start at some basic passes

pass/cfg.go

@@ -0,0 +1,83 @@
+package pass
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/mmcloughlin/avo"
+)
+
+// LabelTarget populates the LabelTarget of the given function. This maps from
+// label name to the following instruction.
+func LabelTarget(fn *avo.Function) error {
+	target := map[avo.Label]*avo.Instruction{}
+	for idx := 0; idx < len(fn.Nodes); idx++ {
+		// Is this a label?
+		lbl, ok := fn.Nodes[idx].(avo.Label)
+		if !ok {
+			continue
+		}
+		// Check for a duplicate label.
+		if _, found := target[lbl]; found {
+			return fmt.Errorf("duplicate label \"%s\"", lbl)
+		}
+		// Advance to next node.
+		if idx == len(fn.Nodes)-1 {
+			return errors.New("function ends with label")
+		}
+		idx++
+		// Should be an instruction.
+		i, ok := fn.Nodes[idx].(*avo.Instruction)
+		if !ok {
+			return errors.New("instruction should follow a label")
+		}
+		target[lbl] = i
+	}
+	fn.LabelTarget = target
+	return nil
+}
+
+// CFG constructs the call-flow-graph of each function.
+func CFG(fn *avo.Function) error {
+	is := fn.Instructions()
+	n := len(is)
+
+	// Populate successors.
+	for i := 0; i < n; i++ {
+		cur := is[i]
+		var nxt *avo.Instruction
+		if i+1 < n {
+			nxt = is[i+1]
+		}
+
+		// If it's a branch, locate the target.
+		if cur.IsBranch {
+			lbl := cur.TargetLabel()
+			if lbl == nil {
+				return errors.New("no label for branch instruction")
+			}
+			target, found := fn.LabelTarget[*lbl]
+			if !found {
+				return errors.New("unknown label")
+			}
+			cur.Succ = append(cur.Succ, target)
+		}
+
+		// Otherwise, could continue to the following instruction.
+		switch {
+		case cur.IsTerminal:
+		case cur.IsBranch && !cur.IsConditional:
+		default:
+			cur.Succ = append(cur.Succ, nxt)
+		}
+	}
+
+	// Populate predecessors.
+	for _, i := range is {
+		for _, s := range i.Succ {
+			s.Pred = append(s.Pred, i)
+		}
+	}
+
+	return nil
+}