#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: 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 simonpj): Let's leave aside the deferred-type-error case for now. Other than that, I claim that '''every evidence variable should be non- bottom''', and so * Can be evaluated strictly, and/or * Its evaluation can be discarded if its payload is not required This is true even of recursively-defined dictionaries; see `Note [Recursive superclasses]` in `TcInstDcls`. (A recursively-defined dictionary may be infinite, so you can't evaluate it hyper-strictly, but it should not be bottom.) But dictionaries still ''can'' be defined recursively (unlike say unboxed values). Now, I believe all this is true of the programs generated by source Haskell, but nothing in Core checks this invariant; and in Core you could certainly declare a bottom dictionary `(let d = d in ...)`. I don't know how to make it a statically-checkable invariant of Core, but I expect that it'd be possible to make a decent conservative approximation that was enough to capture the programs generated from source Haskell. There's even an old flag `-fdicts-strict` that I would like to get back to, which flirted with using call-by-value for evidence variables; c.f. #2436 That leaves deferred type errors. But let's not allow the tail to wag the dog. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | 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: | -------------------------------------+------------------------------------- Changes (by mpickering): * cc: mpickering (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | 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 Simon Peyton Jones ): In [changeset:"954cbc7c106a20639960f55ebb85c5c972652d41/ghc" 954cbc7c/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="954cbc7c106a20639960f55ebb85c5c972652d41" Drop dead Given bindings in setImplicationStatus Trac #13032 pointed out that we sometimes generate unused bindings for Givens, and (worse still) we can't always discard them later (we don't drop a case binding unless we can prove that the scrutinee is non-bottom. It looks as if this may be a major reason for the performace problems in #14338 (see comment:29). This patch fixes the problem at source, by pruning away all the dead Givens. See Note [Delete dead Given evidence bindings] Remarkably, compiler allocation falls by 23% in perf/compiler/T12227! I have not confirmed whether this change actualy helps with }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_compile/T13032 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * testcase: => typecheck/should_compile/T13032 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_compile/T13032 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I fixed the test case by suppressing the extra `-ddump-ds` output when `-dno-debug-output` is enabled. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_compile/T13032 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"efce943ca20b55b18f948681e6b44fd892dbddd2/ghc" efce943c/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="efce943ca20b55b18f948681e6b44fd892dbddd2" Add -ddump-ds-preopt This allows you to see the output immediately after desugaring but before any optimisation. I've wanted this for some time, but I was triggered into action by Trac #13032 comment:9. Interestingly, the change means that with -dcore-lint we will now Lint the output before the very simple optimiser; and this showed up Trac #14749. But that's not the fault of -ddump-ds-preopt! }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13032#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler