#11126: Entered absent arg in a Repa program -------------------------------------+------------------------------------- Reporter: tuplanolla | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by jscholl): I have a theory what is happening here: The important values here are {{{arr2}}} and some value called {{{get}}}. In the original definition {{{get}}} evaluates {{{arr2}}} and uses some component of it. Now {{{arr2}}} just is some tuple type ({{{ADelayed}}}) with two lazy fields. It is constructed by evaluating another value {{{arr}}} and then building this tuple. The second field of the tuple is just a thunk. So the simplifier inlines {{{arr2}}} in {{{get}}} and thus avoids building the tuple thing of {{{arr2}}}. Now the last reference to {{{arr2}}} was dropped. In the next run the simplifier replaces {{{arr2}}} with {{{absentError}}}. Shortly after that a reference to {{{arr2}}} appears again in the code. So what went wrong? {{{get}}} had an unfolding! In this unfolding it mentions {{{arr2}}} and the simplifier spots a position where it wants to inline {{{get}}}. Now we have a reference to {{{arr2}}} again because the unoptimized version of {{{get}}} is inlined. But this time we will optimize in a different way as {{{arr2}}} was incorrectly changed to {{{absentError}}}. So the main question would be: Does the compiler consider unfoldings when determining whether a value is unused and thus can be replaced by {{{absentError}}}? If not this could be the culprit. Sadly the bug relies on a very specific order of events in the compiler, so it will be hard to find a smaller test case for this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11126#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler