[GHC] #11798: Explicit recompilation required to change compilation options (was: Recompiling with -fhpc flag added does nothing)

[GHC]

#11798: Explicit recompilation required to change compilation options
-------------------------------------+-------------------------------------
        Reporter:  drathier          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.10.3
      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:                    |
-------------------------------------+-------------------------------------
Changes (by dfeuer):

 * type:  bug => feature request
 * architecture:  x86_64 (amd64) => Unknown/Multiple
 * milestone:   => 8.4.1


Old description:

> When you recompile without modifiying any source code, nothing happens,
> as expected. The file is already compiled, after all. If you then add the
> -fhpc flag, it still doesn't recompile, even though it now should
> generate .tix files. These files are now unexpectedly missing, until the
> source files are modified or the .o and .hi files are deleted, and the
> code is recompiled again with the -fhpc flag.
>
> reproduce:
> 1. compile program
> 2. compile program with -fhpc
> note: missing .tix file
> 3. delete .hi and .o files
> 4. compile program with -fhpc
> 5. success; .tix file is generated

New description:

 When you recompile without modifying any source code, nothing happens, as
 expected. The file is already compiled, after all. If you then change
 compilation options, such as adding the `-fhpc` or `-O2` flag, it still
 doesn't recompile, even though the program might now be expected to behave
 or perform differently. Users can work around this using `-fforce-recomp`,
 but perhaps they shouldn't have to, or perhaps they should be warned that
 this may be necessary. One option might be to record "significant"
 compilation options in the `.hi` file, to determine whether they have
 changed in a way that suggests recompilation may be in order.

--

Comment:

 The original ticket claimed this was a bug recompiling with `-fhpc`, but
 in fact this is the way GHC behaves with compilation option changes in
 general. I've taken the liberty of modifying the title and description and
 making this a feature request, because I think there's a reasonable
 feature request here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11798#comment:1>
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