Unveiling the Mystery: LLVM JIT "Symbol Not Found" Errors
Have you ever encountered the frustrating "Symbol Not Found" error when using LLVM's Just-In-Time (JIT) compilation? This can be a common problem when working with dynamically generated code, but understanding the underlying causes and solutions can greatly simplify your development process.
The Scenario:
Imagine you're building a dynamic application that generates code at runtime using LLVM's JIT compiler. This code might include functions, variables, or other symbols that need to be resolved at runtime. You run your application, but encounter the dreaded "Symbol Not Found" error. What went wrong?
Code Example:
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/ExecutionEngine/ExecutionEngine.h"
#include "llvm/IR/IRBuilder.h"
int main() {
llvm::LLVMContext Context;
llvm::IRBuilder<> Builder(Context);
// Create a function
auto* Function = llvm::Function::Create(
llvm::FunctionType::get(llvm::Type::getInt32Ty(Context), {}, false),
llvm::Function::ExternalLinkage, "my_function",
llvm::Module::newModule("MyModule", Context));
// Define the function body
auto* EntryBlock = llvm::BasicBlock::Create(Context, "entry", Function);
Builder.SetInsertPoint(EntryBlock);
Builder.CreateRet(llvm::ConstantInt::get(llvm::Type::getInt32Ty(Context), 42));
// Create an execution engine
std::unique_ptr<llvm::ExecutionEngine> Engine(
llvm::EngineBuilder(std::move(Function->getParent()))
.setEngineKind(llvm::EngineKind::JIT)
.create());
// Error: "Symbol Not Found" when calling my_function
auto result = (int)Engine->runFunction(Function, {});
return 0;
}
In this example, the my_function is defined but not linked into the JIT execution environment. Calling Engine->runFunction will fail with the "Symbol Not Found" error.
The Root of the Problem:
The "Symbol Not Found" error often occurs because the JIT compiler doesn't have access to the code that defines the symbol you're trying to access. This can happen due to:
- Missing Definitions: You might have declared a function or variable but haven't provided its actual implementation.
- Mismatched Linkages: The symbol you're trying to access might have internal linkage, which restricts its visibility outside the module it's defined in.
- Incorrect Compilation: The code containing the symbol might have been compiled without generating proper symbol information for the JIT to find.
- Initialization Order: The function or variable might be defined before the JIT has been initialized, leading to an unresolved reference.
Solutions and Best Practices:
- Ensure Symbol Definitions: Make sure all symbols you're using in your JIT-compiled code are defined within the module or are externally linked.
- Use Proper Linkages: For symbols that need to be accessible outside the module, use
llvm::GlobalValue::ExternalLinkage. - Enable Symbol Information: During compilation, ensure that symbol information is generated by using the
-gflag. - Order Matters: Initialize the JIT engine before generating code and defining symbols.
Debugging Tips:
- Symbol Table: Examine the symbol table of the JIT's compiled module to verify that the symbol you're looking for is present.
- Print IR: Use
llvm::raw_ostreamto print the generated LLVM Intermediate Representation (IR) and check for any errors or missing definitions.
Additional Resources:
By understanding the causes and solutions for the "Symbol Not Found" error, you can streamline your development process and confidently build dynamic applications using LLVM's JIT compiler.
