[GHC] #15646: ghci takes super long time to find the type of large fractional number

[GHC]

#15646: ghci takes super long time to find the type of large fractional number
-------------------------------------+-------------------------------------
        Reporter:  Johannkokos       |                Owner:
                                     |  JulianLeviston
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  GHCi              |              Version:  8.4.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by JulianLeviston):

 I've been trying to build a profiled compiler.

 I adjusted `build.mk` by changing the `BuildFlavour` thusly: `BuildFlavour
 = prof` and commented out the `stage = 2`setting I had.
 I also added this to the bottom, to put profiling hooks across the
 typechecker:

 {{{
 define add_mods_flag =
   $(foreach mod,$(2),$(eval $(basename $(mod))_HC_OPTS += $(1)))
 endef

 $(call add_mods_flag,-fprof-auto,$(wildcard compiler/typecheck/*.hs))
 }}}

 I keep getting this error after `make clean && make -j4` in either the
 `ghc/compiler` or `ghc/` directories:

 {{{
 GHC error in desugarer lookup in GHC.Real:
   Can't find interface-file declaration for type constructor or class
 mkRational
     Probable cause: bug in .hi-boot file, or inconsistent .hi file
     Use -ddump-if-trace to get an idea of which file caused the error
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.7.20181012 for x86_64-apple-darwin):
         initDs

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 What should I do next?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:49>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler