Happy path

[Aleksey Shipilev]

On 03/15/2018 10:16 AM, Roland Westrelin wrote:
> 
> Here is a follow up patch:
> 
> http://cr.openjdk.java.net/~roland/shenandoah/happypath/webrev.01/

Nice!

A few observations:

 *) Matrix pre-barriers are still being eliminated from the happy path. hotspot_gc_shenandoah fails
because of that with partial heuristics. Fixing that probably rewires the patch a bit.

 *) 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

Thanks,
-Aleksey