RFR: AARCH64: Changes to HotSpot shared code

[Dean Long]

On 11/11/2014 11:02 AM, Andrew Haley wrote:
> http://cr.openjdk.java.net/~aph/aarch64-JDK-8064611/hotspot.patch
>
> Everything except cpu/ and os_cpu/.
>
> Most of this is obvious and trivial, with a few exceptions.
>
> In memory/metaspace.cpp, we allocated the memory for metadata in a
> different way.  This is because we want to be able to decode and
> encode compressed metadata pointers with a single instruction, and we
> can always do that iff the base address is of a particular form.
>
> In opto/, we have made some changes in order to be able to use AArch64
> store release instructions for volatile field stores.  These don't
> require leading or trailing barriers.  We have tried several times to
> do this without changing shared code, but it is impossible with the
> current back-end interface.
Is this something ppc64 can also take advantage of?  I hope Vladimir can 
suggest
a more flexible way to do this, perhaps with a runtime flag.
> In several places a release store is used where the AArch64 memory
> model makes it unnecessary.  From earlier emails on this list we
> discovered that the only architecture which requires this release
> store is IA64, and OpenJDK does not support it anyway.  We should
> perhaps look at re-engineering the way that memory barriers and memory
> accesses are handled in HotSpot with a view to pushing all these
> architecture-dependent assumptions out to the back ends.
I agree.  More comments below.
> Andrew.
c1_Canonicalizer.cpp
     Can this be handled in the back-end?  I imagine other platforms, 
such as x86, have similar limitations.
c1_LIR.cpp
     It looks like you need a temp for convert because your backend 
because you're checking the FPSR.
     What happens if you ignore the FPSR, do you get a wrong result?
c1_LinearScan.cpp
     I'm not familiar with what the changed code is doing.  Can you 
explain why it applies to x86 and aarch64?
c1_Runtime1.cpp
     This will break our closed port that NOP instructions for 
patching.  How about moving your deopt-instead-of-patch
     support into Runtime1::patch_code() and enable it with a read-only 
platform-specific developer runtime flag
     (see INTPRESSURE for example)?
compiledIC.hpp
     You should be able to use set_inst_mark()/cbuf.insts_mark() to set 
and retrieve the mark address.
arguments.cpp
     I wish there was a way to fix ReservedCodeCacheSize in the back-end.

dl