[GHC] #11146: Manual eta expansion leads to orders of magnitude less allocations
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While working on deterministic free variable computation I run into some performance issues. I narrowed it down to code that when eta expanded performed less allocations. I ended up writing a benchmark and for large examples the eta expanded versions performs orders of magnitude less allocations. The results of my test: {{{ EtaExpanded test3 50,450,112 bytes allocated in the heap real 0.89 EtaReduced test3 3,661,740,240 bytes allocated in the heap real 3.66 }}} The whole source is here: https://github.com/niteria/deterministic-fvs, I'm also attaching the two interesting implementations. I experimented with it a little and GHC eta-expanded when I didn't have mutually recursive bindings. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * Attachment "FVCommon.hs" added. FVCommon.hs -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * Attachment "FV.hs" added. EtaExpanded/FV.hs -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * Attachment "FV.2.hs" added. EtaReduced/FV.hs -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * Attachment "Tests.hs" added. Tests.hs -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * cc: simonmar, simonpj (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 nomeata): Let me be cheeky: Where is the bug in the report? It is not a secret that eta-expanded definitions tend to perform better, unless you make use of sharing somewhere. So maybe you did expect the compiler to make that transformation for you? But for that to happen, the compiler needs to be very sure that it is safe to do so (e.g. no loss of sharing), and in a modular compiler, that is often not possible. Do you have a (hopefully small and self-contained) bit of code where you think “duh, obviously the compiler should be a allowed to eta-expand here”, but it does not? Do things are better if all the relevant code is in one module, with an explicit export list that does not include any of the functions you’d like to see eta-expanded? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria):
Where is the bug in the report?
Do you have a (hopefully small and self-contained) bit of code where you
I'm not convinced this is a bug. I've created this in response to this comment: https://phabricator.haskell.org/D1537#inline-12820. think “duh, obviously the compiler should be a allowed to eta-expand here”, but it does not? I can't tell if GHC should be allowed to eta-expand in my example, but the fact that it decided to eta-expand in a very similar situation where I didn't have mutually recursive functions suggests that it might be able to.
Do things are better if all the relevant code is in one module, with an explicit export list that does not include any of the functions you’d like to see eta-expanded?
Yes, it's enough to not export the functions I want eta-expanded. Unfortunately that's exactly what I want do in Phab:D1537 because I'm composing a computation from smaller pieces from several modules. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 osa1): Could you also add `Types.hs`? It's used in all of the modules you added. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 niteria): * Attachment "Types.hs" added. Types.hs -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 nomeata):
Yes, it's enough to not export the functions I want eta-expanded. Unfortunately that's exactly what I want do in Phab:D1537 because I'm composing a computation from smaller pieces from several modules.
Then GHC (likely Call Arity \o/) seems to be smart enough to do the expansion if it is allowed to operate on the whole code at once, as desired. But with the code spread over several modules, there isn’t really much GHC can do here. You could try to make it `INLINE` more aggressively, or simply do the eta-expansion as you do now. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 osa1): I just tried `INLINE`ing all the functions defined in `FV` and `FV2`, it didn't make any difference. (maybe because GHC was already inlining?) (tried with -O2) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 osa1): * cc: osa1 (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11146: Manual eta expansion leads to orders of magnitude less allocations -------------------------------------+------------------------------------- Reporter: niteria | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * failure: None/Unknown => Runtime performance bug -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11146#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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