[serializer] Explicitly declare unused byte codes

Creates a macro that in debug builds generates case statements for
unused bytes codes (marked UNREACHABLE). This will catch the case where
a byte code declared to be unused is actually used. Should be easier to
maintain than the existing comments.

Change-Id: I0b5d830be88b7ef747975657283c1b1e98182360
Reviewed-on: https://chromium-review.googlesource.com/928650Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51482}

src/snapshot/deserializer.cc

@@ -647,12 +647,17 @@ bool Deserializer<AllocatorT>::ReadData(Object** current, Object** limit,
         break;
       }
 
+#ifdef DEBUG
+#define UNUSED_CASE(byte_code) \
+  case byte_code:              \
+    UNREACHABLE();
+      UNUSED_SERIALIZER_BYTE_CODES(UNUSED_CASE)
+#endif
+#undef UNUSED_CASE
+
 #undef SIXTEEN_CASES
 #undef FOUR_CASES
 #undef SINGLE_CASE
-
-      default:
-        UNREACHABLE();
     }
   }
   CHECK_EQ(limit, current);

src/snapshot/serializer-common.h

@@ -116,6 +116,40 @@ class SerializerDeserializer : public RootVisitor {
   void RestoreExternalReferenceRedirectors(
       const std::vector<CallHandlerInfo*>& call_handler_infos);
 
+#define UNUSED_SERIALIZER_BYTE_CODES(V) \
+  V(0x1d)                               \
+  V(0x1e)                               \
+  V(0x55)                               \
+  V(0x56)                               \
+  V(0x57)                               \
+  V(0x75)                               \
+  V(0x76)                               \
+  V(0x77)                               \
+  V(0x78)                               \
+  V(0x79)                               \
+  V(0x7a)                               \
+  V(0x7b)                               \
+  V(0x7c)                               \
+  V(0x7d)                               \
+  V(0x7e)                               \
+  V(0x7f)                               \
+  V(0xf0)                               \
+  V(0xf1)                               \
+  V(0xf2)                               \
+  V(0xf3)                               \
+  V(0xf4)                               \
+  V(0xf5)                               \
+  V(0xf6)                               \
+  V(0xf7)                               \
+  V(0xf8)                               \
+  V(0xf9)                               \
+  V(0xfa)                               \
+  V(0xfb)                               \
+  V(0xfc)                               \
+  V(0xfd)                               \
+  V(0xfe)                               \
+  V(0xff)
+
   // ---------- byte code range 0x00..0x7f ----------
   // Byte codes in this range represent Where, HowToCode and WhereToPoint.
   // Where the pointed-to object can be found:
@@ -196,8 +230,6 @@ class SerializerDeserializer : public RootVisitor {
   // Used for embedder-allocated backing stores for TypedArrays.
   static const int kOffHeapBackingStore = 0x1c;
 
-  // 0x1d, 0x1e unused.
-
   // Used for embedder-provided serialization data for embedder fields.
   static const int kEmbedderFieldsData = 0x1f;
 
@@ -217,8 +249,6 @@ class SerializerDeserializer : public RootVisitor {
   static const int kHotObjectWithSkip = 0x58;
   static const int kHotObjectMask = 0x07;
 
-  // 0x55..0x57, 0x75..0x7f unused.
-
   // ---------- byte code range 0x80..0xff ----------
   // First 32 root array items.
   static const int kNumberOfRootArrayConstants = 0x20;
@@ -241,8 +271,6 @@ class SerializerDeserializer : public RootVisitor {
   static const int kFixedRepeat = 0xe0;
   static const int kFixedRepeatStart = kFixedRepeat - 1;
 
-  // 0xf0..0xff unused.
-
   // ---------- special values ----------
   static const int kAnyOldSpace = -1;