[GHC] #8275: Loopification breaks profiling

[GHC]

#8275: Loopification breaks profiling
----------------------------------------+----------------------------------
        Reporter:  jstolarek            |            Owner:
            Type:  bug                  |           Status:  patch
        Priority:  high                 |        Milestone:  7.8.1
       Component:  Profiling            |          Version:  7.7
      Resolution:                       |         Keywords:
Operating System:  Unknown/Multiple     |     Architecture:
 Type of failure:  Building GHC failed  |  Unknown/Multiple
       Test Case:                       |       Difficulty:  Unknown
        Blocking:                       |       Blocked By:  8456
                                        |  Related Tickets:
----------------------------------------+----------------------------------
Changes (by parcs):

 * status:  new => patch


Comment:

 I have attached a patch that fixes loopification with profiling. The
 problem is that `nodeReg` aka R1 may get clobbered inside the body of a
 closure. With profiling, upon entering a closure, `enterFunCCS` is called
 with R1 passed to it. But since R1 may get clobbered inside the body of a
 closure, and since a self-recursive tail call does not restore R1, a
 subsequent call to `enterFunCCS` may be passed a bogus value from R1.

 The solution is to not pass `nodeReg` aka R1 to `enterFunCCS`. Instead, we
 pass `node`, the callee-saved temporary that stores the original value of
 R1. This way R1 may get clobbered and loopification will not care.

 I don't know whether it's necessary to re-run the LDV code after looping.
 Not doing so certainly does not cause a segfault, anyway.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8275#comment:32>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler