runtime: always incorporate hash seed at start of hash computation

Otherwise we can get predictable collisions.

R=golang-dev, dave, patrick, rsc
CC=golang-dev
https://golang.org/cl/7051043

src/pkg/runtime/alg.c

@@ -19,13 +19,13 @@ runtime·memhash(uintptr *h, uintptr s, void *a)
 	uintptr hash;
 
 	b = a;
-	hash = M0;
+	hash = M0 ^ *h;
 	while(s > 0) {
 		hash = (hash ^ *b) * M1;
 		b++;
 		s--;
 	}
-	*h = (*h ^ hash) * M1;
+	*h = hash;
 }
 
 void
@@ -355,7 +355,7 @@ void
 runtime·interhash(uintptr *h, uintptr s, void *a)
 {
 	USED(s);
-	*h = (*h ^ runtime·ifacehash(*(Iface*)a)) * M1;
+	*h = runtime·ifacehash(*(Iface*)a, *h ^ M0) * M1;
 }
 
 void
@@ -389,7 +389,7 @@ void
 runtime·nilinterhash(uintptr *h, uintptr s, void *a)
 {
 	USED(s);
-	*h = (*h ^ runtime·efacehash(*(Eface*)a)) * M1;
+	*h = runtime·efacehash(*(Eface*)a, *h ^ M0) * M1;
 }
 
 void

src/pkg/runtime/iface.c

@@ -546,10 +546,10 @@ runtime·assertE2E2(InterfaceType* inter, Eface e, Eface ret, bool ok)
 }
 
 static uintptr
-ifacehash1(void *data, Type *t)
+ifacehash1(void *data, Type *t, uintptr h)
 {
 	Alg *alg;
-	uintptr size, h;
+	uintptr size;
 	Eface err;
 
 	if(t == nil)
@@ -563,7 +563,6 @@ ifacehash1(void *data, Type *t)
 		runtime·newErrorString(runtime·catstring(runtime·gostringnocopy((byte*)"hash of unhashable type "), *t->string), &err);
 		runtime·panic(err);
 	}
-	h = 0;
 	if(size <= sizeof(data))
 		alg->hash(&h, size, &data);
 	else
@@ -572,17 +571,17 @@ ifacehash1(void *data, Type *t)
 }
 
 uintptr
-runtime·ifacehash(Iface a)
+runtime·ifacehash(Iface a, uintptr h)
 {
 	if(a.tab == nil)
-		return 0;
-	return ifacehash1(a.data, a.tab->type);
+		return h;
+	return ifacehash1(a.data, a.tab->type, h);
 }
 
 uintptr
-runtime·efacehash(Eface a)
+runtime·efacehash(Eface a, uintptr h)
 {
-	return ifacehash1(a.data, a.type);
+	return ifacehash1(a.data, a.type, h);
 }
 
 static bool

src/pkg/runtime/runtime.h

@@ -644,8 +644,8 @@ void	runtime·freemcache(MCache*);
 void	runtime·mallocinit(void);
 bool	runtime·ifaceeq_c(Iface, Iface);
 bool	runtime·efaceeq_c(Eface, Eface);
-uintptr	runtime·ifacehash(Iface);
-uintptr	runtime·efacehash(Eface);
+uintptr	runtime·ifacehash(Iface, uintptr);
+uintptr	runtime·efacehash(Eface, uintptr);
 void*	runtime·malloc(uintptr size);
 void	runtime·free(void *v);
 bool	runtime·addfinalizer(void*, void(*fn)(void*), uintptr);