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+# The tier 2 execution engine
+
+## General idea
+
+When execution in tier 1 becomes "hot", that is the counter for that point in
+the code reaches some threshold, we create an executor and execute that
+instead of the tier 1 bytecode.
+
+Since each executor must exit, we also track the "hotness" of those
+exits and attach new executors to those exits.
+
+As the program executes, and the hot parts of the program get optimized,
+a graph of executors forms.
+
+## Superblocks and Executors
+
+Once a point in the code has become hot enough, we want to optimize it.
+Starting from that point we project the likely path of execution,
+using information gathered by tier 1 to guide that projection to
+form a "superblock", a mostly linear sequence of micro-ops.
+Although mostly linear, it may include a single loop.
+
+We then optimize this superblock to form an optimized superblock,
+which is equivalent but more efficient.
+
+A superblock is a representation of the code we want to execute,
+but it is not in executable form.
+The executable form is known as an executor.
+
+Executors are semantically equivalent to the superblock they are
+created from, but are in a form that can be efficiently executable.
+
+There are two execution engines for executors, and two types of executors:
+* The hardware which runs machine code executors created by the JIT compiler.
+* The tier 2 interpreter runs bytecode executors.
+
+It would be very wasteful to support both a tier 2 interpreter and
+JIT compiler in the same process.
+For now, we will make the choice of engine a configuration option,
+but we could make it a command line option in the future if that would prove useful.
+
+
+### Tier 2 Interpreter
+
+For platforms without a JIT and for testing, we need an interpreter
+for executors. It is similar in design to the tier 1 interpreter, but has a
+different instruction set, and does not adapt.
+
+### JIT compiler
+
+The JIT compiler converts superblocks into machine code executors.
+These have identical behavior to interpreted executors, except that
+they consume more memory for the generated machine code and are a lot faster.
+
+## Transfering control
+
+There are three types of control transfer that we need to consider:
+* Tier 1 to tier 2
+* Tier 2 to tier 1
+* One executor to another within tier 2
+
+Since we expect the graph of executors to span most of the hot
+part of the program, transfers from one executor to another should
+be the most common.
+Therefore, we want to make those transfers fast.
+
+### Tier 2 to tier 2
+
+#### Cold exits
+
+All side exits start cold and most stay cold, but a few become
+hot. We want to keep the memory consumption small for the many
+cold exits, but those that become hot need to be fast.
+However we cannot know in advance, which will be which.
+
+So that tier 2 to tier 2 transfers are fast for hot exits,
+exits must be implemented as executors. In order to patch
+executor exits when they get hot, a pointer to the current
+executor must be passed to the exit executor.
+
+#### Handling reference counts
+
+There must be an implicit reference to the currently executing
+executor, otherwise it might be freed.
+Consequently, we must increment the reference count of an
+executor just before executing it, and decrement it just after
+executing it.
+
+We want to minimize the amount of data that is passed from
+one executor to the next. In the JIT, this reduces the number
+of arguments in the tailcall, freeing up registers for other uses.
+It is less important in the interpreter, but following the same
+design as the JIT simplifies debugging and is good for performance.
+
+Provided that we incref the new executor before executing it, we
+can jump directly to the code of the executor, without needing
+to pass a reference to that executor object.
+However, we do need a reference to the previous executor,
+so that it can be decref'd and for handling of cold exits.
+To avoid messing up the JIT's register allocation, we pass a
+reference to the previous executor in the thread state's
+`previous_executor` field.
+
+#### The interpreter
+
+The tier 2 interpreter has a variable `current_executor` which
+points to the currently live executor. When transfering from executor
+`A` to executor `B` we do the following:
+(Initially `current_executor` points to `A`, and the refcount of
+`A` is elevated by one)
+
+1. Set the instruction pointer to start at the beginning of `B`
+2. Increment the reference count of `B`
+3. Start executing `B`
+
+We also make the first instruction in `B` do the following:
+1. Set `current_executor` to point to `B`
+2. Decrement the reference count of `A` (`A` is referenced by `tstate->previous_executor`)
+
+The net effect of the above is to safely decrement the refcount of `A`,
+increment the refcount of `B` and set `current_executor` to point to `B`.
+
+#### In the JIT
+
+Transfering control from one executor to another is done via tailcalls.
+
+The compiled executor should do the same, except that there is no local
+variable `current_executor`.
+
+### Tier 1 to tier 2
+
+Since the executor doesn't know if the previous code was tier 1 or tier 2,
+we need to make a transfer from tier 1 to tier 2 look like a tier 2 to tier 2
+transfer to the executor.
+
+We can then perform a tier 1 to tier 2 transfer by setting `current_executor`
+to `None`, and then performing a tier 2 to tier 2 transfer as above.
+
+### Tier 2 to tier 1
+
+Each micro-op that might exit to tier 1 contains a `target` value,
+which is the offset of the tier 1 instruction to exit to in the
+current code object.
+
+## Counters
+
+TO DO.
+The implementation will change soon, so there is no point in
+documenting it until then.
+