[GHC] #8144: Interface hashes include time stamp of included .h file when CPP is used

[GHC]

#8144: Interface hashes include time stamp of included .h file when CPP is used
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        Reporter:  nh2                           |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  Compiler                      |          Version:  7.6.3
      Resolution:                                |         Keywords:
Operating System:  Unknown/Multiple              |     Architecture:
 Type of failure:  Compile-time performance bug  |  Unknown/Multiple
       Test Case:                                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by nh2):

 ''UsageFile'' was introduced with ''addDependentFile'' in
 https://github.com/ghc/ghc/commit/b994313.

 The modification time being only (obtained) in second resolution is what
 triggers the inconsistent behaviour.

 Regarding that commit, I am also not sure how
 https://github.com/ghc/ghc/commit/b994313#L3R1283 works:

 {{{
 checkModUsage _this_pkg UsageFile{ usg_file_path = file, usg_mtime =
 old_mtime } = do
   new_mtime <- liftIO $ getModificationTime file
   return $ old_mtime /= new_mtime
 }}}

 I might be wrong, but it looks to me as if this can only ever return False
 (''UpToDate'' in current GHC head) '''because''' the mtime is so coarse -
 in other words if your computer is too slow, a second will pass and this
 will never be considered up to date?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8144#comment:5>
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