RFR(M): 8155949: Support relaxed semantics in cmpxchg

[David Holmes]

On 10/05/2016 9:04 PM, David Holmes wrote:
> Hi Hiroshi,
>
> On 10/05/2016 8:44 PM, Hiroshi H Horii wrote:
>> Hi All,
>>
>> Can I please request reviews for the following change?
>>
>> Code change:
>> http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.01/
>
> Changes look good. I'm currently running them through our internal build
> system. I will sponsor this and push the change through JPRT.

Still a problem on Solaris sparc:

"/opt/jprt/T/P1/102505.daholme/s/hotspot/src/share/vm/runtime/atomic.inline.hpp", 
line 96: Error: Could not find a match for static Atomic::cmpxchg(signed 
char, volatile signed char*, signed char).
1 Error(s) detected.

Needs this patch:

diff -r 68853ef19be9 src/share/vm/runtime/atomic.inline.hpp
--- a/src/share/vm/runtime/atomic.inline.hpp
+++ b/src/share/vm/runtime/atomic.inline.hpp
@@ -92,7 +92,7 @@

  #ifndef VM_HAS_SPECIALIZED_CMPXCHG_BYTE
  // See comment in atomic.cpp how to override.
-inline jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte 
*dest, jbyte comparand)
+inline jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte 
*dest, jbyte comparand, cmpxchg_memory_order order)
  {
    return cmpxchg_general(exchange_value, dest, comparand);
  }

David
-----

> Just need another reviewer to chime in - given you and Martin are both
> contributors. Or are you the main contributor with Martin being a reviewer?
>
> Thanks,
> David
>
> PS. It's my night now so I'll be signing off and will pick this up in
> the morning.
>
>> This change follows the discussion started from these mails.
>> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-April/018960.html
>>
>> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-April/019148.html
>>
>> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-May/019320.html
>>
>>
>> Description:
>> This change provides relaxed compare-and-exchange by introducing
>> relaxed memory order. As described in atomic_linux_ppc.inline.hpp,
>> the current implementation of cmpxchg is fence_cmpxchg_acquire.
>> This implementation is useful for general purposes because twice calls of
>> sync before and after cmpxchg will provide strict consistency.
>> However, they sometimes cause overheads because sync instructions are
>> very expensive in the current POWER chip design.
>>
>> We confirmed this change improves performance of copy_to_survivor
>> in the parallel GC. However, we will need more investigation of GC
>> by more experts. So, We would like to request a review of the change
>> of cmpxchg first (as Martin requested).
>> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-April/019188.html
>>
>>
>> Summary of source code changes:
>>
>> * src/share/vm/runtime/atomic.hpp
>>      - Defines enum memory_order and adds a parameter to cmpxchg.
>>
>> * src/share/vm/runtime/atomic.cpp
>> * src/os_cpu/bsd_x86/vm/atomic_bsd_x86.inline.hpp
>> * src/os_cpu/bsd_zero/vm/atomic_bsd_zero.inline.hpp
>> * src/os_cpu/linux_aarch64/vm/atomic_linux_aarch64.inline.hpp
>> * src/os_cpu/linux_sparc/vm/atomic_linux_sparc.inline.hpp
>> * src/os_cpu/linux_x86/vm/atomic_linux_x86.inline.hpp
>> * src/os_cpu/linux_zero/vm/atomic_linux_zero.inline.hpp
>> * src/os_cpu/solaris_sparc/vm/atomic_solaris_sparc.inline.hpp
>> * src/os_cpu/solaris_x86/vm/atomic_solaris_x86.inline.hpp
>> * src/os_cpu/windows_x86/vm/atomic_windows_x86.inline.hpp
>>      - Added a parameter for each cmpxchg function to follow
>>         the change of atomic.hpp. Their implementations are not changed.
>>
>> * src/os_cpu/aix_ppc/vm/atomic_aix_ppc.inline.hpp
>> * src/os_cpu/linux_ppc/vm/atomic_linux_ppc.inline.hpp
>>      - Added a parameter for each cmpxchg function to follow
>>         the change of atomic.hpp. In addition, implementations
>>         are changed corresponding to the specified memory_order.
>>
>> Regards,
>> Hiroshi
>> -----------------------
>> Hiroshi Horii, Ph.D.
>> IBM Research - Tokyo
>>