#10218: GHC creates incorrect code which throws <<loop>> -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: yes Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): OK here is a nice small test case, which does not (unlike the test above) depend on `lens`! {{{ module Main where {-# NOINLINE foo #-} foo :: Bool -> Int -> Int -> Int foo True _ x = 1 foo False _ x = x+1 {-# NOINLINE bar #-} bar :: Int -> (Int,Int) bar x = let y1 = x * 2 y2 = x * 2 in (foo False y1 y2,foo False y2 y1) main = print (fst p + snd p) where p = bar 3 }}} Compile with `-O -feager-blackholing` and you get `<<loop>`. Add `-fno- cse` or `-flate-dmd-anal` restores correct behaviour. Points to note * `foo` uses its second argument zero times, and its third argument exactly once. * So the two calls to `foo` in `bar` use `y1` exactly once and `y2` exactly once. * But when `y1` and `y2` are CSE'd, the usage goes up to twice; and that is the problem. I'm validating a simple fix in CSE, which zaps the demand-info on binders which are potentially shared. It's a bit brutal. But another run of the demand analyser (which has other advantages) restores everything again. I'm validating now; will commit next week. Really sorry to have taken two weeks of your time to find this bug. But at least your efforts will be rewarded by a real fix. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10218#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler