Include standard heap object header when debug-printing String objects

This brings %DebugPrint(string) closer to %DebugPrint(object) by also
including the pointer, object kind, and RO/old space. Especially the
pointer can be useful while debugging.

One could consider going even further end printing full details of
the string object, e.g. first and second pointers for cons strings.

Before:

 $ out/debug/d8 --allow-natives-syntax -e '%DebugPrint("abc");'
 DebugPrint: #abc
 0x263f080402cd: [Map] in ReadOnlySpace
 [...]

After:

 $ out/debug/d8 --allow-natives-syntax -e '%DebugPrint("abc");'
 DebugPrint: 0xa830824ffe1: [String] in OldSpace: #abc
 0xa83080402cd: [Map] in ReadOnlySpace
 [...]

Drive-by: Document string printing functions.
Drive-by: Use PrintUC16 in spots that don't want a full debug print.

Bug: v8:10581
Change-Id: Ided59047b9c3edc1830ce7721376dddfd24fad1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228509Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68136}

src/diagnostics/objects-printer.cc

@@ -97,19 +97,28 @@ void Object::Print(std::ostream& os) const {  // NOLINT
   }
 }
 
-void HeapObject::PrintHeader(std::ostream& os, const char* id) {  // NOLINT
-  os << reinterpret_cast<void*>(ptr()) << ": [";
+namespace {
+
+void PrintHeapObjectHeaderWithoutMap(HeapObject object, std::ostream& os,
+                                     const char* id) {  // NOLINT
+  os << reinterpret_cast<void*>(object.ptr()) << ": [";
   if (id != nullptr) {
     os << id;
   } else {
-    os << map().instance_type();
+    os << object.map().instance_type();
   }
   os << "]";
-  if (ReadOnlyHeap::Contains(*this)) {
+  if (ReadOnlyHeap::Contains(object)) {
     os << " in ReadOnlySpace";
-  } else if (GetHeapFromWritableObject(*this)->InOldSpace(*this)) {
+  } else if (GetHeapFromWritableObject(object)->InOldSpace(object)) {
     os << " in OldSpace";
   }
+}
+
+}  // namespace
+
+void HeapObject::PrintHeader(std::ostream& os, const char* id) {  // NOLINT
+  PrintHeapObjectHeaderWithoutMap(*this, os, id);
   if (!IsMap()) os << "\n - map: " << Brief(map());
 }
 
@@ -1046,6 +1055,8 @@ void JSMessageObject::JSMessageObjectPrint(std::ostream& os) {  // NOLINT
 }
 
 void String::StringPrint(std::ostream& os) {  // NOLINT
+  PrintHeapObjectHeaderWithoutMap(*this, os, "String");
+  os << ": ";
   os << PrefixForDebugPrint();
   PrintUC16(os, 0, length());
   os << SuffixForDebugPrint();

src/interpreter/interpreter.cc

@@ -194,14 +194,14 @@ void InterpreterCompilationJob::CheckAndPrintBytecodeMismatch(
     MaybeHandle<String> maybe_name = parse_info()->literal()->GetName(isolate);
     Handle<String> name;
     if (maybe_name.ToHandle(&name) && name->length() != 0) {
-      name->StringPrint(std::cerr);
+      name->PrintUC16(std::cerr);
     } else {
       std::cerr << "anonymous";
     }
     Object script_name = script->GetNameOrSourceURL();
     if (script_name.IsString()) {
       std::cerr << " ";
-      String::cast(script_name).StringPrint(std::cerr);
+      String::cast(script_name).PrintUC16(std::cerr);
       std::cerr << ":" << parse_info()->literal()->start_position();
     }
 #endif

src/objects/objects.cc

@@ -5802,7 +5802,7 @@ void Dictionary<Derived, Shape>::Print(std::ostream& os) {
     if (!dictionary.ToKey(roots, i, &k)) continue;
     os << "\n   ";
     if (k.IsString()) {
-      String::cast(k).StringPrint(os);
+      String::cast(k).PrintUC16(os);
     } else {
       os << Brief(k);
     }

src/objects/string.h

@@ -352,12 +352,20 @@ class String : public TorqueGeneratedString<String, Name> {
   // For use during stack traces.  Performs rudimentary sanity check.
   bool LooksValid();
 
-  // Dispatched behavior.
+  // Printing utility functions.
+  // - PrintUC16 prints the raw string contents to the given stream.
+  //   Non-printable characters are formatted as hex, but otherwise the string
+  //   is printed as-is.
+  // - StringShortPrint and StringPrint have extra formatting: they add a
+  //   prefix and suffix depending on the string kind, may add other information
+  //   such as the string heap object address, may truncate long strings, etc.
   const char* PrefixForDebugPrint() const;
   const char* SuffixForDebugPrint() const;
   void StringShortPrint(StringStream* accumulator);
   void PrintUC16(std::ostream& os, int start = 0, int end = -1);  // NOLINT
   void PrintUC16(StringStream* accumulator, int start, int end);
+
+  // Dispatched behavior.
 #if defined(DEBUG) || defined(OBJECT_PRINT)
   char* ToAsciiArray();
 #endif