[cfe-dev] JIT doens't resolve address

[Lang Hames via cfe-dev]

HI Bjoern,

There are two kinds of symbol lookup in the JIT:

(1) You want to find a symbol defined JIT'd code. This is what "findSymbol"
on the JIT class does.

(2) The JIT wants to fix up a module that contains references to symbols
defined elsewhere (either in your program, or in other JIT'd code). This is
the SymbolResolver's job.

So assuming your main function in main.ll is trivial, e.g.

int main() {
  return 0;
}

then your findSymbol call will return the address of the JIT'd main without
ever needing to look anything up in the resolver.

If, on the other hand, your main function contains an external reference,
e.g.

int main() {
  printf("Hello, World!"); // Reference to externally defined function
printf.
  return 0;
}

Then you'll receive a call back on your resolver looking for the address of
printf.

Hope this helps!

Cheers,
Lang.

On Thu, Apr 20, 2017 at 6:43 AM, via cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Hello LLVM-World,
>
> I was following the "Building a JIT in LLVM"-Tutorial and tried to load a
> normal main. My code is the following:
> class Jitter
> {
> private:
>   std::unique_ptr<TargetMachine> TM;
>   const DataLayout DL;
>   ObjectLinkingLayer<> ObjectLayer;
>   IRCompileLayer<decltype(ObjectLayer)> CompileLayer;
>
> public:
>   typedef decltype(CompileLayer)::ModuleSetHandleT ModuleHandle;
>
>   Jitter() : TM(EngineBuilder().selectTarget()), DL(TM->
> createDataLayout()),
>       CompileLayer(ObjectLayer, SimpleCompiler(*TM))
>   {printf("!");
>         llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr);
>   }
>
>   TargetMachine &getTargetMachine() { return *TM; }
>
>   ModuleHandle addModule(std::unique_ptr<Module> &&M) {
>   // Build our symbol resolver:
>   // Lambda 1: Look back into the JIT itself to find symbols that are
> part of
>   //           the same "logical dylib".
>   // Lambda 2: Search for external symbols in the host process.
>   auto Resolver = createLambdaResolver(
>       [&](const std::string &Name)
>       {
>                 printf("FLUSH :0\n");
>
>         if (auto Sym = CompileLayer.findSymbol(Name, false))
>           return Sym;
>         return JITSymbol(nullptr);
>       },
>       [](const std::string &S)
>           {
>                   printf("PLUSH :0\n");
>
>         if (auto SymAddr =
>               RTDyldMemoryManager::getSymbolAddressInProcess(S))
>           return JITSymbol(SymAddr, JITSymbolFlags::Exported);
>         return JITSymbol(nullptr);
>       });
>
>   // Build a singleton module set to hold our module.
>   std::vector<std::unique_ptr<Module>> Ms;
>   Ms.push_back(std::move(M));
>
>   // Add the set to the JIT with the resolver we created above and a newly
>   // created SectionMemoryManager.
>   return CompileLayer.addModuleSet(std::move(Ms),
>                                    make_unique<SectionMemoryManager>(),
>                                    std::move(Resolver));
> }
>
> JITSymbol findSymbol(const std::string Name) {
>   std::string MangledName;
>   raw_string_ostream MangledNameStream(MangledName);
>   Mangler::getNameWithPrefix(MangledNameStream, Name, DL);
>   printf("Tzearch for: %s\n\n", MangledNameStream.str());
>   return CompileLayer.findSymbol(MangledNameStream.str(), false);
> }
>
> void removeModule(ModuleHandle H) {
>   CompileLayer.removeModuleSet(H);
> }
>
> };
>
> And calling from main with:
>
> int main()
> {
>         llvm::InitializeNativeTarget();
>         llvm::InitializeNativeTargetAsmPrinter();
>         llvm::InitializeNativeTargetAsmParser();
>
>         llvm::LLVMContext context;
>         llvm::SMDiagnostic dia;
>
>         std::unique_ptr<llvm::Module> M = llvm::parseIRFile("./jit_main.
> ll", dia, context);
>         Jitter jit;
>         printf("Wuff?");
>         Jitter::ModuleHandle h = jit.addModule(std::move(M));
>         printf("KNUFF!\n");
>
>         printf("Kuchen! 0x%p\n", jit.findSymbol("main").getAddress());
>
>         system("PAUSE");
>         return 0;
> }
> The Code runs without a fail, but when the programm tries to resolve
> "main" the address is 0. The strange thing: the printf "FLUSH :0\n" and "PLUSH
> :0\n" are never called, so did the code never compiled? What I'm doing
> wrong?
>
> Kind regards
> Björn
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789
> Geschäftsführer: Hiroshi Kawamura, Dr Hiroshi Nakamura, Markus Bode, Heiko
> Lampert, Takashi Nagano, Takeshi Fukushima.
> _______________________________________________
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> cfe-dev at lists.llvm.org
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>
>
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