[GHC] #9400: poor performance when compiling modules with many Text literals at -O1

[GHC]

#9400: poor performance when compiling modules with many Text literals at -O1
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              Reporter:  rwbarton    |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:  invalid     |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Compile-    |       Blocked By:
  time performance bug               |  Related Tickets:  #9370
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Just to add a bit more detail:

  * The special handling for singleton string literals is in
 `MkCore.mkStringExprFS`. Omittign the `lengthFS str == 1` case looks very
 plausible to me.

  * Notice that this same function sometimes generates `unpackCStringUtf8`
 rather than `unpackCString`. Rules that recognise only the latter will
 stumble if the former is produced. I have no clue what to do about this --
 Unicode experts may.

  * The function `CoreSubst.exprIsConApp_maybe` is the place where we say
 "does this expression look like a constructor application?".  Adding an
 extra case to `go`, below the ones for `dataConWorkId` and
 `DFunFunfolding` should let us dynamically expand a call to
 `unpackCString` into a cons cell. And that will actually be ''better''
 than now, because it should eliminate
 {{{
   case "foo" of
     'f' : 'o' : xs -> ....
 }}}


 Would someone care to try?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9400#comment:5>
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