#13695: Highly nested UNPACKed data causes panic ---------------------------------+---------------------------------------- Reporter: ryanreich | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: Operating System: Linux | Architecture: Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #3990 | Differential Rev(s): Wiki Page: | ---------------------------------+---------------------------------------- Comment (by RyanGlScott): Ah, it seems that rebuilding with slight changes is crucial to triggering this bug. I've attached simplified versions of your original programs that remove the Template Haskell stuff and make clear what specific change is needed to make GHC panic. In particular, to reproduce this bug, do the following with either GHC 8.2.1 or HEAD: 1. Run `/opt/ghc/8.2.1/bin/ghc BigWord2.hs -O2` 2. Edit `BigWord2.hs`, and uncomment the block of code at the bottom (the one that starts with `type instance BigWord 5 = BigWord5` and ends with `instance LowHigh BigWord4 where ...`) 3. Run `/opt/ghc/8.2.1/bin/ghc BigWord2.hs -O2` again: {{{ $ /opt/ghc/8.2.1/bin/ghc BigWord2.hs -O2 [3 of 3] Compiling BigWord2 ( BigWord2.hs, BigWord2.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.2.0.20170505 for x86_64-unknown-linux): urk! lookup local fingerprint D:R:DoubleWordDoubleWord4 [cESeWE :-> (BigWord2, 34dc4d50eaf3c3f1fec88a99dae21850), cESfxE :-> (BigWord3, 2b1c1772a861f399344dc06c29e7e7c0), cESfxF :-> (BigWord4, df345db7e7bc5c1dd0c73d6b038bc339), cESfxY :-> (BigWord5, 2b3cbb09e91bec36a2a2c359191139d2), cESg1l :-> (R:DoubleWordDoubleWord5, 00000000000000000000000000000000), cESg1n :-> (D:R:DoubleWordDoubleWord6, 00000000000000000000000000000000), cESg1v :-> (D:R:BigWord5, 560a80c635c7dd211457b5d4dab86d51), dESfxY :-> (BigWord5, 775d4a6ae2e1194445cb6e4252de8b91), iESfxY :-> (BigWord5, fc430fea13ebb9a94e051713cccf16ee), iESg1p :-> ($WBigWord5, c95ed7d25584a6ae4abaf9ae169cd259)] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1134:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in ghc:Outputable pprPanic, called at compiler/iface/MkIface.hs:508:37 in ghc:MkIface Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13695#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler