[llvm-dev] lld-link with MSVC6 object files

Zachary Turner via llvm-dev
2019-09-30T22:01:51Z

I would expect it to be able to link the object file, even if it ignored
debug info.  It's a bit strange that it complains about bad file magic.

It might be tricky to get debug information working and produce a valid PDB
file since that is pretty old and the format has changed both with how it
was stored in the object file itself as well as the format of the PDB
file.

My guess is that the "magic" it's complaining about is not the
magic of the
object file itself but rather the first 4 bytes of the .debug$S (or was it
the .debug$T?) section.  Perhaps a simple fix in this case is that instead
of erroring out if we encounter an "older" magic, we just link as if
debug
info was not present to begin with.

This will at least make it work.  If you want to actually consume the debug
info though, you're in for a fun ride :)

On Mon, Sep 30, 2019 at 2:19 PM Paul Moran via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> It sounds like perhaps it might mostly work with some tweaks - given its
> complaining about bad file magic. I'll see if I can get lld-link to
build
> locally and hack out the magic checks to see if it works.
>
> On Mon, Sep 30, 2019 at 10:14 PM David Blaikie <dblaikie at
gmail.com> wrote:
>
>>
>>
>> On Mon, Sep 30, 2019 at 2:07 PM Paul Moran <bankybooks at
gmail.com> wrote:
>>
>>> MSVC 6 is 1998 not 1989 :)
>>>
>>
>> Ah, I just glanced briefly at the Wikipedia article (
>> https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B ) & misread
the
>> "C 6.0" and didn't notice it was distinct from
"Visual C++ 6.0" - thanks
>> for the catch!
>>
>>
>>>
>>> The latest MSVC linker can link these object files. Is this just
because
>>> it has support for C13 types and some other code path for whatever
MSVC6
>>> uses? After some digging around it appears to be this format:
>>>
>>>
>>>
https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#coff-file-header-object-and-image
>>>
>>>
>>> Which is COFF object file format? Does lld link support this
format?
>>>
>>
>> COFF is still the windows object file format, and the Windows support
in
>> lld is COFF support, yeah. I guess there might be some format
variations
>> that haven't been implemented in lld, though. It's mostly an
"on demand"
>> sort of approach.
>>
>>
>>>
>>>
>>> On Mon, Sep 30, 2019 at 7:39 PM Alexandre Ganea <
>>> alexandre.ganea at ubisoft.com> wrote:
>>>
>>>> The CodeView library in LLVM only supports Codeview C13 types,
that is,
>>>> MSVC 7.0 / Visual Studio 2002 or after.
>>>>
>>>>
>>>>
>>>> *De :* llvm-dev <llvm-dev-bounces at lists.llvm.org> *De
la part de*
>>>> David Blaikie via llvm-dev
>>>> *Envoyé :* September 30, 2019 2:38 PM
>>>> *À :* Paul Moran <bankybooks at gmail.com>; Rui Ueyama
<ruiu at google.com>
>>>> *Cc :* llvm-dev at lists.llvm.org
>>>> *Objet :* Re: [llvm-dev] lld-link with MSVC6 object files
>>>>
>>>>
>>>>
>>>> MSVC 6 as in the Visual Studio released in 1989? Yes, I imagine
that's
>>>> a bit outside the intended support window.
>>>>
>>>>
>>>>
>>>> On Mon, Sep 30, 2019 at 11:18 AM Paul Moran via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> Hi,
>>>>
>>>>
>>>>
>>>> I have a question about lld-link. What obj file formats should
it
>>>> support? When I try to use an obj from msvc 6.0 it complains
that the file
>>>> magic is not valid.
>>>>
>>>>
>>>>
>>>> However when running  llvm-objdump it reports:
>>>>
>>>>
>>>>
>>>> test1.obj:      file format COFF-i386
>>>>
>>>> Disassembly of section .text:
>>>> 0000000000000000 _main:
>>>>        0:       68 00 00 00 00  pushl   $0
>>>>        5:       e8 00 00 00 00  calll   0 <_main+0xa>
>>>>        a:       83 c4 04        addl    $4, %esp
>>>>        d:       33 c0   xorl    %eax, %eax
>>>>
>>>>       f:       c3      retl
>>>>
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> Paul
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
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>>>>
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