Happy path

[Roman Kennke]

Am 19.03.2018 um 14:18 schrieb Roland Westrelin:
> 
>>  *) On the ye-old SpscQueue benchmark, and this is how it looks with the patch:
>>  http://cr.openjdk.java.net/~shade/shenandoah/happy-path/happypath-1.perfasm
>>
>>  It does coalescing well on non-membar paths. There are things to improve, e.g. back-to-back tests:
>>
>>   0.00%    0.02%   │╰│││  0x00007f8ab0b717f2: jmpq   0x00007f8ab0b7176f
>>   1.88%    2.40%   ↘ │││  0x00007f8ab0b717f7: cmpb   $0x0,0x400(%r15)
>>   0.21%    0.31%     │││  0x00007f8ab0b717ff: jne    0x00007f8ab0b71871
>>   0.00%    0.00%     │││  0x00007f8ab0b71801: cmpb   $0x0,0x400(%r15)
>>   0.67%    0.82%     │││  0x00007f8ab0b71809: jne    0x00007f8ab0b71889
>>   1.88%    2.15%     │││  0x00007f8ab0b7180b: mov    %r12d,0x10(%rcx,%r14,4)
>>   2.19%    2.27%     │││  0x00007f8ab0b71810: cmpb   $0x0,0x400(%r15)
>>   0.01%    0.00%     ╰││  0x00007f8ab0b71818: je     0x00007f8ab0b71730
>>                       ││  0x00007f8ab0b7181e: testb  $0x34,0x400(%r15)
>>                       ╰│  0x00007f8ab0b71826: je     0x00007f8ab0b71720
> 
> I think they are caused by membars that don't show up in the generated
> code because they are nop on x86. Anyway, we can also clone membars:
> 
> http://cr.openjdk.java.net/~roland/shenandoah/happypath-membars/webrev.00/
> 
> with the hack below.

Is this about the WB membars? I eliminated them recently, this should
make life easier.

Roman