[GHC] #10225: GHC does not specialize based on type equality
#10225: GHC does not specialize based on type equality -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.10.1 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Based on my testing, it seems that if I have a function like f :: (v ~ V2) => v -> v this will be slower than f :: v -> v It would be great if GHC could do this optimization. The reason I want this is that I have written some very generic functions in my program, which are actually only used for one type. SPECIALIZE doesn't seem to do enough, as it's hard to get those GHC to "see" all of those types. As a compromise, I tried to tell GHC what those types actually are via type equalities, so that I can keep type constraints around for the future. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10225 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10225: GHC does not specialize based on type equality -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Description changed by yongqli: Old description:
Based on my testing, it seems that if I have a function like
f :: (v ~ V2) => v -> v
this will be slower than
f :: v -> v
It would be great if GHC could do this optimization. The reason I want this is that I have written some very generic functions in my program, which are actually only used for one type. SPECIALIZE doesn't seem to do enough, as it's hard to get those GHC to "see" all of those types. As a compromise, I tried to tell GHC what those types actually are via type equalities, so that I can keep type constraints around for the future.
New description: Based on my testing, it seems that if I have a function like f :: (v ~ V2) => v -> v this will be slower than f :: V2 -> V2 It would be great if GHC could do this optimization. The reason I want this is that I have written some very generic functions in my program, which are actually only used for one type. SPECIALIZE doesn't seem to do enough, as it's hard to get those GHC to "see" all of those types. As a compromise, I tried to tell GHC what those types actually are via type equalities, so that I can keep type constraints around for the future. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10225#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10225: GHC does not specialize based on type equality -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): Can you give a concrete example of what you mean? Does `f` need the fact that `(v ~ V2)`? If not, I'm surprised that the former is slower than the latter. Can you give a test case? The smaller the better Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10225#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10225: GHC does not specialize based on type equality -------------------------------------+------------------------------------- Reporter: yongqli | Owner: Type: feature request | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 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): * status: new => infoneeded * failure: None/Unknown => Runtime performance bug Comment: yongqli: can you supply a test case? Thanks. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10225#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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