[llvm-dev] getSymbolAddressInProcess returning null

Mon Apr 4 11:37:43 PDT 2016

Hi Russell,

Orc provides a way to supply your own custom code for resolving symbols -
> for some reason this is packaged with parameters for memory manager and a
> set of modules rather than a single module - maybe they figured instead of
> providing various custom options and complicating the API, best to just
> provide one big function that lets you customise everything, you can always
> supply basic default parameters for things you don't want to customise -
> which makes sense - and that's what Kaleidoscope uses.

You pretty much nailed it. While Orc and MCJIT provide the
same functionality, their design goals are different. MCJIT is a black-box
that takes in modules and spits out linked code. It's easy to use, but any
customization requires new API to be exposed to enable/disable it, and
because any change to the black box affects all MCJIT clients changes are
considered relatively high-risk. Orc was designed as a component-library
for building things like MCJIT - you take the bits you want off the shelf,
wire them together, and you've got a custom JIT that does what you want
without affecting any other JIT users - changes and customization are cheap
and low-risk. In that spirit, addModuleSet provides a very flexible
interface which you can then wrap in something more friendly if you don't
need all its features.

Orc also provides a class that contains a basic system for resolving
> symbols if you don't want to customise, and that works by just supplying it
> with each symbol you're going to want to look up, together with the
> associated function pointer - and that's what you used.

MappingLayer<T> provides a StringMap symbol table, similar to what I
suggested in a previous email. Once you've added a symbol, it will appear
in findSymbols as if you'd JIT'd it. When your resolver looks back into the
JIT to search for definitions (a standard first step for a resolver) it'll
find anything you've mapped.

I prefer customizing the resolver (as in the example I gave in the other
thread), rather than using the mapping layer. It feels cleaner to keep a
distinction between the symbols owned by the JIT (which are the only ones
you can find with findSymbols, unless you add the mapping layer), and the
symbols the JIT needs (i.e. those found via the resolver). That said,
there's no real performance difference and it's easy to switch between one
scheme and the other once you have something that works, so go with
whatever works for you.

Cheers,
Lang.

On Sat, Apr 2, 2016 at 7:19 AM, Russell Wallace via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks! Comparing your code to Kaleidoscope, they are superficially very
> different but when you boil it down, you can see the common elements. I
> think the most significant difference is that where you have
>
> auto compiler = std::make_unique<SimpleOrcJit>(*targetMachine);
> compiler->addGlobalMapping("myAlloc", (void*)myAlloc);
>
> Kaleidoscope has
>
>   auto resolver = createLambdaResolver(
>       [&](const std::string &name) {
>         if (auto sym = compileLayer->findSymbol(name, false))
>           return RuntimeDyld::SymbolInfo(sym.getAddress(), sym.getFlags());
>         return RuntimeDyld::SymbolInfo(nullptr);
>       },
>       [](const std::string &S) { return nullptr; });
>   compileLayer->addModuleSet(std::vector<Module *>(1, module),
>                              new SectionMemoryManager(),
> std::move(resolver));
>
> I'm going to guess the reason for this difference is something along the
> lines of:
>
> Orc provides a way to supply your own custom code for resolving symbols -
> for some reason this is packaged with parameters for memory manager and a
> set of modules rather than a single module - maybe they figured instead of
> providing various custom options and complicating the API, best to just
> provide one big function that lets you customise everything, you can always
> supply basic default parameters for things you don't want to customise -
> which makes sense - and that's what Kaleidoscope uses.
>
> Orc also provides a class that contains a basic system for resolving
> symbols if you don't want to customise, and that works by just supplying it
> with each symbol you're going to want to look up, together with the
> associated function pointer - and that's what you used.
>
> Is that a more or less accurate summary of the situation?
>
>
> On Sat, Apr 2, 2016 at 12:53 PM, Stefan Gränitz <llvm-dev at lists.llvm.org>
> wrote:
>
>> Hi Russel, I guess you're using ORC? Then you need a mapping layer. Have
>> a look at this line:
>>
>> https://github.com/weliveindetail/JitFromScratch/blob/example/own-externals/JitFromScratch.cpp#L38
>>
>> Best,
>> Stefan
>>
>> Am 02.04.16 um 11:43 schrieb Russell Wallace via llvm-dev:
>>
>> Tried that, still didn't work. Then I tried making a direct API call,
>>
>> GetProcAddress(GetModuleHandle(0),"foo")
>>
>> And this works if and only if __declspec(dllexport) is supplied. So it
>> looks like we were both right.
>>
>> On Sat, Apr 2, 2016 at 9:29 AM, NAKAMURA Takumi <geek4civic at gmail.com>
>> wrote:
>>
>>> Have you tried to add dllexport?
>>>
>>> On Sat, Apr 2, 2016 at 4:23 PM Russell Wallace via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> I've finally managed to extract from Kaleidoscope one particular thing
>>>> that it seems to me should be working and isn't. Given the global
>>>> declaration
>>>>
>>>> extern "C" void foo() {}
>>>>
>>>> within the same program I have
>>>>
>>>> RTDyldMemoryManager::getSymbolAddressInProcess("foo")
>>>>
>>>> And it's returning null. (LLVM 3.8, Windows 7 x64.) What am I missing?
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>>
>>
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>>
>>
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>
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