Horn Clause -based VM and Frontend for Lever 0.10.0

The paper by Jorge Navas, "Horn-Clauses as an Intermediate Representation for Program Analysis", referred to in a previous post, turned out to be a gold nugget.

I adjusted Navas design for a virtual machine and wrote a bytecode compiler for it. The results are great so far. Here's some actual output from the frontend:

proc1(v4; v5) <-
  global("=="; v6), call(v6, v4, 1; v7),  is_true(v7;)? proc2(;v5).
  global("=="; v6), call(v6, v4, 1; v7), ¬is_true(v7;)? global("=="; v8), call(v8, v4, 2; v9),  is_true(v9;)? proc2(;v5).
  global("=="; v6), call(v6, v4, 1; v7), ¬is_true(v7;)? global("=="; v8), call(v8, v4, 2; v9), ¬is_true(v9;)? global("+"; v10), global("fibonacci"; v11), global("-"; v12), call(v12, v4, 2; v13), call(v11, v13; v14), global("fibonacci"; v15), global("-"; v16), call(v16, v4, 1; v17), call(v15, v17; v18), call(v10, v14, v18; v5).
proc2(;v19) <-
  move(1; v19).

This is code for the fibonacci function. It is quite long because it needs to fetch several global variables. Here's the same cleaned out a bit and transformed into a form equivalent to what Navas provide:

fibonacci(v4; v5) <-
  v4 == 1, v5 = 1.
  v4 != 1, v4 == 2, v5 = 1.
  v4 != 1, v4 != 2, fibonacci(v4-2; v14), fibonacci(v4-1; v18), v5 = v14 + v18.

Every clause is a possible scenario that the program can take.


The virtual machine runs off from a list of procedures. Each procedure has inputs and outputs and they can be called by other procedures in the same program. Loops are implemented by tail calls, therefore tail calls have to be optimized here.

Most instructions have a list of inputs and outputs. The clauses are encoded by instructions that jump forward in the procedure when they are triggered. In the location of every jump there is a terminator instruction that may give some additional parameters for the guard.

The iterators, pattern matching, control flow and exceptions are implemented by these guards.

For example, the fibonacci above would be encoded like this:

inputs=[v4], outputs=[v5] tmp=[v14, v18]
o_is_true(v4 == 1) output=[] guard=L2
v5 = 1
guard L2 output=[]
o_is_true(v4 == 2) output=[] guard=L3
v5 = 1
guard L3 output=[]
fibonacci(v4 - 2) output=[v14]
fibonacci(v4 - 1) output=[v18]
v5 = v14 + v18
guard (end of procedure)

If there's a raise -instruction, it is right before guard.

The IR clauses are required to be in SSA-form before translation to VM bytecode. The frontend builds the bytecode in reverse simultaneously using linear register allocation scheme to fit the variable flow into the VM and calculating the jumps that guards have to do.

The source locations are encoded as lists of integers. This may be familiar if you're heard of javascript's source maps because they use a similar method to encode source location into code that was transpiled to javascript:

[byte_count, col0, lno0, col1, lno1, sources_index]

The encoding to VM code is entirely decodeable back to the IR that produced it.

Variable reflow in the IR

The whole translation unit contains every function and their closures. The program that produces the IR can annotate the code with flow-variables instead of variables that satisfy the constraints required by the IR. The flow variables are then converted to constrained variable flow.

The flow-variable conversion takes place in two passes. The first pass computes define-sets for each procedure by flowing the defined variables forwards until a fixed point is reached and none of the define-sets change any longer.

The second pass starts inserting variables to procedure based on where they are originating from. This pass also works along the closure frame boundaries and therefore solves the lexical scope variable flow as well.

At this point the flow variables are written away from the program. Then redundant move instructions and redundant procedures are removed.

Desugaring and control flow construction

Since we do not need to worry about the variable scope, and for control flow we basically piece together procedure blocks, it could be theoretically possible to directly encode the program into the necessary form. Though I prefer doing an intermediary step and first produce these kind of constructs from the parse tree. It allows me to desugar some of those constructs.

 BranchExcept(statements, exception_var, handler_statements)
 RelationalCall(callee, input_exprs, output_slots)
 Repeat(loop_headers, statements)
 Assign(slot, value_expr)
 InplaceAssign(slot, op_expr, value_expr)
 SimpleStatement(opcode, input_exprs)
 Cond(cond, statements, statements)
 Call(callee, input_exprs)

loop header:
 ForHeader(slot, rest, value, cond)

cond, expr:
 Match(value, pattern_expr, match_slots)
 And(cond_a, cond_b)
 Or(cond_a, cond_b)

 Call(callee, input_exprs)
 Function(slots, default_exprs, return_flowvars, statements)
 Let([(flow-variable, expr)], expr)
 ProgExpr(statements, expr)

match_slot, slot, expr:
 Record({strings: slots})
 Pattern(pattern_expr, slots)

match_slot, slot, expr, cond:
 Attr(base_expr, attr_expr)
 Item(base_expr, indexer_expr)

These combinators cover the large parts of the language, although I may add some smaller combinators in the mix.

If the branchexcept and return/continue/break are used in the same mix, the branchexcept has to flag every control flow path and "lift" the flags back into control flow outside it. This operation creates additional flow variables.

Scope does not need to be particularly handled. The Symbol is retrieving flow variables from a simple table. If the table doesn't have a flow variable for the symbol, then it creates one if the symbol is allowed to define a variable. Otherwise it generates a fetch to the global variables of the module.

To allow recursive definitions, the pattern Assign(Symbol(), Function|Generator) must be specially treated. This ensures the closure frames are merged into a recursive shape if that is necessary.

The continuations and the context are two main elements in building the IR from the combinators. The context takes care of the scope and the state. The continuations point out places where you can insert instructions. Jump to a continuation generates a procedure, if the continuation was not pointing to a procedure.

The combinators roughly form classes by what is required from them.

Since the tuple is a slot, there are some cases where it can be multiplexed or hidden, such that no tuple is generated. Furthermore, a function returning only obvious tuples returns multiple variables instead of tuples. And it is checked that when a function returns tuples, it returns them in only one size.

Furthermore I am so glad that I learned about Navas' work. There are still probably tons of things I could learn from him.