RFR(s): Wrong alignment in CDS related Klass* reallocating code

[Fedor Burdun]

Hello all,

I've found an inconsistency in CDS related Klass* reallocating code, in method

        static void ArchiveCompactor::allocate(MetaspaceClosure::Ref* ref, bool read_only) (src/hotspot/share/memory/metaspaceShared.cpp:1122)

the alignment = BytesPerWord has been used, that is fine for 64bit platforms, but may cause problems on 32bit ones.
For example, in case of ARM gcc compiles accessors for u8/long long types using ldrd/strd instructions that requires a double-word alignment on ARMv5TE and older.


Moreover the method (although it used only for compressed oops)

        inline bool check_klass_alignment(Klass* obj) in src/hotspot/share/oops/klass.inline.hpp

checks classes to be aligned to KlassAlignment const, not to the BytesPerWord one.

So I would like to suggest the fix below:

diff --git a/src/hotspot/share/memory/metaspaceShared.cpp b/src/hotspot/share/memory/metaspaceShared.cpp
--- a/src/hotspot/share/memory/metaspaceShared.cpp
+++ b/src/hotspot/share/memory/metaspaceShared.cpp
@@ -1123,7 +1123,7 @@
     address obj = ref->obj();
     int bytes = ref->size() * BytesPerWord;
     char* p;
-    size_t alignment = BytesPerWord;
+    size_t alignment = KlassAlignmentInBytes;
     char* oldtop;
     char* newtop;


Thanks,
Fedor