Use same Task for Task.CompletedTask and ATMB.Completed

[benaadams]

Keep it hotter in cache rather than alternating between the two for manually and compiler generated async methods

/cc @stephentoub

[stephentoub]

Have you confirmed the asm for this is just as good?

[benaadams]

Yep, ends up identical

  [12 IL=0141 TR=000050 060036BE] [profitable inline] AsyncTaskMethodBuilder:SetResult():this
    [13 IL=0011 TR=000240 060036D0] [profitable inline] AsyncTaskMethodBuilder`1:SetResult(ref):this
      [0 IL=0026 TR=000260 060036CE] [FAILED: too many basic blocks] AsyncTaskMethodBuilder`1:SetExistingTaskResult(struct):this
      ...
; Total bytes of code 339, prolog size 29 for method <Main>d__0:MoveNext():this

Its going via a generic rather than non-generic that causes the speed bump

[benaadams]

Yep, ends up identical

More or less

-      BAA4040000           mov      edx, 0x4A4
+      BA84020000           mov      edx, 644
       E89B83B25F           call     CORINFO_HELP_GETSHARED_NONGCSTATIC_BASE
-      48BA80190AD3E5010000 mov      rdx, 0x1E5D30A1980
+      48BA381647D6FC010000 mov      rdx, 0x1FCD6471638
       488B12               mov      rdx, gword ptr [rdx]

[benaadams]

CI looks quite broken

coreclr-ci
 Failure
 ran 20 seconds ago in less than 5 seconds

Pipeline configuration error:
/azure-pipelines.yml: Could not find /eng/platform-matrix.yml in repository self hosted on 
https://api.github.com using commit 16a840c2fb45164924979bace155dc2ca5522bd9. 
GitHub reported the error, "Not Found"

[benaadams]

Code gen ends up identical in usage (other than referring to same object) and a change in .cctors

Total bytes of diff: -31 (-0.00% of base)
    diff is an improvement.

Total byte diff includes -55 bytes from reconciling methods
        Base had    1 unique methods,       55 unique bytes
        Diff had    0 unique methods,        0 unique bytes

Top file improvements by size (bytes):
         -31 : System.Private.CoreLib.dasm (-0.00% of base)

1 total files with size differences (1 improved, 0 regressed), 0 unchanged.

Top method regressions by size (bytes):
          24 (13.87% of base) : System.Private.CoreLib.dasm - Task:.cctor()

Top method improvements by size (bytes):
         -55 (-100.00% of base) : System.Private.CoreLib.dasm - AsyncTaskMethodBuilder:.cctor() (1 base, 0 diff methods)

2 total methods with size differences (1 improved, 1 regressed), 18365 unchanged.

[benaadams]

Follow up that takes this a bit further #24431

[benaadams]

Closing in preference of #24431