[MLton] Question on profile.fun

[Matthew Fluet]

> > > So, there is an insane shift after flatten and possibly a minor
> > > shift after contify3.  I don't know if this actually lays the blame
> > > squarely at the feet of Flatten.flatten (or possibly
> > > Shrink.shrinkFunction).
> > 
> > You could determine this by separating out the call to shrink from
> > flatten into a separate pass and then dropping profile labels in
> > between flattening and shrinking.
> 
> Here are the results of that experiment:
> 
>      1.97 dropProfileR
>           flatten (* without shrinking *)
>      1.94 dropProfileFlatten
>           shrink
>     12.94 dropProfileS
> 
> 
> > > Flatten.flatten doesn't appear to be sensitive to the presence of
> > > profiling statements in the program.  But, there is something
> > > definitely going on there.
> > 
> > Maybe.  At least the shrinker does refer to profile annotations.
> 
> Well, the above is encouraging, at least from the point of view that it 
> confirms that flatten itself isn't really sensitive to the presence of 
> profiling annotations.

Two things jumped out at me from looking at the pre/post shrink pass.

* We don't (and presumably can't) eliminate continuation eta-blocks that 
  have profile statements:

  In the input program we have:

    Enter Time.make <basis>/system/time.sml: 33
    bigQuot_0 (global_17, global_16, tuple_5) NonTail {cont = L_1161,
						       handler = Handle L_902}
  L_902 (x_1620)
    Leave Time.make <basis>/system/time.sml: 33
    L_903 (x_1620)
  L_903 (x_1629)
    L_906 (global_2, x_1629)


  When profile statements are removed, then the shrinker simplifies the 
  above to:

    bigQuot_0 (global_17, global_16, tuple_5) NonTail {cont = L_1161,
						       handler = Handle L_903}
  L_903 (x_1629)
    L_906 (global_2, x_1629)

  With the profile statements, nothing changes.

  But, I don't think there is anything to do in this case, as there isn't 
  anywhere to move the profile statement.


* The other situation is that we don't simplify Case blocks that 
  have profile statements.

  In the input program we have:

    Enter not <basis>/misc/basic.sml: 16
    case x_1643 of
      true => L_917 | false => L_909
  L_909 ()
    Leave not <basis>/misc/basic.sml: 16
    L_910 (global_20)
  L_911 ()
    L_912 (global_119)
  L_913 (x_1631)
    x_1634 = (x_1631, x_1633)
    x_1632 = ::_7 (x_1633, x_1631)
    L_914 (x_1632)
  L_914 (x_1635)
    Leave List.mapPartial.fn <basis>/list/list.sml: 68
    Leave List.foldl.loop <basis>/list/list.sml: 39
    loop_76 (x_1635, x_1636)
  L_915 ()
    L_914 (x_1633)
  L_912 (x_1637)
    Leave List.filter.fn <basis>/list/list.sml: 73
    case x_1637 of
      NONE_4 => L_915 | SOME_3 => L_913
  L_916 ()
    x_1638 = SOME_3 (x_1639)
    L_912 (x_1638)
  L_910 (x_1640)
    Leave StreamIOExtraFile.closeOut.fn <basis>/io/stream-io.fun: 890
    case x_1640 of
      true => L_916 | false => L_911
  L_917 ()
    Leave not <basis>/misc/basic.sml: 16
    L_910 (global_43)
  

  When profile statements are removed, then the shrinker simplifies the 
  above to:
  
    case x_1643 of
      true => L_917 | false => L_909
  L_909 ()
    x_1632 = ::_7 (x_1633, x_1641)
    loop_76 (x_1632, x_1642)
  L_917 ()
    loop_76 (x_1633, x_1642)


  With the profile statements, we only eliminate the assignment to x_1634: 

    Enter not <basis>/misc/basic.sml: 16
    case x_1643 of
      true => L_917 | false => L_909
  L_909 ()
    Leave not <basis>/misc/basic.sml: 16
    L_910 (global_20)
  L_917 ()
    Leave not <basis>/misc/basic.sml: 16
    L_910 (global_43)
  L_910 (x_1640)
    Leave StreamIOExtraFile.closeOut.fn <basis>/io/stream-io.fun: 890
    case x_1640 of
      true => L_916 | false => L_911
  L_911 ()
    L_912 (global_119)
  L_916 ()
    x_1638 = SOME_3 (x_1641)
    L_912 (x_1638)
  L_912 (x_1637)
    Leave List.filter.fn <basis>/list/list.sml: 73
    case x_1637 of
      NONE_4 => L_915 | SOME_3 => L_913
  L_913 (x_1631)
    x_1632 = ::_7 (x_1633, x_1631)
    L_914 (x_1632)
  L_915 ()
    L_914 (x_1633)
  L_914 (x_1635)
    Leave List.mapPartial.fn <basis>/list/list.sml: 68
    Leave List.foldl.loop <basis>/list/list.sml: 39
    loop_76 (x_1635, x_1642)

  This seems much more significant.  It also seems that it could be 
  accomodated by duplicating the profile statements when a trace through a 
  Case block is taken.  We specifically rule this out by only simplifying 
  Case blocks where 0 = Vector.length statements.