[cfe-dev] JIT doens't resolve address

[via cfe-dev]

Hello Lang,

the Code for my jit_main is a normal printf-HelloWorld program. I opended 
the file with notepad and found the following:


; Function Attrs: noinline norecurse uwtable
define i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([35 x 
i8], [35 x i8]* 
@"\01??_C at _0CD@CMJEAMCD at Planschbeckilein?5sagt?5Halloilein@", i32 0, i32 
0))
  %call1 = call i32 @system(i8* getelementptr inbounds ([6 x i8], [6 x 
i8]* @"\01??_C at _05DIAHPDGL@PAUSE?$AA@", i32 0, i32 0))
  ret i32 0
}

So... there is a main... Anyway! The code for the main is this one:

#include <stdio.h>
#include <stdlib.h>

int main()
{
        printf("Planschbeckilein sagt Halloilein!\n");
        system("PAUSE");

        return 0;
}

Kind regards
Björn



From:   Lang Hames <lhames at gmail.com>
To:     bjoern.gaier at horiba.com
Cc:     Clang Dev <cfe-dev at lists.llvm.org>
Date:   20.04.2017 22:54
Subject:        Re: [cfe-dev] JIT doens't resolve address



Hi Bjoern,

Sorry - I just noticed that the address for your "main" function has come 
back as '0'. In this case the answer is even simpler: The JIT never found 
the function at all, and so definitely didn't have any external symbols to 
resolve, so it never called the resolver.

A failure to find main has three likely causes:

(1) jit_main.ll does not define main,
(2) jit_main.ll defines main with private/internal linkage (the JIT can't 
see private/internal symbols in general)
(3) jit_main.ll defines main with external linkage, but a system mangling 
is applied (e.g. on MacOSX 'main' is mangled to '_main'). The mangler code 
in your findSymbol function *should* correct for this, but this may fail 
if the default data layout for your TargetMachine varies from the Module's 
DataLayout.

Can you share the contents of your jit_main.ll Module?

Cheers,
Lang.
   

On Thu, Apr 20, 2017 at 1:35 PM, Lang Hames <lhames at gmail.com> wrote:
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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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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