[GHC] #2731: Avoid unnecessary evaluation when unpacking constructors

[GHC]

#2731: Avoid unnecessary evaluation when unpacking constructors
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  CodeGen
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by AndreasK):

 * version:  6.8.3 => 8.5


Comment:

 If anyone wonders this is still the case in GHC-8.4 / HEAD.

 Changing the type of `T` to `data T = MkT !(Bool,Bool)` also doesn't
 change anything.

 Cmm Code.
 {{{
      {offset
        c1pY:
            if ((Sp + -8) < SpLim) (likely: False) goto c1q2; else goto
 c1q3;
        c1q2:
            R2 = R2;
            R1 = foo_closure;
            call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8;
        c1q3:
            I64[Sp - 8] = c1pV;
            R1 = R2;
            Sp = Sp - 8;
            if (R1 & 7 != 0) goto c1pV; else goto c1pW;
        c1pW:
            call (I64[R1])(R1) returns to c1pV, args: 8, res: 8, upd: 8;
        c1pV:
            I64[Sp] = c1q1;
            R1 = P64[R1 + 7];
            if (R1 & 7 != 0) goto c1q1; else goto c1q5;
        c1q5:
            call (I64[R1])(R1) returns to c1q1, args: 8, res: 8, upd: 8;
        c1q1:
            R1 = P64[R1 + 7] & (-8);
            Sp = Sp + 8;
            call (I64[R1])(R1) args: 8, res: 0, upd: 8;
      }
 }}}

 To solve this we would need to track the evaluation requirements of
 binders in STG. Something it seems we currently don't do.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2731#comment:17>
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