use object addresses as unique IDs

Using a global counter for unique IDs means that eventually, every long-running program will reuse IDs of live objects. Go's garbage collection currently is not compacting; if it ever becomes so, we can modify contains.Set or choose a new method to obtain unique IDs.

go.mod

@@ -5,8 +5,8 @@ go 1.14
 require (
 	github.com/zephyrtronium/contains v0.0.0-20191109101209-403ebe7b0b60
 	gitlab.com/variadico/lctime v0.0.0-20190211022338-49aae8a53d11
-	golang.org/x/sys v0.0.0-20200302150141-5c8b2ff67527
+	golang.org/x/sys v0.0.0-20200409092240-59c9f1ba88fa
 	golang.org/x/text v0.3.2
-	golang.org/x/tools v0.0.0-20200313205530-4303120df7d8
+	golang.org/x/tools v0.0.0-20200410194907-79a7a3126eef
 	gopkg.in/yaml.v2 v2.2.8
 )

go.sum

@@ -1,4 +1,5 @@
 github.com/yuin/goldmark v1.1.25/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
+github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/zephyrtronium/contains v0.0.0-20190813150354-f7df53f08000 h1:rsL4vYsl1EVDklYnVDq2etRQv7jZtFu2gkv4OzKKFbo=
 github.com/zephyrtronium/contains v0.0.0-20190813150354-f7df53f08000/go.mod h1:9VfULMvEEtBLQ93TyMwEFa2rMZj232ZMs7HA/dx31Eg=
 github.com/zephyrtronium/contains v0.0.0-20191109101209-403ebe7b0b60 h1:spBs4SQ3oIUAGBh3fZLGWjQ6iAiEtmxmM5o68l5mTH8=
@@ -7,6 +8,7 @@ gitlab.com/variadico/lctime v0.0.0-20190211022338-49aae8a53d11 h1:Ly1b+G8N/Hhmty
 gitlab.com/variadico/lctime v0.0.0-20190211022338-49aae8a53d11/go.mod h1:oBjcCtUE+aWYiW4NuGN+eK3OK3efpcyKphbDv1Y508M=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/mod v0.2.0 h1:KU7oHjnv3XNWfa5COkzUifxZmxp1TyI7ImMXqFxLwvQ=
 golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
@@ -19,6 +21,8 @@ golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe h1:6fAMxZRR6sl1Uq8U61gxU+kPT
 golang.org/x/sys v0.0.0-20190924154521-2837fb4f24fe/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200302150141-5c8b2ff67527 h1:uYVVQ9WP/Ds2ROhcaGPeIdVq0RIXVLwsHlnvJ+cT1So=
 golang.org/x/sys v0.0.0-20200302150141-5c8b2ff67527/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200409092240-59c9f1ba88fa h1:mQTN3ECqfsViCNBgq+A40vdwhkGykrrQlYe3mPj6BoU=
+golang.org/x/sys v0.0.0-20200409092240-59c9f1ba88fa/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
@@ -29,8 +33,11 @@ golang.org/x/tools v0.0.0-20191101200257-8dbcdeb83d3f/go.mod h1:b+2E5dAYhXwXZwtn
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20200313205530-4303120df7d8 h1:gkI/wGGwpcG5W4hLCzZNGxA4wzWBGGDStRI1MrjDl2Q=
 golang.org/x/tools v0.0.0-20200313205530-4303120df7d8/go.mod h1:Sl4aGygMT6LrqrWclx+PTx3U+LnKx/seiNR+3G19Ar8=
+golang.org/x/tools v0.0.0-20200410194907-79a7a3126eef h1:RHORRhs540cYZYrzgU2CPUyykkwZM78hGdzocOo9P8A=
+golang.org/x/tools v0.0.0-20200410194907-79a7a3126eef/go.mod h1:EkVYQZoAsY45+roYkvgYkIh4xh/qjgUK9TdY2XT94GE=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

internal/object.go

@@ -5,16 +5,13 @@ import (
 	"os"
 	"strings"
 	"sync"
-	"sync/atomic"
 	"time"
+	"unsafe" // for UniqueID
 
 	"github.com/zephyrtronium/contains"
 )
 
 // Object is the basic type of Io. Everything is an Object.
-//
-// Always use NewObject, ObjectWith, or a type-specific constructor to obtain
-// new objects. Creating objects directly will result in arbitrary failures.
 type Object struct {
 	// slots is the set of messages to which this object responds.
 	slots actualSlots
@@ -29,9 +26,6 @@ type Object struct {
 	Value interface{}
 	// tag is the type indicator of the object.
 	tag Tag
-
-	// id is the object's unique ID.
-	id uintptr
 }
 
 // Tag is a type indicator for iolang objects. Tag values must be comparable.
@@ -86,7 +80,6 @@ func (o *Object) Clone() *Object {
 		protos: protoLink{p: o},
 		Value:  v,
 		tag:    o.Tag(),
-		id:     nextObject(),
 	}
 }
 
@@ -125,7 +118,10 @@ func (o *Object) Tag() Tag {
 
 // UniqueID returns the object's unique ID.
 func (o *Object) UniqueID() uintptr {
-	return o.id
+	// This implementation depends on Go's garbage collector remaining
+	// non-compacting. Should that change, then either this or contains.Set
+	// will need to change as well.
+	return uintptr(unsafe.Pointer(o))
 }
 
 // BasicTag is a special Tag type for basic primitive types which do not have
@@ -148,16 +144,6 @@ func (t BasicTag) String() string {
 	return string(t)
 }
 
-// objcounter is the global counter for object IDs. All accesses to this must
-// be atomic.
-var objcounter uintptr
-
-// nextObject increments the object counter and returns its value as a unique
-// ID for a new object.
-func nextObject() uintptr {
-	return atomic.AddUintptr(&objcounter, 1)
-}
-
 // initObject sets up the "base" object that is the first proto of all other
 // built-in types.
 func (vm *VM) initObject() {
@@ -254,7 +240,6 @@ func (vm *VM) ObjectWith(slots Slots, protos []*Object, value interface{}, tag T
 	r := &Object{
 		Value: value,
 		tag:   tag,
-		id:    nextObject(),
 	}
 	r.SetProtos(protos...)
 	vm.definitelyNewSlots(r, slots)
@@ -266,7 +251,6 @@ func (vm *VM) ObjectWith(slots Slots, protos []*Object, value interface{}, tag T
 func (vm *VM) NewObject(slots Slots) *Object {
 	r := &Object{
 		protos: protoLink{p: vm.BaseObject},
-		id:     nextObject(),
 	}
 	vm.definitelyNewSlots(r, slots)
 	return r

internal/vm.go

@@ -77,15 +77,15 @@ func NewVM(args ...string) *VM {
 	haveVM = true // TODO: atomic?
 
 	vm := VM{
-		Lobby: &Object{id: nextObject()},
+		Lobby: &Object{},
 
-		Core:   &Object{id: nextObject()},
-		Addons: &Object{id: nextObject()},
+		Core:   &Object{},
+		Addons: &Object{},
 
-		BaseObject: &Object{id: nextObject()},
-		True:       &Object{id: nextObject()},
-		False:      &Object{id: nextObject()},
-		Nil:        &Object{id: nextObject()},
+		BaseObject: &Object{},
+		True:       &Object{},
+		False:      &Object{},
+		Nil:        &Object{},
 
 		Control: make(chan RemoteStop, 1),