[orc-rt] Initial ORC Runtime design documentation. (#168681)

This document aims to lay out the high level design and goals of the ORC
runtime, and the relationships between key components.

Author: lhames

orc-rt/docs/Design.md

@@ -0,0 +1,119 @@
+# ORC Runtime Design
+
+The ORC runtime provides APIs for *executor* processes in an ORC JIT session
+(as opposed to the LLVM ORC libraries which provide APIs for *controller*
+processes). This includes support for both JIT'd code itself, and for users
+of JIT'd code.
+
+## Background
+
+LLVM's On Request Compilation (ORC) APIs support cross-process loading of JIT'd
+code. We call the process that defines and links the JIT'd code the *controller*
+and the process that executes JIT'd code the *executor*. Controller processes
+will link LLVM's ORC library, and construct a JIT'd program using an
+llvm::orc::ExecutionSession instance (typically through an convenience wrapper
+like llvm::orc::LLJIT). Executor processes construct an `orc_rt::Session`
+object to manage resources for, and access to, JIT'd code within the executor
+process.
+
+## APIs
+
+### Session
+
+The Session object is the root object for a JIT'd program. It owns the
+ResourceManager instances that manage resources supporting JIT'd code (e.g.
+JIT'd memory, unwind info registrations, dynamic library handles, etc.).
+
+The Session object must be constructed prior to adding any JIT'd code, and must
+outlive execution of any JIT'd code.
+
+An executor may have more than one Session object, in which case each Session
+object must outlive execution of any JIT'd code added to that specific session.
+
+### ControllerAccess
+
+ControllerAccess objects support bidirectional RPC between JIT'd code in the
+executor and the ExecutionSession in the controller.
+
+Calls in both directions are to "wrapper functions" with a fixed signature (a
+function that takes a blob of bytes and returns a blob of bytes as its result).
+ControllerAccess objects can not generally assume anything about the format of
+the bytes being sent (their interpretation is up to the called function). The
+RPC is not fully symmetric: Calls from the controller to the executor specify
+wrapper function *addresses* (i.e. the controller can invoke any code in the
+executor). Calls from the executor to the controller specify *tags*, which are
+addresses in the executor processes that are associated with handlers in the
+controller. This ensures that the executing process can only call deliberately
+exposed entry points in the controller.
+
+ControllerAccess objects may be detached before the session ends, at which point
+JIT'd code may continue executing, but will receive no further calls from the
+controller and can make no further calls to the controller.
+
+### ResourceManager
+
+`ResourceManager` is an interface for classes that manage resources that support
+a JIT'd program, for example memory or loaded dylib handles. It provides two
+operations: `detach` and `shutdown`. The `shutdown` operation will be called at
+`Session` destruction time. The `detach` operation may be called if the
+controller detaches: since this means that no further requests for resource
+allocation or release will occur prior to the end of the Session
+ResourceManagers may implement this operation to abandon any fine-grained
+tracking or pre-reserved resources (e.g. address space).
+
+### TaskDispatcher
+
+Runs Tasks within the ORC runtime. In particular, calls originating from the
+controller (via ControllerAccess) will be dispatched as Tasks.
+
+TaskDispatchers are responsible for ensuring that all dispatched Tasks have
+completed or been destroyed during Session shutdown.
+
+### WrapperFunction
+
+A wrapper function is any function with the following C signature:
+
+```c
+void (orc_rt_SessionRef Session, uint64_t CallId,
+      orc_rt_WrapperFunctionReturn Return,
+      orc_rt_WrapperFunctionBuffer ArgBytes);
+```
+
+where `orc_rt_WrapperFunctionReturn` and `orc_rt_WrapperFunctionBuffer` are
+defined as:
+
+```c
+typedef struct {
+  orc_rt_WrapperFunctionBufferDataUnion Data;
+  size_t Size;
+} orc_rt_WrapperFunctionBuffer;
+
+/**
+ * Asynchronous return function for an orc-rt wrapper function.
+ */
+typedef void (*orc_rt_WrapperFunctionReturn)(
+    orc_rt_SessionRef Session, uint64_t CallId,
+    orc_rt_WrapperFunctionBuffer ResultBytes);
+```
+
+The orc_rt::WrapperFunction class provides APIs for implementing and calling
+wrapper functions.
+
+### SPSWrapperFunction
+
+An SPS wrapper function is a wrapper function that uses the
+SimplePackedSerialization scheme (see documentation in
+orc-rt/include/orc-rt/SimplePackedSerialization.h).
+
+## TODO:
+
+Document...
+
+* C API
+* Error handling
+* RTTI
+* ExecutorAddr / ExecutorAddrRange
+* SimpleNativeMemoryMap
+* Memory Access (unimplemented)
+* Platform classes (unimplemented)
+* Other utilities