[GHC] #14697: Redundant computation in fingerprintDynFlags when compiling many modules

[GHC]

#14697: Redundant computation in fingerprintDynFlags when compiling many modules
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           Reporter:  niteria        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  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:
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 I profiled a build of a production code base with thousands of modules and
 computing `fingerprintDynFlags` is `7%` of time and `14%` of allocations.

 Here's a synthetic test case inspired by what I observed:
 {{{
 SIZE=1000

 for i in $(seq -w 1 $SIZE); do
   echo "module A$i where" > A$i.hs
   echo "data A$i = A$i" >> A$i.hs
 done
 }}}

 This generates a 1000 modules each with one datatype.
 Compiling them with:
 {{{
 inplace/bin/ghc-stage2 A*.hs -optP-D__F{1..10000}__
 }}}
 results in `fingerprintDynFlags` being the top cost centre in the profile.
 AFAICT there's only one module dependent piece that goes into computing
 `fingerprintDynFlags` and the rest is the same between those 1000 modules.

 Now, why would I have so many preprocessor flags?
 This is how the Buck build system currently works. If a Haskell library
 depends on a C++ library then the GHC invocation gets the C++ library's
 directory as include path (`-optP-I -optP-I some/library/path`). This can
 grow quite big.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14697>
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