bytes/hash: initialize all 64 bits of hash seed

Fixes #34925

Change-Id: Iadf12ca47a69b62c3f48d732b430cc85cf62a91c
Reviewed-on: https://go-review.googlesource.com/c/go/+/202577
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.14.html

@@ -180,6 +180,19 @@ TODO
 
 </dl><!-- mime -->
 
+<dl id="math"><dt><a href="/pkg/math/">math</a></dt>
+  <dd>
+    <p><!-- CL 127458 -->
+      The new <a href="/pkg/math/#Fma"><code>Fma</code></a> function
+      computes <code>x*y+z</code> in floating point with no
+      intermediate rounding of the <code>x*y</code>
+      computation. Several architectures implement this computation
+      using dedicated hardware instructions for additional
+      performance.
+    </p>
+
+</dl><!-- math -->
+
 <dl id="plugin"><dt><a href="/pkg/plugin/">plugin</a></dt>
   <dd>
     <p><!-- CL 191617 -->

src/bytes/hash/hash.go

@@ -130,7 +130,9 @@ func MakeSeed(s uint64) Seed {
 // New returns a new Hash object. Different hash objects allocated by
 // this function will very likely have different seeds.
 func New() *Hash {
-	seed := Seed{s: uint64(runtime_fastrand())}
+	s1 := uint64(runtime_fastrand())
+	s2 := uint64(runtime_fastrand())
+	seed := Seed{s: s1<<32 + s2}
 	return &Hash{
 		seed:  seed,
 		state: seed,

src/bytes/hash/hash_test.go

@@ -61,6 +61,20 @@ func TestHashBytesVsString(t *testing.T) {
 	}
 }
 
+func TestHashHighBytes(t *testing.T) {
+	// See issue 34925.
+	const N = 10
+	m := map[uint64]struct{}{}
+	for i := 0; i < N; i++ {
+		h := hash.New()
+		h.AddString("foo")
+		m[h.Hash()>>32] = struct{}{}
+	}
+	if len(m) < N/2 {
+		t.Errorf("from %d seeds, wanted at least %d different hashes; got %d", N, N/2, len(m))
+	}
+}
+
 // Make sure a Hash implements the hash.Hash and hash.Hash64 interfaces.
 var _ basehash.Hash = &hash.Hash{}
 var _ basehash.Hash64 = &hash.Hash{}