NWScript JIT engine: MSIL backend support for SAVE_STATE

Yesterday, I described how the fast action call mechanism improves action call performance for JIT’d programs. For today’s NWScript adventure, let’s dig into how SAVE_STATE operations (script situations) are supported in the MSIL JIT backend.

As you may recall, SAVE_STATE operations (codified by I_SAVE_STATE in the IR instruction set and OP_STORE_STATE/OP_STORE_STATEALL in the NWScript instruction set) are used to allow a copy of the script execution environment’s current execution context to be “forked off” for later use. This is easy to implement in the interpretive script VM environment, but something more elaborate is required for the JIT backend.

The NWScript analyzer promotes resume labels for SAVE_STATE operations into first class subroutines; in the MSIL backend, these subroutines are then emitted as IL-level subroutines. When a SAVE_STATE instruction is encountered, the following steps are taken:

  1. The backend emits IL instructions to save the state of all local variables shared with the resume subroutine. This is performed by boxing copies of these locals into an array< Object ^ >.
  2. The backend sets up a call to a method on the main script class (ScriptProgram), CloneScriptProgram. This method allocates a new ScriptProgram instance derived from the current ScriptProgram object and prepares it for use as a saved state clone. This entails duplicating the contents of all global variables in the parent ScriptProgram object and resetting the various runtime guard counters (such as the recursion depth) to their default, zero values.
  3. The backend sets up a call to a JIT intrinsic, Intrinsic_StoreState. This intrinsic takes the boxed local variable array, the cloned ScriptProgram object, and a “resume method id”. All of these values are stored into a new NWScriptSavedState object that is hung off of the overarching NWScriptProgram object.

Once these steps have been taken, a future action service handler will call an API to receive the last saved state. This API will return the most recently constructed NWScriptSavedState object.

Eventually, the script host may opt to execute the saved state. This is known as executing a “script situation”; to accomplish this, the script host passes the NWScriptSavedState object to the NWScriptProgram object (indirected through a C-level API), asking the NWScriptProgram object to call the resume label with the saved state.

For performance reasons, the NWScriptProgram object does not attempt to call the resume label via a Reflection invocation attempt. Instead, a dispatcher method on the INWScriptGeneratedProgram interface implemented by the ScriptProgram type, ExecuteScriptSituation, is invoked. (Here, the ScriptProgram instance that was created by the CloneScriptProgram call earlier is used, ensuring that a copy of the current global variables is referenced.)

As you’ll recall, ExecuteScriptSituation has a signature looking something like this:

 // Execute a script situation (resume label).

  __in UInt32 ScriptSituationId,
  __in array< Object ^ > ^ Locals

Internally, ExecuteScriptSituation is implemented as essentially a large “switch” block that switches on the mysterious ScriptSituationId parameter (corresponding to the “resume method id” that was passed to Intrinsic_StoreState). This parameter identifies which resume subroutine in the script program should be executed. (When emitting IL code for subroutine, the first resume subroutine is assigned resume method id 0; the next is assigned resume method id 1, and so forth.)

If the ScriptSituationId matches a legal case branch that was emitted into ExecuteScriptSituation, additional code to unbox the Locals array contents into parameters follows. These parameters are simply passed to the resume subroutine for that case statement. At this point, the resume globals are set to their correct values (by virtue of the fact that the ‘this’ pointer is set to the cloned ScriptProgram instance), and the resume locals are, similarly, set up correctly as subroutine parameters.

The rest, as they say, is history; the resume label continues on as normal, executing whatever operations it wishes.

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