CODEWITHSUNDEEP
llvmextern
baum
baum

Reputation: 1029

LLVM Kaleidoscope tutorial failing on local extern

I am working through the LLVM Kaleidoscope tutorial. Everything is working fine except for local externs (as opposed to things like the math functions).

[c34n10 kaleidoscope] ./toy
ready> extern sin(x); sin(1);
ready> Read extern:
declare double @sin(double)

ready> ready> Evaluated to 0.841471
ready> extern putchard(x); putchard(120);
ready> Read extern:
declare double @putchard(double)

ready> ready> Failure value returned from cantFail wrapped call
UNREACHABLE executed at /gpfs/loomis/project/fas/manohar/emb99/llvm/include/llvm/Support/Error.h:732!
Aborted (core dumped)

putchard is declared in my code, per the tutorial, as

/// putchard - putchar that takes a double and returns 0.
extern "C" DLLEXPORT double putchard(double X) {
  fputc((char)X, stderr);
  return 0;
}

Other posts online suggest that this issue could be caused by not compiling with -rdynamic, but I am.

The actual error is occuring with the following code

auto ExprSymbol = TheJIT->findSymbol("__anon_expr");
assert(ExprSymbol && "Function not found");

// cast to double-returning function
double (*FP)() = (double (*)())(intptr_t)cantFail(ExprSymbol.getAddress());
fprintf(stderr, "Evaluated to %f\n", FP());

and some investigation with GDB reveals that the hasError flag of ExprSymbol.getAddress() is true, which is why the cantFail is failing. As to why that flag is set, I'm at a loss.

It appears not to be an issue with the function itself: from within GDB, I can successfully run call putchard(120), so the symbol definitely exists in the executable.

Finally, my makefile looks like

LIBS=core orcjit native analysis executionengine instcombine object runtimedyld scalaropts support
FLAGS=`llvm-config --cxxflags --ldflags --system-libs --libs $(LIBS)`

%: %.cpp
        clang++ -v -g3 -O0 $^ -o $@ $(FLAGS) -rdynamic

Upvotes: 2

Views: 694

Answers (1)

Ye Feng
Ye Feng

Reputation: 46

I ran into exactly the same issue in LLVM 8.

The problem is the symbol resolver doesn't try to find the requested symbol in local process's address, and an Expected<T> requires mandatory checking for errors.

I changed KaleidoscopeJIT.h a bit to get over it.

Replace the KaleidoscopeJIT's constructor with this one:

KaleidoscopeJIT()
      : Resolver(createLegacyLookupResolver(
            ES,
            [this](const std::string &Name) {
              auto symbol = ObjectLayer.findSymbol(Name, true);
              if (!symbol)
              {
                if (auto SymAddr = RTDyldMemoryManager::getSymbolAddressInProcess(Name))
                  return JITSymbol(SymAddr, JITSymbolFlags::Exported);
              }
              return symbol;
            },
            [](Error Err) { cantFail(std::move(Err), "lookupFlags failed"); })),
        TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()),
        ObjectLayer(ES,
                    [this](VModuleKey) {
                      return ObjLayerT::Resources{
                          std::make_shared<SectionMemoryManager>(), Resolver};
                    }),
        CompileLayer(ObjectLayer, SimpleCompiler(*TM)) {
    llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr);
  }

The only change is, if no symbol is found after calling ObjectLayer.findSymbol(), it will turn to RTDyldMemoryManager::getSymbolAddressInProcess() to find and create the JITSymbol object.

Upvotes: 3

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