runtime: coalesce 0-size allocations

Fixes #3996.

R=ken2
CC=golang-dev
https://golang.org/cl/7001052

src/pkg/runtime/malloc.goc

@@ -697,6 +697,13 @@ runtime·new(Type *typ, uint8 *ret)
 
 	if(raceenabled)
 		m->racepc = runtime·getcallerpc(&typ);
+
+	if(typ->size == 0) {
+		// All 0-length allocations use this pointer.
+		// The language does not require the allocations to
+		// have distinct values.
+		ret = (uint8*)&runtime·zerobase;
+	} else {
 		flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
 		ret = runtime·mallocgc(typ->size, flag, 1, 1);
 
@@ -706,6 +713,7 @@ runtime·new(Type *typ, uint8 *ret)
 			}
 			runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 		}
+	}
 
 	FLUSH(&ret);
 }
@@ -719,6 +727,13 @@ runtime·cnew(Type *typ)
 
 	if(raceenabled)
 		m->racepc = runtime·getcallerpc(&typ);
+
+	if(typ->size == 0) {
+		// All 0-length allocations use this pointer.
+		// The language does not require the allocations to
+		// have distinct values.
+		ret = (uint8*)&runtime·zerobase;
+	} else {
 		flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
 		ret = runtime·mallocgc(typ->size, flag, 1, 1);
 
@@ -728,6 +743,8 @@ runtime·cnew(Type *typ)
 			}
 			runtime·settype(ret, (uintptr)typ | TypeInfo_SingleObject);
 		}
+	}
+
 	return ret;
 }