cppgc: Trace full pointers in in-construction objects.

On-heap objects may have TracedReferences, which are uncompressed
pointers that still need to be conservatively scanned (another CL).

Bug: chromium:1325007
Change-Id: If05add36191978486f315dc1ef142a7ed8d4a7bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3804808
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82152}

src/heap/cppgc/visitor.cc

@@ -35,33 +35,34 @@ namespace {
 
 void TraceConservatively(ConservativeTracingVisitor* conservative_visitor,
                          const HeapObjectHeader& header) {
-#if defined(CPPGC_POINTER_COMPRESSION)
-  using PointerType = uint32_t;
-#else   // !defined(CPPGC_POINTER_COMPRESSION)
-  using PointerType = uintptr_t;
-#endif  // !defined(CPPGC_POINTER_COMPRESSION)
-
   const auto object_view = ObjectView<>(header);
-  PointerType* object = reinterpret_cast<PointerType*>(object_view.Start());
-  for (size_t i = 0; i < (object_view.Size() / sizeof(PointerType)); ++i) {
-    PointerType maybe_ptr = object[i];
+  uintptr_t* word = reinterpret_cast<uintptr_t*>(object_view.Start());
+  for (size_t i = 0; i < (object_view.Size() / sizeof(uintptr_t)); ++i) {
+    uintptr_t maybe_full_ptr = word[i];
 #if defined(MEMORY_SANITIZER)
     // |object| may be uninitialized by design or just contain padding bytes.
     // Copy into a local variable that is not poisoned for conservative marking.
     // Copy into a temporary variable to maintain the original MSAN state.
-    MSAN_MEMORY_IS_INITIALIZED(&maybe_ptr, sizeof(maybe_ptr));
+    MSAN_MEMORY_IS_INITIALIZED(&maybe_full_ptr, sizeof(maybe_full_ptr));
 #endif
-    if (maybe_ptr > SentinelPointer::kSentinelValue) {
+    // First, check the full pointer.
+    if (maybe_full_ptr > SentinelPointer::kSentinelValue)
+      conservative_visitor->TraceConservativelyIfNeeded(
+          reinterpret_cast<Address>(maybe_full_ptr));
 #if defined(CPPGC_POINTER_COMPRESSION)
-      // We know that all on-heap pointers are compressed, so don't check full
-      // pointers.
-      Address decompressed_ptr =
-          static_cast<Address>(CompressedPointer::Decompress(maybe_ptr));
-#else   // !defined(CPPGC_POINTER_COMPRESSION)
-      Address decompressed_ptr = reinterpret_cast<Address>(maybe_ptr);
+    // Then, check for compressed pointers.
+    auto decompressed_low = reinterpret_cast<Address>(
+        CompressedPointer::Decompress(static_cast<uint32_t>(maybe_full_ptr)));
+    if (decompressed_low >
+        reinterpret_cast<void*>(SentinelPointer::kSentinelValue))
+      conservative_visitor->TraceConservativelyIfNeeded(decompressed_low);
+    auto decompressed_high = reinterpret_cast<Address>(
+        CompressedPointer::Decompress(static_cast<uint32_t>(
+            maybe_full_ptr >> (sizeof(uint32_t) * CHAR_BIT))));
+    if (decompressed_high >
+        reinterpret_cast<void*>(SentinelPointer::kSentinelValue))
+      conservative_visitor->TraceConservativelyIfNeeded(decompressed_high);
 #endif  // !defined(CPPGC_POINTER_COMPRESSION)
-      conservative_visitor->TraceConservativelyIfNeeded(decompressed_ptr);
-    }
   }
 }