[GHC] #10120: Unnecessary code duplication from case analysis

[GHC]

#10120: Unnecessary code duplication from case analysis
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by bgamari):

 I'm unsure of whether it's reasonable to expect the compiler to know that
 it's not worth specializing the case analysis on the character `c`. For
 instance, `do_something` may also branch on the character; if the compiler
 specialized then it could eliminate the branch inside of `do_something`.

 However, it would be nice if the compiler gave us the tools to force the
 sort of behavior we want. For instance, consider an identity-like function
 `evalThis :: a -> a`. This would prevent the compiler from "breaking up"
 the term in the course of optimization. For instance,

 {{{#!hs
 f c = let pred = evalThis $ c == 'a' || c == 'b' || c == 'c' :: Bool
       in case pred of
              True -> ...
              False -> ...
 }}}

 would prevent the compiler from using the "insides" of `pred` in the case
 analysis, ensuring that somewhere in the resulting executable there would
 be boolean value `pred` which would then be matched against. This is
 obviously a pretty big hammer, but it might be nice to have every once in
 a while.

 Perhaps such a facility already exists?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10120#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler