pass: naive implementation of liveness

ast.go

@@ -45,9 +45,13 @@ type Instruction struct {
 	// CFG.
 	Pred []*Instruction
 	Succ []*Instruction
+
+	// LiveIn/LiveOut are sets of live register IDs pre/post execution.
+	LiveIn  map[reg.ID]bool
+	LiveOut map[reg.ID]bool
 }
 
-func (i Instruction) node() {}
+func (i *Instruction) node() {}
 
 func (i Instruction) TargetLabel() *Label {
 	if !i.IsBranch {

build/context.go

@@ -25,7 +25,7 @@ func (c *Context) Function(name string) {
 	c.file.Functions = append(c.file.Functions, c.function)
 }
 
-func (c *Context) Instruction(i avo.Instruction) {
+func (c *Context) Instruction(i *avo.Instruction) {
 	c.node(i)
 }
 

internal/gen/build.go

@@ -33,7 +33,7 @@ func (b *build) instruction(i inst.Instruction) {
 	// Context method.
 	b.Printf("func (c *Context) %s(%s) {\n", i.Opcode, s.ParameterList())
 	b.Printf("if inst, err := x86.%s(%s); err == nil", i.Opcode, s.Arguments())
-	b.Printf(" { c.Instruction(*inst) }")
+	b.Printf(" { c.Instruction(inst) }")
 	b.Printf(" else { c.AddError(err) }\n")
 	b.Printf("}\n")
 

pass/reg.go

@@ -0,0 +1,62 @@
+package pass
+
+import (
+	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/reg"
+)
+
+// Liveness computes register liveness.
+func Liveness(fn *avo.Function) error {
+	is := fn.Instructions()
+
+	// Initialize to empty sets.
+	for _, i := range is {
+		i.LiveIn = map[reg.ID]bool{}
+		i.LiveOut = map[reg.ID]bool{}
+	}
+
+	// Iterative dataflow analysis.
+	for {
+		changes := false
+
+		for _, i := range is {
+			// in[n] = use[n] UNION (out[n] - def[n])
+			nin := len(i.LiveIn)
+			for _, r := range i.InputRegisters() {
+				i.LiveIn[r.ID()] = true
+			}
+			def := map[reg.ID]bool{}
+			for _, r := range i.OutputRegisters() {
+				def[r.ID()] = true
+			}
+			for id := range i.LiveOut {
+				if !def[id] {
+					i.LiveIn[id] = true
+				}
+			}
+			if len(i.LiveIn) != nin {
+				changes = true
+			}
+
+			// out[n] = UNION[s IN succ[n]] in[s]
+			nout := len(i.LiveOut)
+			for _, s := range i.Succ {
+				if s == nil {
+					continue
+				}
+				for id := range s.LiveIn {
+					i.LiveOut[id] = true
+				}
+			}
+			if len(i.LiveOut) != nout {
+				changes = true
+			}
+		}
+
+		if !changes {
+			break
+		}
+	}
+
+	return nil
+}

printer.go

@@ -73,7 +73,7 @@ func (p *GoPrinter) function(f *Function) {
 
 	for _, node := range f.Nodes {
 		switch n := node.(type) {
-		case Instruction:
+		case *Instruction:
 			p.printf("\t%s\t%s\n", n.Opcode, joinOperands(n.Operands))
 		case Label:
 			p.printf("%s:\n", n)

reg/reg_test.go

@@ -22,3 +22,15 @@ func TestSpecBytes(t *testing.T) {
 		}
 	}
 }
+
+func TestVirtualPhysicalHaveDifferentIDs(t *testing.T) {
+	// Confirm that ID() returns different results even when virtual and physical IDs are the same.
+	var v Virtual = virtual{id: 42}
+	var p Physical = register{id: 42}
+	if uint16(v.VirtualID()) != uint16(p.PhysicalID()) {
+		t.Fatal("test assumption violated: VirtualID and PhysicalID should agree")
+	}
+	if v.ID() == p.ID() {
+		t.Errorf("virtual and physical IDs should be different")
+	}
+}

reg/types.go

@@ -1,6 +1,9 @@
 package reg
 
-import "fmt"
+import (
+	"fmt"
+	"math"
+)
 
 type Size uint
 
@@ -23,7 +26,7 @@ type Family struct {
 	registers []Physical
 }
 
-func (f *Family) define(s Spec, id uint16, name string) Physical {
+func (f *Family) define(s Spec, id PID, name string) Physical {
 	r := register{
 		id:   id,
 		kind: f.Kind,
@@ -34,7 +37,7 @@ func (f *Family) define(s Spec, id uint16, name string) Physical {
 	return r
 }
 
-func (f *Family) Virtual(id uint16, s Size) Virtual {
+func (f *Family) Virtual(id VID, s Size) Virtual {
 	return virtual{
 		id:   id,
 		kind: f.Kind,
@@ -46,7 +49,14 @@ type private interface {
 	private()
 }
 
+type (
+	ID  uint32
+	VID uint16
+	PID uint16
+)
+
 type Register interface {
+	ID() ID
 	Kind() Kind
 	Bytes() uint
 	Asm() string
@@ -54,18 +64,22 @@ type Register interface {
 }
 
 type Virtual interface {
-	VirtualID() uint16
+	VirtualID() VID
 	Register
 }
 
 type virtual struct {
-	id   uint16
+	id   VID
 	kind Kind
 	Size
 }
 
-func (v virtual) VirtualID() uint16 { return v.id }
-func (v virtual) Kind() Kind        { return v.kind }
+func (v virtual) VirtualID() VID { return v.id }
+func (v virtual) Kind() Kind     { return v.kind }
+
+func (v virtual) ID() ID {
+	return (ID(math.MaxUint16) << 16) | ID(v.VirtualID())
+}
 
 func (v virtual) Asm() string {
 	// TODO(mbm): decide on virtual register syntax
@@ -75,22 +89,23 @@ func (v virtual) Asm() string {
 func (v virtual) private() {}
 
 type Physical interface {
-	PhysicalID() uint16
+	PhysicalID() PID
 	Mask() uint16
 	Register
 }
 
 type register struct {
-	id   uint16
+	id   PID
 	kind Kind
 	name string
 	Spec
 }
 
-func (r register) PhysicalID() uint16 { return r.id }
-func (r register) Kind() Kind         { return r.kind }
-func (r register) Asm() string        { return r.name }
-func (r register) private()           {}
+func (r register) PhysicalID() PID { return r.id }
+func (r register) ID() ID          { return ID(r.id) }
+func (r register) Kind() Kind      { return r.kind }
+func (r register) Asm() string     { return r.name }
+func (r register) private()        {}
 
 type Spec uint16