[GHC] #4900: DEPENDS pragma

[GHC]

#4900: DEPENDS pragma
-------------------------------------+------------------------------------
        Reporter:  cdsmith           |            Owner:
            Type:  feature request   |           Status:  patch
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  TH_Depends        |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by simonmar):

 I don't want to block this fix, since there's a real problem.  On the
 other hand, the fix is not a simple one because both proposed
 implementations change some key assumption about the flow of data, in what
 is already a complicated part of the compiler.

 I think I'm ok with the idea of adding the usg_file information to the
 ModSummary during the downsweep ''after'' a previous upsweep. I think this
 is the first patch, correct?  But what we need is some careful
 documentation about how it works, preferably with a worked example ("the
 second downsweep adds the following information to the ModSummary, but it
 doesn't cause recompilation because...").  I'm concerned about things like
 re-summarising modules unnecessarily.  The other thing we need is some
 good tests (the recomp tests are under `driver`), testing for both too
 much and too little recompilation.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4900#comment:65>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler