Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner AT cygwin DOT com Mail-Followup-To: cygwin AT cygwin DOT com Delivered-To: mailing list cygwin AT cygwin DOT com Message-ID: <40700B4C02ABD5119F000090278766443BF292@hplex1.hpl.hp.com> From: "Boehm, Hans" To: "'tprince AT computer DOT org'" , tromey AT redhat DOT com Cc: "Billinghurst, David (CRTS)" , java AT gcc DOT gnu DOT org, cygwin AT cygwin DOT com Subject: RE: Object alignment, was: cygwin failures - assertion "!(addr & FLAGS)" failed: Date: Mon, 6 May 2002 10:57:38 -0700 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" > From: Tim Prince [mailto:tprince AT computer DOT org] > Stack alignment matters when you use . I didn't > take the > question as being one where gcj and libgcj were to be built > without regard to > other consequences, although I'm surprised if gcj doesn't > allocate anything > on stack. > To clarify a bit more: The Java locking code needs to steal 3 bits from the address of Java objects in order to be able to atomically update all the most important fields in a "thin lock". Thus it's critical that the least 3 significant bits in the addresses of Java objects are not important. Currently this is enforced by insisting that all Java objects used for synchronization be 8-byte aligned. We have talked about changing the code on some platforms so that the only requirement becomes that 2 different object addresses used for synchronization must be at least 8 bytes apart. This change is a bit tricky, but I don't think it's terribly expensive. It may make sense to do this for 3.2. (It may also be possible to get by with 2 bits instead of 3. But that requires rethinking some very tricky code.) AFAIK, Gcj currently does not try to turn heap allocations of Java objects into stack allocations. But if it did so (and ideally it should), we probably still wouldn't need 8 byte stack alignment, since it's unlikely you would be able to perform this optimization for objects that are shared between threads. Thus you should be able to replace any synchronization on these objects by (at most) suitable memory barriers. Hans -- Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple Bug reporting: http://cygwin.com/bugs.html Documentation: http://cygwin.com/docs.html FAQ: http://cygwin.com/faq/