Program counter trees

This explains the how “program counter” variables are used to memorise the control flow location(s) a Blech program resides in and how such program counters are managed in a tree structure in the code generator.

Program counters

An activity’s program state is given by the position(s) which control flow resides in and the evaluation of local variables and parameters.

This information is stored in a data structure which we call the activity context. In particular, the control flow position(s) are given by those fields in the activity context which we call program counters.

Remember that the control flow of an activity is decomposed into a block graph after causality analysis. The individual blocks are scheduled (ordered) according to a topological sort algorithm. (Details on the block graph are to be documented elsewhere).

The value of a program counter identifies the block that the control flow of an activity is currently in. For block number i the corresponding program counter has value 2i if control flow is about to enter this block in the current reaction step. The program counter has value 2i + 1 if block i will be entered in the next reaction. A program counter which is 0 indicates that control flow has terminated (in the corresponding thread of execution, see below).

An activity may contain concurrent code (expressed by the cobegin statement) and therefore control flow may be forked into several threads of execution. This is why an activity may reside in several control flow points at the same time. Each thread is represented by an individual program counter. Thus there is always at least one program counter for an activity’s root thread and possibly more for subthreads which are forked by cobegin.

PC tree

The Blech compiler constructs a so-called program counter tree (pc tree) for every activity.

type PCtree =
        mainpc: ParamDecl
        thread: Thread
        subPCs: PCtree list

A pc tree corresponds to a thread and maintains the program counter mainpc for this thread. If this thread forks control flow then subPCs contains a list of corresponding subthread program counter trees. The activity context of a single-threaded activity will only have a flat pc tree in which subPCs is an empty list.

Constructing the tree

The only function for constructing pc trees which is used outside the CodeGenContext.fs file is Compilation.addPc. Every time a new block is translated in ActivityTranslator.fs this function is called once:

curComp := Compilation.addPc !curComp block pc

The given pc is a ParamDecl which contains all info about the program counter variable itself. The given block is a Block instance corresponding to this program counter and contains all necessary thread information. addPc returns a new Compilation instance.

The logics of addPc are rather simple:

  • if the given compilation does not yet have an activity context (i.e. we have just started translating the very first block of this activity), create a new context with a new pc tree.
  • if the given compilation has an activity context and its pc tree already contains the given program counter, do nothing and return the unmodified compilation (happens when we translate the second, third,… n-th block along the same thread of execution)
  • otherwise there is an activity context with a pc tree which however does not yet have this particular program counter –> call PCtree.add.

The ‘magic’ happens in PCtree.add:

  • given the thread information, a list of ancestors (root to given thread) is constructed
  • the list is used to navigate through the pc tree to find the ancestor to which a new sub tree is added

Using the pc tree

In ActivityTranslator.fs there is the function

let private findTreeFor (comp: Compilation) (node: Node) -> PCtree

which is vital to work with program counters.

The tree structure is particularly exploited in

let private endThread comp node -> Doc
Last modified May 4, 2021: drafting the module chapter (fa3db01)