runtime: a better fallback hash

For arm and powerpc, as well as x86 without aes instructions.
Contains a mixture of ideas from cityhash and xxhash.

Compared to our old fallback on ARM, it's ~no slower on
small objects and up to ~50% faster on large objects.  More
importantly, it is a much better hash function and thus has
less chance of bad behavior.

Fixes #8737

benchmark                         old ns/op     new ns/op     delta
BenchmarkHash5                    173           181           +4.62%
BenchmarkHash16                   252           212           -15.87%
BenchmarkHash64                   575           419           -27.13%
BenchmarkHash1024                 7173          3995          -44.31%
BenchmarkHash65536                516940        313173        -39.42%
BenchmarkHashStringSpeed          300           279           -7.00%
BenchmarkHashBytesSpeed           478           424           -11.30%
BenchmarkHashInt32Speed           217           207           -4.61%
BenchmarkHashInt64Speed           262           231           -11.83%
BenchmarkHashStringArraySpeed     609           631           +3.61%

Change-Id: I0a9335028f32b10ad484966e3019987973afd3eb
Reviewed-on: https://go-review.googlesource.com/1360
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 0 insertions, 9 deletions

diff --git a/src/runtime/hash_test.go b/src/runtime/hash_test.go
index 41fff98eb0..6b229bdb88 100644
--- a/src/runtime/hash_test.go
+++ b/src/runtime/hash_test.go
@@ -171,9 +171,6 @@ func TestSmhasherCyclic(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
-	if !HaveGoodHash() {
-		t.Skip("fallback hash not good enough for this test")
-	}
 	r := rand.New(rand.NewSource(1234))
 	const REPEAT = 8
 	const N = 1000000
@@ -235,9 +232,6 @@ func TestSmhasherPermutation(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
-	if !HaveGoodHash() {
-		t.Skip("fallback hash not good enough for this test")
-	}
 	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)
@@ -404,9 +398,6 @@ func (k *IfaceKey) name() string {
 
 // Flipping a single bit of a key should flip each output bit with 50% probability.
 func TestSmhasherAvalanche(t *testing.T) {
-	if !HaveGoodHash() {
-		t.Skip("fallback hash not good enough for this test")
-	}
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}