Optimise the %.hi : %.o rule

[ghc-hetmet.git] / rules / hi-rule.mk

diff --git a/rules/hi-rule.mk b/rules/hi-rule.mk
index c30a093..35baffd 100644 (file)
--- a/rules/hi-rule.mk
+++ b/rules/hi-rule.mk
@@ -12,24 +12,61 @@
 
 
 # Here's an interesting rule!
-# The .hi file depends on the .o file,
-# so if the .hi file is dated earlier than the .o file (commonly the case,
-# when interfaces are stable) this rule just makes sure that the .o file,
-# is up to date.  Then it does nothing to generate the .hi file from the
-# .o file, because the act of making sure the .o file is up to date also
-# updates the .hi file (if necessary).
+
+# The .hi file may or may not change when we compile the corresponding
+# .hs file.  If GHC figures out that the .hi file has not changed, it
+# doesn't touch it.  This is a useful optimisation, because it means
+# some modules may not get recompiled if the .hi files of the modules
+# they depend on have not changed.
+#
+# See:
+#   http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoidance
+#
+# So how do we express this dependency to make?  The exact form of
+# this rule is quite fragile.  Here are some versions that don't work
+# very well:
+#
+# %.hi : %.o
+#      @if [ ! -f $@ ] ; then \
+#          echo Panic! $< exists, but $@ does not.; \
+#          exit 1; \
+#      fi
+#
+# This version adds a useful sanity check; but it is also expensive on
+# Windows where spawning a shell takes a while (about 0.3s).  We'd
+# like to avoid the shell if necessary.  This also hides the message
+# "nothing to be done for 'all'", since make thinks it has actually done
+# something.
+#
+# %.hi : %.o
+#
+# This version doesn't work: GNU make knows it has't done anything to
+# update the .hi file, so even if the .o file has been updated, it
+# won't rebuild anything that depends on the .hi file.  So you might
+# think a more correct way is to change the .hs rule:
+#
+# %.hi %.o : %.hs
+#      $(HC) ...
+#
+# this says "compiling %.hs updates both %.hi and %.o", but that's not
+# true, since compiling the .hs file might not update the .hi file, if
+# the .hi file didn't change.  And if we use this version, then make
+# will keep trying to rebuild %.hi if it is out of date with respect
+# to %.hs.
+#
+# Using this form seems to be the best compromise:
+#
+# %.hi : %.o ;
+#
+# the ';' at the end signifies an "empty command" (see the GNU make
+# documentation).  An empty command is enough to get GNU make to think
+# it has updated %.hi, but without actually spawning a shell to do so.
 
 define hi-rule # $1 = way
-%.$$($1_hisuf) : %.$$($1_osuf)
-       @if [ ! -f $$@ ] ; then \
-           echo Panic! $$< exists, but $$@ does not.; \
-           exit 1; \
-       fi
-
-%.$$($1_way_)hi-boot : %.$$($1_way_)o-boot
-       @if [ ! -f $$@ ] ; then \
-           echo Panic! $$< exists, but $$@ does not.; \
-           exit 1; \
-       fi
+
+%.$$($1_hisuf) : %.$$($1_osuf) ;
+
+%.$$($1_way_)hi-boot : %.$$($1_way_)o-boot ;
+
 endef