[llvm-dev] JIT compiler, Windows, external functions like cos

[Stefan Gränitz via llvm-dev]

Hi Marcus

If I understand correctly, you are creating a module with a declaration
for an external function. Then you compile and load the module with the
ExecutionEngine JIT interface. The loader is supposed to resolve your
declaration to an existing definition in the host program, right? Do you
get the error message "Program used external function '...' which could
not be resolved!"?

There is a variety of potential reasons here. The mystery that it works
for sqrt, but not for cos or sin, may be that something in your host
program exposes a definition with its unmangled name only for sqrt,
while the others are mangled or not exposed at all. After all, your host
program links to the C++ stdlib, which provides multiple versions of all
these functions with different signatures!

In general, using host-process symbols directly can be dangerous and
hard to debug. Basically, you are relying on details of your host
compiler and linker. The correct way may be adding another module with
the stdlib implementation that your JITed code should use. The simplest
way may be keeping the host-process calls, but expose your own wrappers
explicitly by address. Using the Orc JIT interface directly, a QND
implementation with LLVM 5.0 looked like this:
https://github.com/weliveindetail/JitFromScratch/commit/e9988a834dd6c35c93268da6ece73a7cc472100a

Note that on Windows you want to declare the wrappers as: extern "C"
__declspec(dllexport)

> After many trials and errors we found out that no LLVM example works
> (“4. Kaleidoscope: Adding JIT and Optimizer Support”, “The Fibonacci
> project“). [...] It seems that the LLVM JIT compiler cannot find
> external functions like “sin” and “cos” nor intrinsic functions under
> Windows. Every (tutorial) example crashes using them.
I can imagine this doesn't get tested so much on Windows.

Best
Stefan

Am 02.01.19 um 19:05 schrieb Dr. Marcus Hoffmann:
> Hello Stefan,
>
> our software FluidSIM (www.fluidsim.de) simulates pneumatic, hydraulic
> and electric circuits. For the mathematical models we use the language
> Modelica (www.modelica.org). We developed our own Modelica simulator
> which solves the dynamical created algebraic differential equation
> systems. One tool is our small JIT compiler, which compiles
> mathematical expressions like “2*x0 + sin(x1)” at runtime.
>
> In the future we want to compile more complex expressions, especially
> entire blocks, like
> X1 = 2*x0 +4
> X2 = 3*sin(x1)
> X3 = x1 + x2
>
> At the moment, we are evaluating the LLVM suite to replace our JIT
> compiler. Our software runs under the Windows platform (32 and 64
> Bit). We are using the Microsoft Visual Studio 2017 Version 15.9.4.
> For our evaluation we use LLVM 7.0.1. We created the LLVM suite for 32
> Bit with the cmake tools. We wrote a small parser and took the LLVM
> tutorials as starting point to implement our LLVM JIT compiler. All
> arithmetic expressions work fine. But we failed to integrate external
> functions like “cos” and “sin”. Only “sqrt” worked. After many trials
> and errors we found out that no LLVM example works (“4. Kaleidoscope:
> Adding JIT and Optimizer Support”, “The Fibonacci project“).
>
> It seems that the LLVM JIT compiler cannot find external functions
> like “sin” and “cos” nor intrinsic functions under Windows. Every
> (tutorial) example crashes using them. Is there any solution for this
> problem (compiler switches, libraries to include, a simple Visual
> Studio project)?
>
> The following code works for “sqrt” but not for “cos”. It just
> encapsulates an external call. We use
> Microsoft Visual Studio 2017 Version 15.9.4.
> LLVM 7.0.1
> Solution Configuration: Debug, 32 Bit, Switch: /MTd
>
> using namespace llvm;
>
> typedef double(__cdecl *JitCompiledFn)(double);
>
> int main()
> {
> // "sqrt" works.
> //const char externalFnName[] = "sqrt";
>
> // "cos", "sin", etc. fails.
> const char externalFnName[] = "cos";
>
> InitializeNativeTarget();
> InitializeNativeTargetAsmPrinter();
> InitializeNativeTargetAsmParser();
>
> LLVMContext context;
>
> IRBuilder<> builder(context);
>
> std::unique_ptr<llvm::Module> module(new Module("TestModule", context));
> Module* pModule = module.get();
>
> auto externalFn_IR =
> cast<Function>(pModule->getOrInsertFunction("externalFn",
> Type::getDoubleTy(context), Type::getDoubleTy(context)));
> Value* x = externalFn_IR->arg_begin();
> x->setName("x");
>
> BasicBlock *entryBlock = BasicBlock::Create(context, "EntryBlock",
> externalFn_IR);
> builder.SetInsertPoint(entryBlock);
>
> std::vector<Type *> args(1, Type::getDoubleTy(context));
> FunctionType *FT = FunctionType::get(Type::getDoubleTy(context), args,
> false);
> auto externalFn_llvm = Function::Create(FT, Function::ExternalLinkage,
> externalFnName, pModule);
> auto ret = builder.CreateCall(externalFn_llvm, x);
>
> builder.CreateRet(ret);
>
> errs() << "Created Module:\n\n" << *pModule;
>
> auto jitCompiler =
> EngineBuilder(std::move(module)).setOptLevel(CodeGenOpt::Level::Default).create();
>
>
> JitCompiledFn externalFn =
> (JitCompiledFn)jitCompiler->getFunctionAddress(externalFn_IR->getName());
>
> errs() << "\n\nexternalFn(9.0) = ";
>
> double y = externalFn(9.0);
>
> errs() << y << "\n\n";
>
> return 0;
> }
>
>
>
> Kind regards
> Marcus Hoffmann
>
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