#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by rwbarton): * status: new => infoneeded Comment: Could you attach your program (preferably after minimizing it as much as possible)? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by yongqli): See Main.hs here: https://gist.github.com/yongqli/5dc94aeaeda2e24c6637 You need the following packages: {{{ linear, lens, vector, vector-th- unbox, strict }}} Running with {{{ ghc Main.hs -O2 -flate-dmd-anal -fforce-recomp; ./Main }}} will give {{{ Main: Oops! Entered absent arg w_sGY6 MVN V1 Double }}} It takes about 3 days's worth of work to produce a test case like this. Is there a better option than deleting a piece of code and then recompiling? I essentially have to "unwrite" my whole program. Is there a fuzzer-like tool that could conceivably be used? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): Oh VERY interesting! I have reproduced the problem with 7.10, and it is YET ANOTHER bug in the demand analyser. Here is the offending code {{{ dmdAnal' sigs dmd (App fun (Coercion co)) = (fun_ty, App fun' (Coercion co)) where (fun_ty, fun') = dmdAnal sigs dmd fun }}} The incoming demand `dmd` is not modified before passing it on to `fun`. But that is wrong wrong wrong. That gets the demands from the function out of step, so the wrong demand goes to the wrong argument. It's a miracle that this has not been causing chaos for ages. Horror. Coercions are ''value'' arguments and so they must be treated as such. I think the solution is simply to delete these lines so that coercion arguments are handled uniformly; but I want to tread carefully. Thank you for extracting this example. More anon. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by thoughtpolice): * status: infoneeded => new -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"d5773a4939b1feea51ec0db6624c9462751e948a/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="d5773a4939b1feea51ec0db6624c9462751e948a" Teach DmdAnal that coercions are value arguments! The demand analyser was treating coercion args like type args, which meant that the arguments in a strictness signature got out of step with the arguments of a call. Result chaos and disaster. Trac #10288 showed it up. It's hard to get this bug to show up in practice because - functions abstracted over coercions are usually abstracted over *boxed* coercions - we don't currently unbox a boxed-coercion arg because it's GADT (I see how to fix this too) But after floating, optimisation, and so on, Trac #10288 did get a function abstracted over an unboxed coercion, and then the -flate-dmd-anal pass went wrong. I don't think I can come up with a test case, but I don't think it matters too much. Still to come - Fix a second bug, namely that coercion variables are wrongly marked as absent because DmdAnal doesn't check the the free variables of casts. I think this never bites in practice (see the follow-up commit) - Make GADT products work with strictness analysis }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: simonpj Type: bug | Status: merge Priority: highest | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by simonpj): * priority: normal => highest * status: new => merge Comment: yongqli, your test case nailed it -- thank you! It was so helpful to be able to reproduce it, and it's not an easy case to make happen, so I might well have failed to find it, or assumed it was already fixed, without your help. Austin: please merge the above two to 7.10. I'm not adding a regression test because it's hard to provoke; and the fix (which consisted only of deleting code) is pretty robust. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10288: -flate-dmd-anal triggers "Entered absent arg" -------------------------------------+------------------------------------- Reporter: yongqli | Owner: simonpj Type: bug | Status: closed Priority: highest | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime crash | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by bgamari): Note that the commit to the `ghc-7.10` branch has a typo. I've opened Phab:D881 to fix this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10288#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler