diff --git a/compiler/optimizing/code_generator_arm.cc b/compiler/optimizing/code_generator_arm.cc
index 20cdae3..e664089 100644
--- a/compiler/optimizing/code_generator_arm.cc
+++ b/compiler/optimizing/code_generator_arm.cc
@@ -624,6 +624,10 @@
 // probably still be a from-space reference (unless it gets updated by
 // another thread, or if another thread installed another object
 // reference (different from `ref`) in `obj.field`).
+//
+// If `entrypoint` is a valid location it is assumed to already be
+// holding the entrypoint. The case where the entrypoint is passed in
+// is for the GcRoot read barrier.
 class ReadBarrierMarkSlowPathARM : public SlowPathCodeARM {
  public:
   ReadBarrierMarkSlowPathARM(HInstruction* instruction,
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index 598be47..017dd36 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -619,8 +619,10 @@
 // probably still be a from-space reference (unless it gets updated by
 // another thread, or if another thread installed another object
 // reference (different from `ref`) in `obj.field`).
-// If entrypoint is a valid location it is assumed to already be holding the entrypoint. The case
-// where the entrypoint is passed in is for the GcRoot read barrier.
+//
+// If `entrypoint` is a valid location it is assumed to already be
+// holding the entrypoint. The case where the entrypoint is passed in
+// is for the GcRoot read barrier.
 class ReadBarrierMarkSlowPathARM64 : public SlowPathCodeARM64 {
  public:
   ReadBarrierMarkSlowPathARM64(HInstruction* instruction,
diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index 137b554..9276616 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -7136,9 +7136,10 @@
       // Fast path implementation of art::ReadBarrier::BarrierForRoot when
       // Baker's read barrier are used:
       //
-      //   root = *address;
-      //   if (Thread::Current()->GetIsGcMarking()) {
-      //     root = ReadBarrier::Mark(root)
+      //   root = obj.field;
+      //   temp = Thread::Current()->pReadBarrierMarkReg ## root.reg()
+      //   if (temp != null) {
+      //     root = temp(root)
       //   }
 
       // /* GcRoot<mirror::Object> */ root = *address
@@ -7159,8 +7160,11 @@
           instruction, root, /* unpoison_ref_before_marking */ false);
       codegen_->AddSlowPath(slow_path);
 
-      __ fs()->cmpl(Address::Absolute(Thread::IsGcMarkingOffset<kX86PointerSize>().Int32Value()),
-                    Immediate(0));
+      // Test the entrypoint (`Thread::Current()->pReadBarrierMarkReg ## root.reg()`).
+      const int32_t entry_point_offset =
+          CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kX86PointerSize>(root.reg());
+      __ fs()->cmpl(Address::Absolute(entry_point_offset), Immediate(0));
+      // The entrypoint is null when the GC is not marking.
       __ j(kNotEqual, slow_path->GetEntryLabel());
       __ Bind(slow_path->GetExitLabel());
     } else {
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index c5367ce..c3ed416 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -6501,9 +6501,10 @@
       // Fast path implementation of art::ReadBarrier::BarrierForRoot when
       // Baker's read barrier are used:
       //
-      //   root = *address;
-      //   if (Thread::Current()->GetIsGcMarking()) {
-      //     root = ReadBarrier::Mark(root)
+      //   root = obj.field;
+      //   temp = Thread::Current()->pReadBarrierMarkReg ## root.reg()
+      //   if (temp != null) {
+      //     root = temp(root)
       //   }
 
       // /* GcRoot<mirror::Object> */ root = *address
@@ -6524,9 +6525,11 @@
           instruction, root, /* unpoison_ref_before_marking */ false);
       codegen_->AddSlowPath(slow_path);
 
-      __ gs()->cmpl(Address::Absolute(Thread::IsGcMarkingOffset<kX86_64PointerSize>().Int32Value(),
-                                      /* no_rip */ true),
-                    Immediate(0));
+      // Test the `Thread::Current()->pReadBarrierMarkReg ## root.reg()` entrypoint.
+      const int32_t entry_point_offset =
+          CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kX86_64PointerSize>(root.reg());
+      __ gs()->cmpl(Address::Absolute(entry_point_offset, /* no_rip */ true), Immediate(0));
+      // The entrypoint is null when the GC is not marking.
       __ j(kNotEqual, slow_path->GetEntryLabel());
       __ Bind(slow_path->GetExitLabel());
     } else {