[llvm-dev] Relocation design of different architecture

[mats petersson via llvm-dev]

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
COFF::IMAGE_REL_AMD64_*.

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
>
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