#14923: Recompilation avoidance fails after a LANGUAGE change -------------------------------------+------------------------------------- Reporter: akio | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.4.1 Keywords: | Operating System: Linux Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It looks like the recompilation checker can be confused when a LANGUAGE pragma is added or removed. The following shell script demonstrates the issue: {{{ #!/bin/sh cat >Foo.hs <<END {-# LANGUAGE UndecidableInstances #-} module Foo where END rm -f Foo.hi Foo.o ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag sed -i 's/^{-#/--&/' Foo.hs # comment out the pragma ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag ghc -c Foo.hs; ghc --show-iface Foo.hi | grep flag }}} I get this output: {{{ % ./bug.sh flag hash: b1ada48735eec2ef16d5070887aa7f10 flag hash: b1ada48735eec2ef16d5070887aa7f10 flag hash: e7b294090e04f1a84f0bfc9b8cc49322 compilation IS NOT required flag hash: e7b294090e04f1a84f0bfc9b8cc49322 }}} In the third invocation to GHC, I expect the recompilation checker to detect that compilation is not required, because nothing has changed since the second invocation. Also it seems suspicious that the flag hash changes beteen the second run and the third run. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14923> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler