[llvm-dev] JIT compiler and calls to existing functions

[Caldarale, Charles R via llvm-dev]

> From: Philip Reames [mailto:listmail at philipreames.com] 
> Subject: Re: [llvm-dev] JIT compiler and calls to existing functions

> There is no documentation I know of.  Rough sketch:
> 1) Create a subclass of SectionMemoryManager
> 2) Create an instance of this class and add it the EngineBuilder via setMCJITMemoryManager
>  (Make sure everything runs without changes)
> 3) Override the getSymbolAddress method, have your implementation call the base classes impl
>  (Make sure everything runs without changes)
> 4) Add handling to map name->address for any custom functions you want.

Here's what we use for the functions that have statically-known addresses:

First, make LLVM's symbol library available:

    #include <llvm/Support/DynamicLibrary.h>        // for linking in C code

Make sure the symbols from the executable are available:

    sys::DynamicLibrary::LoadLibraryPermanently(nullptr);

For each function whose name and code pointer are known, create its llvm::Function definition, and enter the name and code pointer into the symbol table if not already there:

    fType = FunctionType::get(retType, margs, false);
    pFunc = Function::Create(fType, Function::ExternalLinkage, Twine(name), pMod);
    setFuncAttr(pFunc, margs); // customize as needed
    if (sys::DynamicLibrary::SearchForAddressOfSymbol(name) == NULL) {
        sys::DynamicLibrary::AddSymbol(name, (void*)ptr);
    }

In our class derived from SectionMemoryManager, we have the following:

    uint64_t getSymbolAddress(const std::string& Name) override {
        const char* NameStr = Name.c_str();
        void* ptr;
        if (NameStr[0] == '_' &&
            (ptr = sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr + 1)) != NULL) {
            return uint64_t(ptr);
        }
        /* If Name did not require demangling, or we failed to find the demangled name,
           try again without demangling. */
        return uint64_t(sys::DynamicLibrary::SearchForAddressOfSymbol(NameStr));
    }

 - Chuck