[GHC] #8123: GHCi warns about -eventlog even though it's sometimes necessary

[GHC]

#8123: GHCi warns about -eventlog even though it's sometimes necessary
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        Reporter:  akio              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  GHCi              |                 Version:  7.6.3
      Resolution:                    |                Keywords:
Operating System:  Linux             |            Architecture:  x86_64
 Type of failure:  Incorrect         |  (amd64)
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by rwbarton):

 akio, you seem to have something special about your environment
 (`DYNAMIC_BY_DEFAULT`?) I had to add `-dynamic` to the `ghc` invocations
 to reproduce.

 But a simpler test is
 {{{
 rwbarton at morphism:/tmp$ ghc -c Test.hs
 rwbarton at morphism:/tmp$ ghc -c Test.hs -threaded
 compilation IS NOT required
 rwbarton at morphism:/tmp$ ghc -c Test.hs -debug
 compilation IS NOT required
 rwbarton at morphism:/tmp$ ghc -c Test.hs -eventlog
 rwbarton at morphism:/tmp$
 }}}

 Technically the eventlog way is not actually RTS-only because it adds
 `-DTRACING` to the C compiler and C preprocessor options. If you had CPP
 in Test.hs that checked whether `TRACING` was defined, then recompiling
 the module to an object file (either in ghci or by ghc) really would have
 been necessary.

 So, there are a few possible resolutions:

 1. Make `WayEventLog` a non-RTS only way in `wayRTSOnly`, which would
 suppress this ghci warning, on the grounds that modules can detect (via
 CPP) whether they were built with `-eventlog`. I think this would prevent
 linking `-eventlog` object files with non-`-eventlog` ones, though.

 2. Don't change `wayRTSOnly`, and just change the test for that ghci
 warning to check something other than `wayRTSOnly` (e.g., treat
 `WayEventLog` specially). However this leaves us in the current mildly
 inconsistent state of affairs that `-eventlog` is considered in some sense
 (and is documented to be) a link-time only option, when in fact it affects
 compilation as well.

 3. Implement `WayEventLog` in the same manner as `WayThreaded` and
 `WayDebug`. I don't know exactly what is (perhaps thomie can comment) but
 the latter two ways are not built by automatically enabling CPP options in
 ghc, even though the RTS is in fact full of `#ifdef THREADED` and `#ifdef
 DEBUG`. I assume there is some special logic in the build system for
 these. This potentially breaks people who were using `#ifdef TRACING` in
 their Haskell source, but hopefully there aren't any (it is undocumented
 after all).

 Personally I lean towards 3 if it isn't technically difficult to
 implement.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8123#comment:3>
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