src/cpu/x86/vm/macroAssembler_x86.cpp

rev 10501 : [backport] Check heap stability in C1 WBs
rev 10524 : [backport] Some trivial-ish cleanups
rev 10557 : [backport] Assembler write barriers should consistently check for forwarded objects

@@ -3748,11 +3748,10 @@
 void MacroAssembler::null_check(Register reg, int offset) {
   if (needs_explicit_null_check(offset)) {
     // provoke OS NULL exception if reg = NULL by
     // accessing M[reg] w/o changing any (non-CC) registers
     // NOTE: cmpl is plenty here to provoke a segv
-
     cmpptr(rax, Address(reg, 0));
     // Note: should probably use testl(rax, Address(reg, 0));
     //       may be shorter code (however, this version of
     //       testl needs to be implemented first)
   } else {

@@ -4447,19 +4446,23 @@
 void MacroAssembler::shenandoah_write_barrier(Register dst) {
   assert(UseShenandoahGC && ShenandoahWriteBarrier, "Should be enabled");
 
   Label done;
 
-  // Check for evacuation-in-progress
   Address gc_state(r15_thread, in_bytes(JavaThread::gc_state_offset()));
-  testb(gc_state, ShenandoahHeap::EVACUATION);
 
-  // The read-barrier.
+  // Check for heap stability
+  testb(gc_state, ShenandoahHeap::HAS_FORWARDED | ShenandoahHeap::EVACUATION);
+  jccb(Assembler::zero, done);
+
+  // Heap is unstable, need to perform the read-barrier even if WB is inactive
   if (ShenandoahWriteBarrierRB) {
     movptr(dst, Address(dst, BrooksPointer::byte_offset()));
   }
 
+  // Check for evacuation-in-progress and jump to WB slow-path if needed
+  testb(gc_state, ShenandoahHeap::EVACUATION);
   jccb(Assembler::zero, done);
 
   if (dst != rax) {
     xchgptr(dst, rax); // Move obj into rax and save rax into obj.
   }