Prologue and epilogue

[apt1002]

Prologue and epilogue

[apt1002]

An analysis of goals:

1. We want the Slots (unbounded in number) to be on the stack.
2. We want the Registers and Slots to be dead when Mijit is not running.
3. We want some Registers and Slots to be live in the root Machine::States, and we want to specify the live set in an ergonomic way.
4. We want to allow the Machine to choose the Save data structure used to persist the state of the VM when Mijit is not running.
5. We want to minimise the marshalling code that gets run on entry to and exit from Mijit.

Goal 1 deserves an issue of its own (#26) which also covers goal 2.

The combination of goals 2 and 3 motivates the prologue and epilogue code, whose purpose it to marshal data between the persistent state and the live set.

Goal 4 does not require any support from Mijit beyond providing a single persistent pointer to the prologue and epilogue code. The Machine can use it as a pointer to a Save if it wants; Mijit does not need to understand. So Machine::Save is an unnecessary concept.

Goal 5 essentially boils down to sharing between the persistent state and the live set. This can be done by reference, e.g. via the pointer to the Save. However, it is cheap and probably useful to provide some state that is shared "by value" too. I propose a constant-sized array of words (Globals?) that can be accessed as Values (alongside Registers and Slots) but which are always live and which persist when Mijit is not running. This is a mild generalisation of the single persistent pointer required by goal 4.

In summary, I propose the following design:

• Add a case Global to code::Value, alongside Register and Slot, to represent a 64-bit word that persists when Mijit is not running. This deserves its own issue (#27).
• Machine::Save does not exist.
• Machine::values() does not exist. Instead Mijit defines a set of 64 Values: the first 12 Registers and the first 52 Slots. It is not necessary to include Globals, which are always live.
• Machine::num_globals() returns the number of Globals that the Machine would like to use. This resurrects an earlier Mijit feature, with the difference that the Globals are not considered to be a subset of the Slots.
• Machine::get_code(state) returns (among other things) a 64-bit mask indicating the Values that are live in state. The bitmask selects from the Mijit-defined set.
• Machine::prologue() returns a list of Mijit Actions to marshal data from the Globals to the live values. It is not passed anything on entry. On exit it must have initialised all Values that are live in any State.
• Machine::epilogue() returns a list of Mijit Actions to marshal data from the live values back to the Globals. On entry, it gets all Values that are live in any State; those that are dead are set to a dummy value (0xdeaddeaddeaddead). On exit nothing is live.