[GHC] #8905: Function arguments are always spilled/reloaded if scrutinee is already in WHNF

[GHC]

#8905: Function arguments are always spilled/reloaded if scrutinee is already in
WHNF
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        Reporter:  tibbe                    |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.9
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by simonpj):

 I'm puzzled by why the spill code is before the eval check rather than
 after.  Before spilling etc I'd expect to see:
 {{{
    ...
    if <eval check> goto Leval else goto Ldone

 Leval: call (I64[R1])(R1) returns to Ldone

 Ldone:  // Now it's evaluated
 }}}
 Then the stack-layout stuff should insert spills and reloads ''around the
 call'', but not interfering with the fast path.

 So how come the spills affect the fast path at all?  Is there some special
 code-size-saving magic in the code generator?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8905#comment:8>
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