[llvm-dev] Linking existing functions from JITed code

Lang Hames via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 13 16:41:32 PDT 2015

Hi Andy,

I haven't tested this on Linux, but on MacOS the
RuntimeDyldMemorManager::getSymbolAddressInProcess method should find
symbol addresses in the host program, including symbols from static
archives linked into the program. However, one gotcha is that the symbol
has to be reachable from main, otherwise the linker may strip it from the
final executable.

Do you have a test-case that I could try to reproduce the issue with?

Alternatively, if you run the code under a debugger, do you see symbols
that the JIT failed to find? If the symbols are visible in the debugger but
invisible to the JIT that sounds like a bug in the JIT. If the symbols are
invisible in both that would suggest that the linker is stripping them out.

- Lang.

On Thu, Aug 13, 2015 at 1:12 PM, David Blaikie <dblaikie at gmail.com> wrote:

> +"Lang Hames, Linker of Linkers"
> (Lang developed the ORC JIT you seem to be using & can provide more
> context than I have)
> On Thu, Aug 13, 2015 at 12:43 PM, Andy Somogyi via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi
>> I’ve previously used the ExecutionEngine::addGlobalMapping to make
>> existing functions available to my JITed code.
>> I’m currently using ORC, as MCJIT does not appear to be maintained any
>> longer (the kaleidoscope examples have not worked for some time with
>> MCJIT).
>> I’m using just the basic ORC CompileLayer directly.
>> So, I’ve essentially copied the ExecutionEngine::addGlobalMapping related
>> function to my JIT context, and I create a lambda resolver as such:
>> JITContext::addModule(…) {
>> auto Resolver = createLambdaResolver(
>> [&](const std::string &name) {
>> // look up first in JIT'ed code
>> if (auto sym = findMangledSymbol(name)) {
>> return RuntimeDyld::SymbolInfo(sym.getAddress(),
>> sym.getFlags());
>> return RuntimeDyld::SymbolInfo(nullptr);
>> }
>> // look up in added globals
>> if (auto addr = getPointerToGlobalMapping(name)) {
>> return RuntimeDyld::SymbolInfo(addr, JITSymbolFlags::Exported);
>> }
>> // finally try to look up existing process symbols, note
>> // this works for symbols loaded in shared libraries, but
>> // does NOT seem to find symbols declared in the executable.
>> if (auto Addr =
>> RTDyldMemoryManager::getSymbolAddressInProcess(name)) {
>> return RuntimeDyld::SymbolInfo(Addr, JITSymbolFlags::Exported);
>> }
>> },
>> [](const std::string &S) { return nullptr; }
>> );
>> }
>> Here the getPointerToGlobalMapping function looks in a uint64 StringMap
>> into which values are added via the addGlobalMapping functions.
>> This approach seems to be working, but my question is do you suppose
>> there any are issues with such an approach?
>> The troubling thing is why doesn’t  RTDyldMemoryManager::
>> getSymbolAddressInProcess(name)) return an address for a symbol that is
>> defined in either a static library, or in the executable itself.
>> If this approach is correct, in adding the global values to the context,
>> and looking them up the lambda resolver, in addition to looking up external
>> symbols, and considering that the ORC kaleidoscope examples do in fact
>> allow external function calls (which are broken currently), should they be
>> fixed with this approach?
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