[llvm-dev] Relocation design of different architecture

Siddharth Shankar Swain via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 20 04:05:56 PDT 2017

Thanks for the reply. I was just asking about in general whatever header
files are there in Targets/ for different architectures are not including
any function except this processRelocationRef() to be used in
RuntimeDyldELF.cpp or RuntimeDyldCOFF.cpp or RuntimeDyldMachO.cpp and i
think these files are the ones which are actually doing the relocation and
linking work. So what purpose do these header files inside Targets/
actually serve. Also they include exception handling in form of exception
frames, So can u guide on this issue ?


On Thu, Apr 20, 2017 at 4:02 PM, mats petersson <mats at planetcatfish.com>

> The x86_64 and i386 architectures have different actual relocation
> records. So if you build code for i386, you need one processRelocationRef()
> function (handling the relevant relocations in that model), and when
> producing code for x86_64, there are different relocation records. The two
> files contain the derived form of the class that processes the relocation
> records when dynamically loading JITed code in LLVM - mainly implementing
> the two different forms of symbol entries that refer to the relocations -
> i386 uses COFF::IMAGE_REL_I386_*, in x86_64 the relocation types are
> Conceptually, they do the same thing, it's the details of exactly how and
> where the relocation ends up and how it's recorded by the linker that
> differs.
> Theoretically, one could probably construct a loadable file that doesn't
> care what architecture it is for, but it would end up with a lot of
> redundant & overlapping functionality, and the code to handle every
> different architecture in one huge switch-statement would be rather complex
> (and long!). So splitting the functionality per architecture helps make the
> code clear.
> If you need further help to understand the code, you'll probably need to
> ask a more concrete question, as it is probably not possible to describe
> all the relevant information on this subject in less than 200 pages, never
> mind a simple email-thread.
> --
> Mats
> On 20 April 2017 at 11:13, Siddharth Shankar Swain via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi,
>> Can anyone explain in lib/ExecutionEngine/RuntimeDyld/Targets/ the
>> header files included for different architectures like
>> RuntimeDyldCOFFX86_64.h or RuntimeDyldCOFFI386.h etc, what is the
>> connection of these files for relocation and linking as the linking and
>> relocation for diff architecture is done in RuntimeDyldELF.cpp,
>> RuntimeDyldCOFF.cpp  and it doesn't use any function from these header file
>> except the processRelocationRef(). The header files in Targets/ also
>> handles exceptions, so what is the need for that in relocation and linking
>> process ? Also please help with what this processRelocationRef() actually
>> does ? . Please guide.
>> sincerely,
>> Siddharth
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
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