4 files changed, 36 insertions, 8 deletions

diff --git a/openjdkjvmti/ti_heap.cc b/openjdkjvmti/ti_heap.cc
index 0c06ae1169..81d1fc778f 100644
--- a/openjdkjvmti/ti_heap.cc
+++ b/openjdkjvmti/ti_heap.cc
@@ -1039,7 +1039,9 @@ class FollowReferencesHelper final {
           }
 
           auto& java_info = static_cast<const art::JavaFrameRootInfo&>(info);
-          ref_info->stack_local.slot = static_cast<jint>(java_info.GetVReg());
+          size_t vreg = java_info.GetVReg();
+          ref_info->stack_local.slot = static_cast<jint>(
+              vreg <= art::JavaFrameRootInfo::kMaxVReg ? vreg : -1);
           const art::StackVisitor* visitor = java_info.GetVisitor();
           ref_info->stack_local.location =
               static_cast<jlocation>(visitor->GetDexPc(/* abort_on_failure= */ false));
diff --git a/runtime/java_frame_root_info.cc b/runtime/java_frame_root_info.cc
index dd3be5d415..9a0f184a83 100644
--- a/runtime/java_frame_root_info.cc
+++ b/runtime/java_frame_root_info.cc
@@ -24,7 +24,18 @@ void JavaFrameRootInfo::Describe(std::ostream& os) const {
   const StackVisitor* visitor = stack_visitor_;
   CHECK(visitor != nullptr);
   os << "Type=" << GetType() << " thread_id=" << GetThreadId() << " location=" <<
-      visitor->DescribeLocation() << " vreg=" << vreg_;
+      visitor->DescribeLocation() << " vreg=";
+  if (vreg_ == JavaFrameRootInfo::kUnknownVreg) {
+    os << "Unknown";
+  } else if (vreg_ == JavaFrameRootInfo::kImpreciseVreg) {
+    os << "imprecise";
+  } else if (vreg_ == JavaFrameRootInfo::kProxyReferenceArgument) {
+    os << "Proxy reference argument";
+  } else if (vreg_ == JavaFrameRootInfo::kMethodDeclaringClass) {
+    os << "method declaring class";
+  } else {
+    os << vreg_;
+  }
 }
 
 }  // namespace art
diff --git a/runtime/java_frame_root_info.h b/runtime/java_frame_root_info.h
index 8141ea24fd..c21eee1b32 100644
--- a/runtime/java_frame_root_info.h
+++ b/runtime/java_frame_root_info.h
@@ -18,6 +18,7 @@
 #define ART_RUNTIME_JAVA_FRAME_ROOT_INFO_H_
 
 #include <iosfwd>
+#include <limits>
 
 #include "base/locks.h"
 #include "base/macros.h"
@@ -29,6 +30,20 @@ class StackVisitor;
 
 class JavaFrameRootInfo final : public RootInfo {
  public:
+  static_assert(std::numeric_limits<size_t>::max() > std::numeric_limits<uint16_t>::max(),
+                "No extra space in vreg to store meta-data");
+  // Unable to determine what register number the root is from.
+  static constexpr size_t kUnknownVreg = -1;
+  // The register number for the root might be determinable but we did not attempt to find that
+  // information.
+  static constexpr size_t kImpreciseVreg = -2;
+  // The root is from the declaring class of the current method.
+  static constexpr size_t kMethodDeclaringClass = -3;
+  // The root is from the argument to a Proxy invoke.
+  static constexpr size_t kProxyReferenceArgument = -4;
+  // The maximum precise vreg number
+  static constexpr size_t kMaxVReg = std::numeric_limits<uint16_t>::max();
+
   JavaFrameRootInfo(uint32_t thread_id, const StackVisitor* stack_visitor, size_t vreg)
      : RootInfo(kRootJavaFrame, thread_id), stack_visitor_(stack_visitor), vreg_(vreg) {
   }
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 7adcb11c99..088f997855 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -3757,7 +3757,7 @@ class ReferenceMapVisitor : public StackVisitor {
         }
       }
       mirror::Object* new_ref = klass.Ptr();
-      visitor_(&new_ref, /* vreg= */ -1, this);
+      visitor_(&new_ref, /* vreg= */ JavaFrameRootInfo::kMethodDeclaringClass, this);
       if (new_ref != klass) {
         method->CASDeclaringClass(klass.Ptr(), new_ref->AsClass());
       }
@@ -3830,7 +3830,7 @@ class ReferenceMapVisitor : public StackVisitor {
         mirror::Object* ref = ref_addr->AsMirrorPtr();
         if (ref != nullptr) {
           mirror::Object* new_ref = ref;
-          visitor_(&new_ref, /* vreg= */ -1, this);
+          visitor_(&new_ref, /* vreg= */ JavaFrameRootInfo::kProxyReferenceArgument, this);
           if (ref != new_ref) {
             ref_addr->Assign(new_ref);
           }
@@ -3861,7 +3861,7 @@ class ReferenceMapVisitor : public StackVisitor {
                       size_t stack_index ATTRIBUTE_UNUSED,
                       const StackVisitor* stack_visitor)
           REQUIRES_SHARED(Locks::mutator_lock_) {
-        visitor(ref, -1, stack_visitor);
+        visitor(ref, JavaFrameRootInfo::kImpreciseVreg, stack_visitor);
       }
 
       ALWAYS_INLINE
@@ -3869,7 +3869,7 @@ class ReferenceMapVisitor : public StackVisitor {
                          size_t register_index ATTRIBUTE_UNUSED,
                          const StackVisitor* stack_visitor)
           REQUIRES_SHARED(Locks::mutator_lock_) {
-        visitor(ref, -1, stack_visitor);
+        visitor(ref, JavaFrameRootInfo::kImpreciseVreg, stack_visitor);
       }
 
       RootVisitor& visitor;
@@ -3907,8 +3907,8 @@ class ReferenceMapVisitor : public StackVisitor {
         }
 
         if (!found) {
-          // If nothing found, report with -1.
-          visitor(ref, -1, stack_visitor);
+          // If nothing found, report with unknown.
+          visitor(ref, JavaFrameRootInfo::kUnknownVreg, stack_visitor);
         }
       }