Valgrind: cppgc "Conditional jump or move depends on uninitialised value(s)"

[mmastrac]

Valgrind reports that we're somehow depending on uninitialized values in v8's cppgc heap. Reproducible by running unit tests directly in valgrind.

==3963== 160 errors in context 8 of 10:
==3963== Conditional jump or move depends on uninitialised value(s)
==3963==    at 0x3311FDF: cppgc::internal::ConservativeTracingVisitor::TraceConservativelyIfNeeded(void const*)::$_0::operator()(unsigned char*) const (visitor.cc:94)
==3963==    by 0x2CD9821: v8::internal::(anonymous namespace)::UnifiedHeapConservativeMarkingVisitor::TraceConservativelyIfNeeded(void const*) (cpp-heap.cc:260)
==3963==    by 0x3314319: heap::base::(anonymous namespace)::IteratePointersInStack(heap::base::StackVisitor*, void const*, void const*, void const*) (stack.cc:137)
==3963==    by 0x3314360: heap::base::Stack::IteratePointersUntilMarker(heap::base::StackVisitor*) const (stack.cc:189)
==3963==    by 0x3304AF8: cppgc::internal::MarkerBase::VisitRoots(cppgc::EmbedderStackState) (marker.cc:483)
==3963==    by 0x3304DE9: cppgc::internal::MarkerBase::EnterAtomicPause(cppgc::EmbedderStackState) (marker.cc:294)
==3963==    by 0x2CD8A8D: v8::internal::CppHeap::EnterFinalPause(cppgc::EmbedderStackState) (cpp-heap.cc:821)
==3963==    by 0x2D5E357: v8::internal::MarkCompactCollector::MarkLiveObjects() (mark-compact.cc:2425)
==3963==    by 0x2D5DFCD: v8::internal::MarkCompactCollector::CollectGarbage() (mark-compact.cc:409)
==3963==    by 0x2D14F18: v8::internal::Heap::MarkCompact() (heap.cc:2695)
==3963==    by 0x2D14628: v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::internal::GarbageCollectionReason, char const*) (heap.cc:2381)
==3963==    by 0x2D25B9C: v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags)::$_1::operator()() const (heap.cc:1865)
==3963== 
==3963== 
==3963== 160 errors in context 9 of 10:
==3963== Conditional jump or move depends on uninitialised value(s)
==3963==    at 0x3311FDF: cppgc::internal::ConservativeTracingVisitor::TraceConservativelyIfNeeded(void const*)::$_0::operator()(unsigned char*) const (visitor.cc:94)
==3963==    by 0x3311E5E: cppgc::internal::ConservativeTracingVisitor::TraceConservativelyIfNeeded(void const*) (visitor.cc:106)
==3963==    by 0x2CD9821: v8::internal::(anonymous namespace)::UnifiedHeapConservativeMarkingVisitor::TraceConservativelyIfNeeded(void const*) (cpp-heap.cc:260)
==3963==    by 0x3314319: heap::base::(anonymous namespace)::IteratePointersInStack(heap::base::StackVisitor*, void const*, void const*, void const*) (stack.cc:137)
==3963==    by 0x3314360: heap::base::Stack::IteratePointersUntilMarker(heap::base::StackVisitor*) const (stack.cc:189)
==3963==    by 0x3304AF8: cppgc::internal::MarkerBase::VisitRoots(cppgc::EmbedderStackState) (marker.cc:483)
==3963==    by 0x3304DE9: cppgc::internal::MarkerBase::EnterAtomicPause(cppgc::EmbedderStackState) (marker.cc:294)
==3963==    by 0x2CD8A8D: v8::internal::CppHeap::EnterFinalPause(cppgc::EmbedderStackState) (cpp-heap.cc:821)
==3963==    by 0x2D5E357: v8::internal::MarkCompactCollector::MarkLiveObjects() (mark-compact.cc:2425)
==3963==    by 0x2D5DFCD: v8::internal::MarkCompactCollector::CollectGarbage() (mark-compact.cc:409)
==3963==    by 0x2D14F18: v8::internal::Heap::MarkCompact() (heap.cc:2695)
==3963==    by 0x2D14628: v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::internal::GarbageCollectionReason, char const*) (heap.cc:2381)
==3963== 
==3963== 
==3963== 250 errors in context 10 of 10:
==3963== Conditional jump or move depends on uninitialised value(s)
==3963==    at 0x331430F: heap::base::(anonymous namespace)::IteratePointersInStack(heap::base::StackVisitor*, void const*, void const*, void const*) (stack.cc:136)
==3963==    by 0x3314360: heap::base::Stack::IteratePointersUntilMarker(heap::base::StackVisitor*) const (stack.cc:189)
==3963==    by 0x3304AF8: cppgc::internal::MarkerBase::VisitRoots(cppgc::EmbedderStackState) (marker.cc:483)
==3963==    by 0x3304DE9: cppgc::internal::MarkerBase::EnterAtomicPause(cppgc::EmbedderStackState) (marker.cc:294)
==3963==    by 0x2CD8A8D: v8::internal::CppHeap::EnterFinalPause(cppgc::EmbedderStackState) (cpp-heap.cc:821)
==3963==    by 0x2D5E357: v8::internal::MarkCompactCollector::MarkLiveObjects() (mark-compact.cc:2425)
==3963==    by 0x2D5DFCD: v8::internal::MarkCompactCollector::CollectGarbage() (mark-compact.cc:409)
==3963==    by 0x2D14F18: v8::internal::Heap::MarkCompact() (heap.cc:2695)
==3963==    by 0x2D14628: v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::internal::GarbageCollectionReason, char const*) (heap.cc:2381)
==3963==    by 0x2D25B9C: v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags)::$_1::operator()() const (heap.cc:1865)
==3963==    by 0x2D256F4: void heap::base::Stack::SetMarkerAndCallbackImpl<v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags)::$_1>(heap::base::Stack*, void*, void const*) (stack.h:95)
==3963==    by 0x331442A: ??? (in /mnt/deno2)
==3963==