[GHC] #13829: ghc --make should not relink when we know the binary doesn't change

[GHC]

#13829: ghc --make should not relink when we know the binary doesn't change
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        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13828            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by nh2:

@@ -22,1 +22,6 @@
- Perhaps even earlier (e.g. after parsing).
+ Perhaps even earlier (e.g. after parsing) in this specific case; but
+ certainly after code gen, which I'd expect also to avoid the recompilation
+ if e.g. a comment was added to the file, as the comment won't make it into
+ the program code (unless something like TH `location` or `HasCallStack` is
+ used, which can change the line numbers those generate if a comment pushes
+ them down).

New description:

 I would expect GHC not to relink after touching a file, since no actual
 program code was changed. But it does:

 {{{
 $ echo "main = return ()" > Main.hs

 $ ghc --make Main.hs
 [1 of 1] Compiling Main             ( Main.hs, Main.o )
 Linking Main ...

 $ touch Main.hs

 $ ghc --make Main.hs
 [1 of 1] Compiling Main             ( Main.hs, Main.o )
 Linking Main ...
 }}}

 I would expect that the recompilation avoidance makes GHC stop continuing
 as soon as it has noticed that the same object code (`.o` file) was
 generated as in the first build.

 Perhaps even earlier (e.g. after parsing) in this specific case; but
 certainly after code gen, which I'd expect also to avoid the recompilation
 if e.g. a comment was added to the file, as the comment won't make it into
 the program code (unless something like TH `location` or `HasCallStack` is
 used, which can change the line numbers those generate if a comment pushes
 them down).

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13829#comment:1>
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