llvm-objcopy use. We
know of even more instances where GNU objcopy is super hard to replace.
Most of the users of/people who want to use llvm-objcopy need it to be a
drop in replacement. This all said, GNU objcopy hasn't generally been used
on windows platforms, so this might be a moot point. I suppose to convince
me otherwise you'd have to convince me that supporting the GNU objcopy
options in any reasonable way was detrimental to support for COFF in
llvm-objcopy. COFF specific options are fine, we just need to also support
the non-ELF specific GNU objcopy options. I'm sure there are several
options that llvm-objcopy already supports that just don't make sense on
other formats. In short, you can add extra options but you'd have to
convince me that not supporting most of the GNU objcopy options was a bad
idea.

Separate thoughts on how to structure handeling other formats:

When I started on this project it was recommended to me that I not try and
abstract the operations out over different binary formats. I firmly believe
that was good advice now. I've since imagined setting things up in an LLD
like fashion and having an ELF, COFF, and MachO directory which would more
or less function separately. There are some details to be hashed out on how
exactly that should happen and how the main executable should dispatch to
the different implementations however.

Please assume that tablegen will be used going forward despite that fact
that it isn't now. From there, there are two options:

1. Have a different .td file for every llvm-{objcopy, strip} and {ELF,
COFF, MachO} combination with heavy use of common .td files
2. Only have different .td files for llvm-objcopy and llvm-strip and have
ELF, COFF, and MachO specific options emit errors on non-supported formats

I'm in favor of option 2 but I could be convinced of option 1 as well,
especially if you also convinced me that GNU objcopy was too ELF centric
and that supporting the GNU objcopy options wasn't worth it.

Best,
Jake

On Thu, Mar 8, 2018 at 1:22 AM James Henderson <jh7370.2008 at my.bristol.ac.uk>
wrote:

> Hi,
>
> It's not clear to me what you mean by CLI "subcommands". Would you mind
> giving a brief example?
>
> Up to now, we've been trying to keep llvm-objcopy as close as possible to
> GNU objcopy, to make transitioning between them easier (I'm thinking in
> particular things like DWO generation). There are a small number of edge
> cases/unusual behaviours that we have chosen not to support whilst doing
> this. I guess the obvious question from me is do we need the same drop-in
> replacement for COFF support?
>
> Also, Jake has just posted up a fairly major refactor of the existing
> command-line interface on Phabricator:
>
> https://reviews.llvm.org/D44236
>
> I don't know how your proposal would interact with this, but if the
> approach we're looking at taking looks like this would just make life
> harder, please shout!
>
> James
>
> On 7 March 2018 at 20:34, Saleem Abdulrasool via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> On Wed, Mar 7, 2018 at 9:56 AM Eric Christopher via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi Zach!
>>>
>>> I've been thinking a bit about this for a while now and I'm still of two
>>> opinions:
>>>
>>> On Wed, Mar 7, 2018 at 9:21 AM Zachary Turner via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Currently llvm-objcopy only supports ELF files, and most of it's
>>>> command line flags are ELF / DWARF specific that don't make any sense on
>>>> COFF files.  So a useful set of options for COFF would be largely disjoint,
>>>> with maybe 1-2 overlapping options.  What would be the best way to add this
>>>> in llvm-objcopy?  I can think of 3 options:
>>>>
>>>> 1) Re-write the existing CLI of llvm-objcopy to use subcommands, and
>>>> put the current set of options behind an ELF subcommand.  To me this is the
>>>> cleanest approach, but it's also the most disruptive, as existing users of
>>>> llvm-objcopy would have to retrain themselves to use this new subcommand,
>>>> and tools / scripts may have to be updated as wells.
>>>>
>>>>
>>> I really like this option. I like orderly commands and having an ELF
>>> subcommand is really nice, however...
>>>
>>>
>>
>> While this is tempting, I don’t think that we can break compatibility
>> with existing tools as it is intended to be a replacement.  However, we
>> could do something like we did with readobj and readelf.  Note that the obj
>> copy from binutils also works with COFF, so that should be supported too.
>>
>> 2) Throw in all of the COFF options to the current llvm-objcopy, and just
>>>> have them be mixed with the ELF options.  I think this makes the tool more
>>>> difficult to use and more confusing, but it is admittedly the simplest
>>>> approach.
>>>>
>>>>
>>> This is the sort of thing that people expect from using gnu objcopy and
>>> so I'm reticent to have a tool with no way to get the "command line
>>> expected syntax".
>>>
>>> Mostly what I want is 1 with a shim that gets me 2.
>>>
>>> Thoughts?
>>>
>>
>> I think we are thinking more or less the same thing.
>>
>>
>>> -eric
>>>
>>>
>>>> 3) Make a new tool called llvm-coffcopy / llvm-objcopy-coff, or
>>>> something to that effect.
>>>>
>>>> Anyone have any thoughts or strong preferences?
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
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>>>
>> --
>> Saleem Abdulrasool
>> compnerd (at) compnerd (dot) org
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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