What exactly is a LLVMContext?

Beginners
1

Hi,

I am working on constructing a llvm::Module from a given bitCode string. The string is constructed from a llvm::Module object using WriteBitcodeToFile(), and it is compiled into IR from the ORC API ExecuteAction. However, I am aware that after ExecutionAction I can also take a LLVMContext object from the action object by action.takeLLVMContext, and LLVMContext may include important information for this llvm::Module.

Nevertheless, I can’t find any approach to serialize the LLVMContext. I am wondering if I declare a new one and try to parse the IR string like:

llvmContext context;
llvm::parseIR(ModuleString, context);

Can I get the identical llvm::Module and compile it as it is never serialized?

If not, how can I serialize LLVMContext, and why do we need LLVMContext for parsing an IR file?

Thank you!

2

You’re not supposed to, it’s for storing per-context types, and in your use-case you just create it on demand

3

For example, Type::getInt8PtrTy(LLVMContext& ctx) returns the same pointer if the same LLVMContext is passed in, so you can do fast pointer comparison when checking types

4

You can. However, as explained above you’ll need to redo the work if you need:

  • custom MDKindID
  • ValueHandlers
  • Type Comparisions
  • …

Basically, you can’t/ shouldn’t serialize LLVMContext, or perform operations on IR Units from different LLVMContext

5

Thank you for the answer!

Just want to check my understanding: A LLVMContext is a data structure maintaining metadata for the LLM to process programs. We can’t perform operations on IR units (llvm::Module) with different LLVMContext because LLVMContext owns some metadata related to the IR units.

However, we can declare a new LLVMContext and use that with the bitcode to construct a IR unit. Is this because bitcode contains additional information so that it doesn’t require the original LLVMContext?

6

LLVMContext is basically a container for various global tables that are used to work with IR–it does not contain any IR itself.

For example, the entire Type hierarchy in LLVM has the property that types are “uniqued”: if you call Type::getIntNTy twice with the same LLVMContext and the same integer bitwidth, the resulting pointers will be the same. To guarantee this property, it’s necessary to have a hash table somewhere that maps integer bitwidths to their corresponding Type instance–and you don’t want to eagerly construct them, because it consumes inordinate amounts of memory to keep around a i3245234 type that is legal to create in LLVM but no one is likely to create. This hash table lives in the LLVMContext instance.

Effectively, serialization means storing the sequence of C++ calls necessary to reconstruct the Module from scratch. However, none of those calls are going to be a member function of LLVMContext itself: LLVMContext is generally passed around as a parameter to the internal details of LLVM classes (so that they might use the global tables contained inside it], but it’s usually never used directly by user code. This means that there is nothing to actually serialize in an LLVMContext.

7

Thank you so much! I think I got it.