#13977: ExnStr doesn't propagate "outwards" -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): I think I might see why the demand analyser drops the `x`. Consider `postProcessDmdEnv`, {{{#!hs postProcessDmdEnv :: DmdShell -> DmdEnv -> DmdEnv postProcessDmdEnv ds@(JD { sd = ss, ud = us }) env | Abs <- us = emptyDmdEnv | Str _ _ <- ss , Use One _ <- us = env -- Shell is a no-op | otherwise = mapVarEnv (postProcessDmd ds) env }}} When we get here while demand-analysing the `catchRetry# @Char a` application in `hello` we will have the "shell" equal to `JD {sd = Str ExnStr (), ud = Use One ()}` (as computed as `defer_and_use` by `dmdAnalStar`). The environment we are post-processing is `{aQq->}`. Note that the shell has an `ExnStr` which gets eaten by the `Str _ _ <- ss` case of `postProcessDmdEnv`, meaning we don't transform the environment. I haven't yet done enough reading to know whether this is correct but is seems suspicious. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13977: ExnStr doesn't propagate "outwards" -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:

While looking into #8091 (see ticket:8091#comment:9) and #13916, I noticed something that I found rather surprising. Consider this program,

{{{#!hs hello :: STR Char -> STM Char hello a = a `orElse` pure 'a' }}}

Recall that `catchRetry#` (which `orElse` is defined in terms of) has the following demand signature, {{{ }}} Note the `x` here: this means it is `ExnStr`, since it doesn't bottom even if the first argument does (despite the fact that it's strict).

So, the question is this: What demand should `hello` place on its first argument? I would have thought that it should be precisely the same as the demand that `catchRetry#` places on **its** first argument. However, the demand analyser seems to conclude this: {{{ }}}

Note the lack of an `x`. I believe this may be what causes #13916.

New description: While looking into #8091 (see ticket:8091#comment:9) and #13916, I noticed something that I found rather surprising. Consider this program, {{{#!hs hello :: STM Char -> STM Char hello a = a `orElse` pure 'a' }}} Recall that `catchRetry#` (which `orElse` is defined in terms of) has the following demand signature, {{{ }}} Note the `x` here: this means it is `ExnStr`, since it doesn't bottom even if the first argument does (despite the fact that it's strict). So, the question is this: What demand should `hello` place on its first argument? I would have thought that it should be precisely the same as the demand that `catchRetry#` places on **its** first argument. However, the demand analyser seems to conclude this: {{{ }}} Note the lack of an `x`. I believe this may be what causes #13916. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13977: ExnStr doesn't propagate "outwards" -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3756 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"c940e3b92f4527ca59fcae93f36c869de3e7ccb9/ghc" c940e3b/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="c940e3b92f4527ca59fcae93f36c869de3e7ccb9" dmdAnal: Ensure that ExnStr flag isn't dropped inappropriately This fixes #13977 and consequently #13615. Here an optimization in the demand analyser was too liberal, causing us to drop the ExnStr flag and consequently resulting in incorrect demand signatures. This manifested as a segmentation fault in #13615 as we incorrectly concluded that an application of catchRetry# would bottom. Specifically, we had orElse' :: STM a -> STM a -> STM a orElse' x = catchRetry# x y where y = {- some action -} Where the catchRetry# primop places a demand of on its first argument. However, due to #13977 the demand analyser would assign a demand of on the first argument of orElse'. Note the missing `x`. case orElse' bottomingAction anotherAction of { x -> Just x } being transformed to, case orElse' bottomingAction anotherAction of {} by the simplifier. This would naturally blow up when orElse' returned at runtime, causing the segmentation fault described in #13615. Test Plan: Validate, perhaps add a testcase Reviewers: austin, simonpj Reviewed By: simonpj Subscribers: rwbarton, thomie GHC Trac Issues: #13977, #13615 Differential Revision: https://phabricator.haskell.org/D3756 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13977: ExnStr doesn't propagate "outwards" -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Exceptions Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3756 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * keywords: => Exceptions Comment: I believe this fix may bear on a number of tickets which have come up in the past year or so, * #13380: `raiseIO#` result looks wrong * #13330: `forkIO` has inconsistent behavior under optimization * #11555: `catch` _|_ breaks at `-O1` * #11222: Teach strictness analysis about `catch`-like operations * #13357: Check demand signatures for `catchRetry#` and `catchSTM#` -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler