[GHC] #14372: CMM contains a bunch of tail-merging opportunities

[GHC]

#14372: CMM contains a bunch of tail-merging opportunities
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by heisenbug):

 Replying to [comment:2 nomeata]:
 > >  It would be nice to factor out the common tails, e.g. by branching to
 the first tail already emitted.
 >
 > Branches can be expensive, so this might not be a clear win. Have you
 measured the effect? (You can do that without modifying GHC: Just get your
 hands on the assembly file, make the changes you want to see, and
 compare.)

 These will probably be short, known branches. Since more ''relevant'' code
 now can reside in the instruction cache, I expect less misses and no
 prediction failures.

 >
 > The other idea, detecting blocks that are equal up-to a constant, sounds
 fun. But again, the question is: Is it worth it?

 Well, it makes code straight-line and branchless. Eliminates jump-tables.
 Shrinks code size. Which one of these is a penalty? ;-)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14372#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler