[llvm-dev] Relocation design of different architecture

mats petersson via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 20 06:40:19 PDT 2017

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to answer.

Like I've tried to explain, there is a generic piece of code that
understands how to load code in general (the class RuntimeDyld and related
bits), and then specific implementations that derive from a base class to
do the specific relocation and exception handling for that particular
hardware and file-format - for each supported processor architecture and
file-format, there needs to be a specific class that implements some
functions (processRelocationRef is one of those). Technically, it looks
like it's using a "pImpl" pattern, but the basic principle is the same
either way - generic code handles the generic case, a derived class that
understands how to deal with the specifics is used to actually perform
relocations in that particular case.

Exception handling is also target-specific, so in x86-64 and i386, how
exception information is stored and used is different (I don't know the
exact details in this case as COFF is the file-format used on Windows, and
it's been at least 8 or 10 years since I did any programming at all on a
Windows machine, I know that i386 on Linux uses an exception table, and
x86-64 on linux essentially has debug information [DWARF tables]). The
exception information is used to determine how to unwind the stack and
destroy objects on the way back to the "catch" for that particular
exception. There is code required both to load the exception tables into
memory, and to interpret/use those tables - but I'm not overly familiar
with how that works for JIT'd code. [Actually, looking at the code for
x86-64, it looks like it's mainly SEH (Structured Exception Handling) that
is dealt with - the overall concept still applies, but SEH is a Windows
concept for handling exceptions, which includes hardware exceptions such as
integer division by zero and memory access exceptions - regular C++
exceptions are dealt with separately, and that is what uses what I
described for Linux earlier in this paragraph].

As to WHY different architectures use different relocations and exception
handling tables, that's an ABI design issue - a convention that is based on
the needs and requirements for each architecture, and a bunch of
compromises between simplicity (a very simple table is easy to construct),
space (simple table takes up more space than a more complex table
construction - like a zip file or a text file - the zip file is more
complicated to read, but takes up a lot less space) and code complexity
(save space in table, more complex code most likely). Either way, for a
given platform (OS, Processor, file format), there is a given ABI for
handling exceptions. The loader needs to load the table in the correct way
into the correct part of memory, and when an exception is thrown, the
table(s) need to be understood and acted upon to find the way back to the
relevant place where the exception is caught.

The fact that the classes are declared in different files is similar to my
simple animal example, where you'd have a animal.h for the base class, a
cat.h, dog.h and fish.h for the actual implementations. Obviously, the
specific implementations for the RuntimeDyld belongs in "Target" because
they are dependent on the actual target (which is the combination of
fileformat, OS and processor architecture).


On 20 April 2017 at 14:04, Siddharth Shankar Swain <
h2015096 at pilani.bits-pilani.ac.in> wrote:

> So RuntimeDyldELF.cpp or RuntimeDyldCOFF.cpp or RuntimeDyldMachO.cpp  are
> doing relocation and linking for specific object file format and all
> architectures using that object file format. Am i correct? If that is so
> then these  .cpp files are not using any header file in Targets/ so what
> are these header files in Targets/ made for ? Another thing is that why
> these header files in Targets/ handling exception and that too using a
> different concept of exception frames and exception tables. Please guide
> Thanks,
> Siddharth
> On Thu, Apr 20, 2017 at 6:06 PM, mats petersson <mats at planetcatfish.com>
> wrote:
>> Basic Object Oriented design uses a derived class to implement a
>> functionality of the generic case. It's the same basic principle as:
>> class Animal
>> {
>>     void virtual Say() = 0;
>> };
>> class Cat: public Animal
>> {
>>     void Say() override { cout << "Meow!" << endl; }
>> }
>> class Dog: public Animal
>> {
>>     void Say() override { cout << "Woof!" << endl; }
>> }
>> class Fish: public Animal
>> {
>>     void Say() override { cout << "Blub!" << endl; }
>> }
>> In this case, different types of COFF-architectures use different
>> relocation entries, and based on the architecture, a specific
>> implementation of the RelocationDyldCOFF class is created to perform the
>> relocation.
>> See http://llvm.org/docs/doxygen/html/classllvm_1_1RuntimeDyldCOFF.html
>> for a class diagram of how this is done.
>> The generic code in RuntimeDyld*.cpp only knows that relocations exists,
>> and that they need to be dealt with. Not HOW to actually perform the
>> relocation - just like "Animal" doesn't know what a cat or a dog "says".
>> The processRelocationRef() is called here:
>> http://llvm.org/docs/doxygen/html/RuntimeDyld_8cpp_source.html#l00251
>> Again, it's not clear exactly what you are asking for, so I'm not sure
>> whether my explanation is helpful or not...
>> --
>> Mats
>> On 20 April 2017 at 12:05, Siddharth Shankar Swain <
>> h2015096 at pilani.bits-pilani.ac.in> wrote:
>>> 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 ?
>>> Thanks,
>>> Siddharth
>>> On Thu, Apr 20, 2017 at 4:02 PM, mats petersson <mats at planetcatfish.com>
>>> wrote:
>>>> 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
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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