runtime: fix aeshash of empty string

Aeshash currently computes the hash of the empty string as
hash("", seed) = seed.  This is bad because the hash of a compound
object with empty strings in it doesn't include information about
where those empty strings were.  For instance [2]string{"", "foo"}
and [2]string{"foo", ""} might get the same hash.

Fix this by returning a scrambled seed instead of the seed itself.
With this fix, we can remove the scrambling done by the generated
array hash routines.

The test also rejects hash("", seed) = 0, if we ever thought
it would be a good idea to try that.

The fallback hash is already OK in this regard.

Change-Id: Iaedbaa5be8d6a246dc7e9383d795000e0f562037
Reviewed-on: https://go-review.googlesource.com/14129Reviewed-by: jcd . <jcd@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -2611,21 +2611,6 @@ func genhash(sym *Sym, t *Type) {
 		colasdefn(n.List, n)
 		ni = n.List.N
 
-		// TODO: with aeshash we don't need these shift/mul parts
-
-		// h = h<<3 | h>>61
-		n.Nbody = list(n.Nbody, Nod(OAS, nh, Nod(OOR, Nod(OLSH, nh, Nodintconst(3)), Nod(ORSH, nh, Nodintconst(int64(Widthptr)*8-3)))))
-
-		// h *= mul
-		// Same multipliers as in runtime.memhash.
-		var mul int64
-		if Widthptr == 4 {
-			mul = 3267000013
-		} else {
-			mul = 23344194077549503
-		}
-		n.Nbody = list(n.Nbody, Nod(OAS, nh, Nod(OMUL, nh, Nodintconst(mul))))
-
 		// h = hashel(&p[i], h)
 		call := Nod(OCALL, hashel, nil)
 

src/runtime/asm_386.s

@@ -1012,9 +1012,10 @@ endofpage:
 	RET
 
 aes0:
-	// return input seed
-	MOVL	h+4(FP), AX
-	MOVL	AX, (DX)
+	// Return scrambled input seed
+	AESENC	X7, X6
+	AESENC	X7, X6
+	MOVL	X6, (DX)
 	RET
 
 aes16:

src/runtime/asm_amd64.s

@@ -1003,9 +1003,10 @@ endofpage:
 	RET
 
 aes0:
-	// return input seed
-	MOVQ	h+8(FP), AX
-	MOVQ	AX, (DX)
+	// Return scrambled input seed
+	AESENC	X7, X6
+	AESENC	X7, X6
+	MOVQ	X6, (DX)
 	RET
 
 aes16:

src/runtime/hash_test.go

@@ -561,3 +561,100 @@ func BenchmarkHash16(b *testing.B)    { benchmarkHash(b, 16) }
 func BenchmarkHash64(b *testing.B)    { benchmarkHash(b, 64) }
 func BenchmarkHash1024(b *testing.B)  { benchmarkHash(b, 1024) }
 func BenchmarkHash65536(b *testing.B) { benchmarkHash(b, 65536) }
+
+func TestArrayHash(t *testing.T) {
+	// Make sure that "" in arrays hash correctly.  The hash
+	// should at least scramble the input seed so that, e.g.,
+	// {"","foo"} and {"foo",""} have different hashes.
+
+	// If the hash is bad, then all (8 choose 4) = 70 keys
+	// have the same hash. If so, we allocate 70/8 = 8
+	// overflow buckets.  If the hash is good we don't
+	// normally allocate any overflow buckets, and the
+	// probability of even one or two overflows goes down rapidly.
+	// (There is always 1 allocation of the bucket array.  The map
+	// header is allocated on the stack.)
+	f := func() {
+		// Make the key type at most 128 bytes.  Otherwise,
+		// we get an allocation per key.
+		type key [8]string
+		m := make(map[key]bool, 70)
+
+		// fill m with keys that have 4 "foo"s and 4 ""s.
+		for i := 0; i < 256; i++ {
+			var k key
+			cnt := 0
+			for j := uint(0); j < 8; j++ {
+				if i>>j&1 != 0 {
+					k[j] = "foo"
+					cnt++
+				}
+			}
+			if cnt == 4 {
+				m[k] = true
+			}
+		}
+		if len(m) != 70 {
+			t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
+		}
+	}
+	if n := testing.AllocsPerRun(10, f); n > 6 {
+		t.Errorf("too many allocs %f - hash not balanced", n)
+	}
+}
+func TestStructHash(t *testing.T) {
+	// See the comment in TestArrayHash.
+	f := func() {
+		type key struct {
+			a, b, c, d, e, f, g, h string
+		}
+		m := make(map[key]bool, 70)
+
+		// fill m with keys that have 4 "foo"s and 4 ""s.
+		for i := 0; i < 256; i++ {
+			var k key
+			cnt := 0
+			if i&1 != 0 {
+				k.a = "foo"
+				cnt++
+			}
+			if i&2 != 0 {
+				k.b = "foo"
+				cnt++
+			}
+			if i&4 != 0 {
+				k.c = "foo"
+				cnt++
+			}
+			if i&8 != 0 {
+				k.d = "foo"
+				cnt++
+			}
+			if i&16 != 0 {
+				k.e = "foo"
+				cnt++
+			}
+			if i&32 != 0 {
+				k.f = "foo"
+				cnt++
+			}
+			if i&64 != 0 {
+				k.g = "foo"
+				cnt++
+			}
+			if i&128 != 0 {
+				k.h = "foo"
+				cnt++
+			}
+			if cnt == 4 {
+				m[k] = true
+			}
+		}
+		if len(m) != 70 {
+			t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
+		}
+	}
+	if n := testing.AllocsPerRun(10, f); n > 6 {
+		t.Errorf("too many allocs %f - hash not balanced", n)
+	}
+}