1 files changed, 37 insertions, 9 deletions

diff --git a/compiler/iface/FlagChecker.hs b/compiler/iface/FlagChecker.hs
index 5e4a7092bf..0365be7338 100644
--- a/compiler/iface/FlagChecker.hs
+++ b/compiler/iface/FlagChecker.hs
@@ -10,6 +10,7 @@ import Binary
 import BinIface ()
 import DynFlags
 import HscTypes
+import Module
 import Name
 import Fingerprint
 -- import Outputable
@@ -21,11 +22,12 @@ import System.FilePath (normalise)
 -- | Produce a fingerprint of a @DynFlags@ value. We only base
 -- the finger print on important fields in @DynFlags@ so that
 -- the recompilation checker can use this fingerprint.
-fingerprintDynFlags :: DynFlags -> (BinHandle -> Name -> IO ())
+fingerprintDynFlags :: DynFlags -> Module -> (BinHandle -> Name -> IO ())
                     -> IO Fingerprint
 
-fingerprintDynFlags DynFlags{..} nameio =
-    let mainis   = (mainModIs, mainFunIs)
+fingerprintDynFlags DynFlags{..} this_mod nameio =
+    let mainis   = if mainModIs == this_mod then Just mainFunIs else Nothing
+                      -- see #5878
         -- pkgopts  = (thisPackage dflags, sort $ packageFlags dflags)
         safeHs   = setSafeMode safeHaskell
         -- oflags   = sort $ filter filterOFlags $ flags dflags
@@ -38,12 +40,8 @@ fingerprintDynFlags DynFlags{..} nameio =
         cpp = (map normalise includePaths, sOpt_P settings)
             -- normalise: eliminate spurious differences due to "./foo" vs "foo"
 
-        -- -i, -osuf, -hcsuf, -hisuf, -odir, -hidir, -stubdir, -o, -ohi
-        paths = (map normalise importPaths,
-                   [ objectSuf, hcSuf, hiSuf ],
-                   [ objectDir, hiDir, stubDir, outputHi ])
-                   -- NB. not outputFile, we don't want "ghc --make M -o <file>"
-                   -- to force recompilation when <file> changes.
+        -- Note [path flags and recompilation]
+        paths = [ hcSuf ]
 
         -- -fprof-auto etc.
         prof = if opt_SccProfilingOn then fromEnum profAuto else 0
@@ -51,3 +49,33 @@ fingerprintDynFlags DynFlags{..} nameio =
     in -- pprTrace "flags" (ppr (mainis, safeHs, lang, cpp, paths)) $
        computeFingerprint nameio (mainis, safeHs, lang, cpp, paths, prof)
 
+
+{- Note [path flags and recompilation]
+
+There are several flags that we deliberately omit from the
+recompilation check; here we explain why.
+
+-osuf, -odir, -hisuf, -hidir
+  If GHC decides that it does not need to recompile, then
+  it must have found an up-to-date .hi file and .o file.
+  There is no point recording these flags - the user must
+  have passed the correct ones.  Indeed, the user may
+  have compiled the source file in one-shot mode using
+  -o to specify the .o file, and then loaded it in GHCi
+  using -odir.
+
+-stubdir
+  We omit this one because it is automatically set by -outputdir, and
+  we don't want changes in -outputdir to automatically trigger
+  recompilation.  This could be wrong, but only in very rare cases.
+
+-i (importPaths)
+  For the same reason as -osuf etc. above: if GHC decides not to
+  recompile, then it must have already checked all the .hi files on
+  which the current module depends, so it must have found them
+  successfully.  It is occasionally useful to be able to cd to a
+  different directory and use -i flags to enable GHC to find the .hi
+  files; we don't want this to force recompilation.
+
+The only path-related flag left is -hcsuf.
+-}