cmd/compile: reduce element size of arrays in sparse{map,set}

sparseSet and sparseMap only need 32 bit integers in their
arrays, since a sparseEntry key is also limited to 32 bits.
This appears to reduce the space allocated for at least
one pathological compilation by 1%, perhaps more.

Not necessarily for 1.7, but it saves a little and is very
low-risk.

Change-Id: Icf1185859e9f5fe1261a206b441e02c34f7d02fd
Reviewed-on: https://go-review.googlesource.com/22972
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/ssa/sparsemap.go

@@ -14,13 +14,13 @@ type sparseEntry struct {
 
 type sparseMap struct {
 	dense  []sparseEntry
-	sparse []int
+	sparse []int32
 }
 
 // newSparseMap returns a sparseMap that can map
 // integers between 0 and n-1 to int32s.
 func newSparseMap(n int) *sparseMap {
-	return &sparseMap{nil, make([]int, n)}
+	return &sparseMap{dense: nil, sparse: make([]int32, n)}
 }
 
 func (s *sparseMap) size() int {
@@ -29,14 +29,14 @@ func (s *sparseMap) size() int {
 
 func (s *sparseMap) contains(k ID) bool {
 	i := s.sparse[k]
-	return i < len(s.dense) && s.dense[i].key == k
+	return i < int32(len(s.dense)) && s.dense[i].key == k
 }
 
 // get returns the value for key k, or -1 if k does
 // not appear in the map.
 func (s *sparseMap) get(k ID) int32 {
 	i := s.sparse[k]
-	if i < len(s.dense) && s.dense[i].key == k {
+	if i < int32(len(s.dense)) && s.dense[i].key == k {
 		return s.dense[i].val
 	}
 	return -1
@@ -44,12 +44,12 @@ func (s *sparseMap) get(k ID) int32 {
 
 func (s *sparseMap) set(k ID, v int32) {
 	i := s.sparse[k]
-	if i < len(s.dense) && s.dense[i].key == k {
+	if i < int32(len(s.dense)) && s.dense[i].key == k {
 		s.dense[i].val = v
 		return
 	}
 	s.dense = append(s.dense, sparseEntry{k, v})
-	s.sparse[k] = len(s.dense) - 1
+	s.sparse[k] = int32(len(s.dense)) - 1
 }
 
 // setBit sets the v'th bit of k's value, where 0 <= v < 32
@@ -58,17 +58,17 @@ func (s *sparseMap) setBit(k ID, v uint) {
 		panic("bit index too large.")
 	}
 	i := s.sparse[k]
-	if i < len(s.dense) && s.dense[i].key == k {
+	if i < int32(len(s.dense)) && s.dense[i].key == k {
 		s.dense[i].val |= 1 << v
 		return
 	}
 	s.dense = append(s.dense, sparseEntry{k, 1 << v})
-	s.sparse[k] = len(s.dense) - 1
+	s.sparse[k] = int32(len(s.dense)) - 1
 }
 
 func (s *sparseMap) remove(k ID) {
 	i := s.sparse[k]
-	if i < len(s.dense) && s.dense[i].key == k {
+	if i < int32(len(s.dense)) && s.dense[i].key == k {
 		y := s.dense[len(s.dense)-1]
 		s.dense[i] = y
 		s.sparse[y.key] = i

src/cmd/compile/internal/ssa/sparseset.go

@@ -9,13 +9,13 @@ package ssa
 
 type sparseSet struct {
 	dense  []ID
-	sparse []int
+	sparse []int32
 }
 
 // newSparseSet returns a sparseSet that can represent
 // integers between 0 and n-1
 func newSparseSet(n int) *sparseSet {
-	return &sparseSet{nil, make([]int, n)}
+	return &sparseSet{dense: nil, sparse: make([]int32, n)}
 }
 
 func (s *sparseSet) cap() int {
@@ -28,16 +28,16 @@ func (s *sparseSet) size() int {
 
 func (s *sparseSet) contains(x ID) bool {
 	i := s.sparse[x]
-	return i < len(s.dense) && s.dense[i] == x
+	return i < int32(len(s.dense)) && s.dense[i] == x
 }
 
 func (s *sparseSet) add(x ID) {
 	i := s.sparse[x]
-	if i < len(s.dense) && s.dense[i] == x {
+	if i < int32(len(s.dense)) && s.dense[i] == x {
 		return
 	}
 	s.dense = append(s.dense, x)
-	s.sparse[x] = len(s.dense) - 1
+	s.sparse[x] = int32(len(s.dense)) - 1
 }
 
 func (s *sparseSet) addAll(a []ID) {
@@ -54,7 +54,7 @@ func (s *sparseSet) addAllValues(a []*Value) {
 
 func (s *sparseSet) remove(x ID) {
 	i := s.sparse[x]
-	if i < len(s.dense) && s.dense[i] == x {
+	if i < int32(len(s.dense)) && s.dense[i] == x {
 		y := s.dense[len(s.dense)-1]
 		s.dense[i] = y
 		s.sparse[y] = i