[GHC] #10923: GHC should recompile if flags change

[GHC]

#10923: GHC should recompile if flags change
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      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:                    |
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Changes (by thomie):

 * cc: simonmar (added)


Comment:

 simonmar: you wrote in ticket:437#comment:18 (5 years ago):
 > It's not clear to me that changing optimisation settings should trigger
 recompilation: for example, you might have a large program compiled
 unoptimised, and then decide that you want to optimise just one or two
 modules, so you remove a few .o files, add -O and recompile.

 But these GHC users recently expressed their confusion about the current
 behavior:

 * #haskell user mtesseract:
 > Isn't stack supposed to rebuild a package when I  specify a new set of
 ghc-options on the command line? As in e.g. 'stack build --ghc-
 options=-O0'? It doesn't seem to rebuild anything here, irregardles of the
 ghc-options I specify.

 * reddit user
 [https://www.reddit.com/r/haskell/comments/4ro749/help_cabal_and_ghc_optimzation_flag_questions/
 winterland1989]:
 > when I change my own project's cabal from none to -O2, nothing will be
 recompiled, why?

 Shall we add optimization flags to `fingerprintDynFlags` in
 `compiler/iface/FlagChecker.hs`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10923#comment:1>
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