RFR: Streamline LRB midpath

[Aleksey Shipilev]

On 3/6/19 4:51 PM, Roman Kennke wrote:
> We have some issues with generated code quality in the LRB barrier
> midpath. Specifically, we made a mess on the return path, and we
> generated an explicit null-check where an implicit null-check would do.
> See Aleksey's findings and related discussions here:
> 
> https://mail.openjdk.java.net/pipermail/shenandoah-dev/2019-March/008924.html
> 
> Regarding the null-check, it turned out that I mistakenly removed some
> code from lcm.cpp (see webrev). This potentially affects other
> null-checks too.
> 
> With this patch, the return path is clean and returns straight back to
> where it came from, and the null-check is implicit now.
> 
> http://cr.openjdk.java.net/~rkennke/streamline-lrb-midpath/webrev.00/

Yes, midpath looks much better now. Compare with:
  https://mail.openjdk.java.net/pipermail/shenandoah-dev/2019-March/008924.html

-XX:-UseCompressedOops

               [Verified Entry Point]
  6.53%          0x00007fda43a14530: mov    %eax,-0x14000(%rsp)
  5.04%          0x00007fda43a14537: push   %rbp
  6.81%          0x00007fda43a14538: sub    $0x10,%rsp
  4.55%          0x00007fda43a1453c: mov    0x10(%rsi),%rax
.......................... LRB fastpath check ..........................
  1.22%          0x00007fda43a14540: testb  $0x1,0x20(%r15)
  6.18%  ╭       0x00007fda43a14545: jne    0x00007fda43a1455a
.........│......... LRB fastpath ends, store to %rax follows ...........
  0.41%  │↗  ↗↗  0x00007fda43a14547: movl   $0x2a,0x20(%rax)
 11.89%  ││  ││  0x00007fda43a1454e: add    $0x10,%rsp
  0.50%  ││  ││  0x00007fda43a14552: pop    %rbp
  4.96%  ││  ││  0x00007fda43a14553: test   %eax,0x17646aa7(%rip)
  0.28%  ││  ││  0x00007fda43a14559: retq
---------││--││----------- LRB midpath starts --------------------------
.........││..││............ checking in-cset ...........................
         ↘│  ││  0x00007fda43a1455a: mov    %rax,%r10
          │  ││  0x00007fda43a1455d: shr    $0x17,%r10
          │  ││  0x00007fda43a14561: movabs $0x7fda5308cd88,%r11
          │  ││  0x00007fda43a1456b: cmpb   $0x0,(%r11,%r10,1)
          ╰  ││  0x00007fda43a14570: je     0x00007fda43a14547
.............││.......... checking is-forwarded ........................
           ╭ ││  0x00007fda43a14572: mov    -0x8(%rax),%rdi
           │ ││  0x00007fda43a14576: cmp    %rax,%rdi
           │╭││  0x00007fda43a14579: je     0x00007fda43a14580
           ││││  0x00007fda43a1457b: mov    %rdi,%rax
           ││╰│  0x00007fda43a1457e: jmp    0x00007fda43a14547
...........││.│............ slowpath call ..............................
           │↘ │  0x00007fda43a14580: movabs $0x7fda59985d90,%r10
           │  │  0x00007fda43a1458a: callq  *%r10
           │  ╰  0x00007fda43a1458d: jmp    0x00007fda43a14547
           ↘     0x00007fda43a1458f: mov    $0xfffffff6,%esi

-XX:+UseCompressedOops:

              [Verified Entry Point]
  4.07%         0x00007fbb9c470950: mov    %eax,-0x14000(%rsp)
  8.91%         0x00007fbb9c470957: push   %rbp
  2.08%         0x00007fbb9c470958: sub    $0x10,%rsp
  4.90%         0x00007fbb9c47095c: mov    0xc(%rsi),%r11d
  4.90%         0x00007fbb9c470960: mov    %r11,%r9
  1.95%         0x00007fbb9c470963: shl    $0x3,%r9
.......................... LRB fastpath check ..........................
  2.47%         0x00007fbb9c470967: testb  $0x1,0x20(%r15)
  3.14%  ╭      0x00007fbb9c47096c: jne    0x00007fbb9c470982
.........│......... LRB fastpath ends, store to %r9 follows ...........
  4.68%  │↗ ↗↗  0x00007fbb9c47096e: movl   $0x2a,0xc(%r9)
  7.37%  ││ ││  0x00007fbb9c470976: add    $0x10,%rsp
  2.06%  ││ ││  0x00007fbb9c47097a: pop    %rbp
  2.69%  ││ ││  0x00007fbb9c47097b: test   %eax,0x198e467f(%rip)
  3.73%  ││ ││  0x00007fbb9c470981: retq
---------││-││----------- LRB midpath starts --------------------------
.........││.││............ checking in-cset ...........................
         ↘│ ││  0x00007fbb9c470982: mov    %r9,%r10
          │ ││  0x00007fbb9c470985: shr    $0x17,%r10
          │ ││  0x00007fbb9c470989: movabs $0x7fbbac0871c0,%r8
          │ ││  0x00007fbb9c470993: cmpb   $0x0,(%r8,%r10,1)
          ╰ ││  0x00007fbb9c470998: je     0x00007fbb9c47096e
............││.......... checking is-forwarded ........................
           ╭││  0x00007fbb9c47099a: mov    -0x8(%r12,%r11,8),%r9
           │││  0x00007fbb9c47099f: lea    (%r12,%r11,8),%r10
           │││  0x00007fbb9c4709a3: cmp    %r10,%r9
           │╰│  0x00007fbb9c4709a6: jne    0x00007fbb9c47096e
...........│.│............ slowpath call ..............................
           │ │  0x00007fbb9c4709a8: mov    %r9,%rdi
           │ │  0x00007fbb9c4709ab: movabs $0x7fbbb467fd90,%r10
           │ │  0x00007fbb9c4709b5: callq  *%r10
           │ │  0x00007fbb9c4709b8: mov    %rax,%r9
           │ ╰  0x00007fbb9c4709bb: jmp    0x00007fbb9c47096e

Thanks,
-Aleksey