runtime,hash/maphash: reuse hashSets to save memory pressure

Might help with OOMs on 32-bit platforms

Change-Id: Idd5129c61ecdfeedd5a9a18fce85dbba27cab946
Reviewed-on: https://go-review.googlesource.com/c/go/+/574475
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 27 insertions, 25 deletions

diff --git a/src/hash/maphash/smhasher_test.go b/src/hash/maphash/smhasher_test.go
index f34cea8e80..28cdae0444 100644
--- a/src/hash/maphash/smhasher_test.go
+++ b/src/hash/maphash/smhasher_test.go
@@ -112,6 +112,8 @@ func (s *hashSet) check(t *testing.T) {
 	if float64(collisions) > expected+SLOP*(3*stddev+1) {
 		t.Errorf("unexpected number of collisions: got=%d mean=%f stddev=%f", collisions, expected, stddev)
 	}
+	// Reset for reuse
+	s.list = s.list[:0]
 }
 
 // a string plus adding zeros must make distinct hashes
@@ -212,8 +214,8 @@ func TestSmhasherCyclic(t *testing.T) {
 	r := rand.New(rand.NewSource(1234))
 	const REPEAT = 8
 	const N = 1000000
+	h := newHashSet()
 	for n := 4; n <= 12; n++ {
-		h := newHashSet()
 		b := make([]byte, REPEAT*n)
 		for i := 0; i < N; i++ {
 			b[0] = byte(i * 79 % 97)
@@ -238,18 +240,18 @@ func TestSmhasherSparse(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
-	sparse(t, 32, 6)
-	sparse(t, 40, 6)
-	sparse(t, 48, 5)
-	sparse(t, 56, 5)
-	sparse(t, 64, 5)
-	sparse(t, 96, 4)
-	sparse(t, 256, 3)
-	sparse(t, 2048, 2)
-}
-func sparse(t *testing.T, n int, k int) {
-	b := make([]byte, n/8)
 	h := newHashSet()
+	sparse(t, h, 32, 6)
+	sparse(t, h, 40, 6)
+	sparse(t, h, 48, 5)
+	sparse(t, h, 56, 5)
+	sparse(t, h, 64, 5)
+	sparse(t, h, 96, 4)
+	sparse(t, h, 256, 3)
+	sparse(t, h, 2048, 2)
+}
+func sparse(t *testing.T, h *hashSet, n int, k int) {
+	b := make([]byte, n/8)
 	setbits(h, b, 0, k)
 	h.check(t)
 }
@@ -276,15 +278,15 @@ func TestSmhasherPermutation(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
-	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7}, 8)
-	permutation(t, []uint32{0, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 8)
-	permutation(t, []uint32{0, 1}, 20)
-	permutation(t, []uint32{0, 1 << 31}, 20)
-	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 6)
+	h := newHashSet()
+	permutation(t, h, []uint32{0, 1, 2, 3, 4, 5, 6, 7}, 8)
+	permutation(t, h, []uint32{0, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 8)
+	permutation(t, h, []uint32{0, 1}, 20)
+	permutation(t, h, []uint32{0, 1 << 31}, 20)
+	permutation(t, h, []uint32{0, 1, 2, 3, 4, 5, 6, 7, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 6)
 }
-func permutation(t *testing.T, s []uint32, n int) {
+func permutation(t *testing.T, h *hashSet, s []uint32, n int) {
 	b := make([]byte, n*4)
-	h := newHashSet()
 	genPerm(h, b, s, 0)
 	h.check(t)
 }
@@ -418,8 +420,8 @@ func windowed(t *testing.T, k key) {
 	}
 	const BITS = 16
 
+	h := newHashSet()
 	for r := 0; r < k.bits(); r++ {
-		h := newHashSet()
 		for i := 0; i < 1<<BITS; i++ {
 			k.clear()
 			for j := 0; j < BITS; j++ {
@@ -438,18 +440,18 @@ func TestSmhasherText(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
-	text(t, "Foo", "Bar")
-	text(t, "FooBar", "")
-	text(t, "", "FooBar")
+	h := newHashSet()
+	text(t, h, "Foo", "Bar")
+	text(t, h, "FooBar", "")
+	text(t, h, "", "FooBar")
 }
-func text(t *testing.T, prefix, suffix string) {
+func text(t *testing.T, h *hashSet, prefix, suffix string) {
 	const N = 4
 	const S = "ABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrst0123456789"
 	const L = len(S)
 	b := make([]byte, len(prefix)+N+len(suffix))
 	copy(b, prefix)
 	copy(b[len(prefix)+N:], suffix)
-	h := newHashSet()
 	c := b[len(prefix):]
 	for i := 0; i < L; i++ {
 		c[0] = S[i]