RFR (S) 8019968: Reference CAS induces GC store barrier even on failure

Aleksey Shipilev aleksey.shipilev at oracle.com
Mon Aug 10 09:13:37 UTC 2015

Hi Vladimir!

On 07/31/2015 06:03 AM, Vladimir Kozlov wrote:
> I think the test is wrong. It should be:
> if_then(load_store, BoolTest::eq, oldval, PROB_STATIC_FREQUENT);

Um, no? I remember eyeballing the assembly to confirm this.

For LS_cmpxchg, we are inlining "*boolean* cas(...)", so the load_store
seems to have a boolean value, but "oldval" is oop. In other words,
"load_store != 0" tests "(boolean)load_store != false".

Current VM produces:

 13.46%   45.85%    │││  0x00007fa06809cdaf: lock cmpxchg %r8d,(%rdi)
 14.91%    4.41%    │││  0x00007fa06809cdb4: sete   %r11b
  0.07%             │││  0x00007fa06809cdb8: movzbl %r11b,%r11d
                              <UNCONDITIONAL STORE BARRIER HERE>
  0.06%             │││  0x00007fa06809cdd1: test   %r11d,%r11d

                         ; CAS fail, jump to respin
                    │╰│  0x00007fa06809cdd4: je     0x00007fa06809ccf0

Patched VM piggybacks on the same result:

  1.97%    0.05%  │││  0x00007fe618af4115: lock cmpxchg %ebx,(%r9)
 50.59%   90.01%  │││  0x00007fe618af411a: sete   %r11b
  0.05%    0.01%  │││  0x00007fe618af411e: movzbl %r11b,%r11d
  3.02%    1.80%  │││  0x00007fe618af4122: test   %r11d,%r11d

                       ; CAS success, jump to store barrier
                  │╰│  0x00007fe618af4125: jne    0x00007fe618af4070

                       ; CAS fail, jump to respin
                  │ ╰  0x00007fe618af412b: jmpq   0x00007fe618af4082

Your suggestion seems to ignore the test completely (GVN helped?), and
while it's still technically correct with emitting the barrier always,
it defeats the purpose of the change:

  2.35%    4.97%  │ ││  0x00007f7790aefd26: lock cmpxchg %r10d,(%rbx)
 48.54%   86.32%  │ ││  0x00007f7790aefd2b: mov    $0x1,%eax
  0.03%           │╭││  0x00007f7790aefd30: je     0x00007f7790aefd3b
                  ││││  0x00007f7790aefd36: mov    $0x0,%eax
                              <UNCONDITIONAL STORE BARRIER HERE>
  2.16%    0.01%  │ ││  0x00007f7790aefd53: cmp    $0x0,%eax

                        ; CAS fail, jump back to respin
                  │ ╰│  0x00007f7790aefd56: je     0x00007f7790aefd10

                        ; CAS success, follow to exit

Also, AFAIU, performance results would look different if we screwed the
success check. But they seem to be coherent with our expectations: when
CAS fails, either the conditional card marking or this change helps, and
the change does not help when CAS succeeds.


> Thanks,
> Vladimir
> On 7/29/15 2:57 AM, Aleksey Shipilev wrote:
>> On 07/29/2015 12:24 PM, Andrew Dinn wrote:
>>> On 29/07/15 09:58, Aleksey Shipilev wrote:
>>>> I would like to suggest a fix for:
>>>> https://bugs.openjdk.java.net/browse/JDK-8019968
>>>> In short, current reference CAS intrinsic blindly emits
>>>> post_barrier, ignoring the CAS result. In some cases, notably
>>>> contended CAS spin-loops, we fail the CAS a lot, and thus go for a
>>>> post_barrier excessively. Instead, we can conditionalize on the
>>>> result of the store itself, and put the post_barrier only on
>>>> success path: http://cr.openjdk.java.net/~shade/8019968/webrev.01/
>>>> More performance results here:
>>>> http://cr.openjdk.java.net/~shade/8019968/notes.txt
>>> Nice! The code looks fine and your test results are very convincing.
>>> I'll be interested to see how this looks on AArch64.
>> Thanks Andrew!
>> The change passes JPRT, so AArch64 build is available. The benchmark JAR
>> mentioned in the issue comments would run without intervention, taking
>> around 40 minutes. You are very welcome to try, while Reviewers are
>> taking a look. I can do that only next week.
>>> That said, I am afraid you still need a Reviewer!
>> That reminds me I haven't spelled out what testing was done:
>>   * JPRT on all open platforms
>>   * Targeted benchmarks
>>   * Eyeballing the generated x86 assembly
>> Thanks,
>> -Aleksey

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