RFR: Add comments in shenandoah_store_check on direct heap field use

[Aleksey Shipilev]

As suggested by aph last week:

------------- 8< -----------------------------------------------------------------------

$ hg diff
diff -r 533d1c8a56d2 src/cpu/aarch64/vm/macroAssembler_aarch64.cpp
--- a/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp	Fri Sep 29 13:46:48 2017 +0000
+++ b/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp	Mon Oct 02 12:45:25 2017 +0200
@@ -5651,6 +5651,9 @@

   // During evacuation and evacuation only, we can have the stores of cset-values
   // to non-cset destinations. Everything else is covered by storeval barriers.
+  // Poll the heap directly: that would be the least performant, yet more reliable way,
+  // because it will also capture the errors in thread-local flags that may break the
+  // write barrier.
   mov(tmp1, ShenandoahHeap::evacuation_in_progress_addr());
   ldrw(tmp1, Address(tmp1));
   cbnzw(tmp1, done);
diff -r 533d1c8a56d2 src/cpu/x86/vm/macroAssembler_x86.cpp
--- a/src/cpu/x86/vm/macroAssembler_x86.cpp	Fri Sep 29 13:46:48 2017 +0000
+++ b/src/cpu/x86/vm/macroAssembler_x86.cpp	Mon Oct 02 12:45:25 2017 +0200
@@ -6446,6 +6446,9 @@

   // During evacuation and evacuation only, we can have the stores of cset-values
   // to non-cset destinations. Everything else is covered by storeval barriers.
+  // Poll the heap directly: that would be the least performant, yet more reliable way,
+  // because it will also capture the errors in thread-local flags that may break the
+  // write barrier.
   movptr(tmp, (intptr_t) ShenandoahHeap::evacuation_in_progress_addr());
   movbool(tmp, Address(tmp, 0));
   testbool(tmp);

------------- 8< -----------------------------------------------------------------------

Turns out, we poll any flags only on this path in the checking code.

Also, I note the inconsistency here: we should use ldrb/cbnz in aarch64, right?

Thanks,
-Aleksey