#13376: GHC fails to specialize a pair of polymorphic INLINABLE functions -------------------------------------+------------------------------------- Reporter: jberryman | 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: #8668 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by jberryman: Old description:
This is a boiled down version of a library I'm working on. It's possible this is the same issue as #8668 which seems to have stalled. Hopefully this example is simpler and useful in that case. Also likely the same as this https://github.com/jmoy/testing-specialize
I have a library which defines the classes `H` and `S`; library consumers are likely to define their own `H` instances, and import `S` instances declared by ''other'' library authors (not me), who will depend on my `H`.
Performance depends on all of it getting fully-specialized `hash` (i.e. for each combination of `H` and `S` that the consumer uses). But I don't really want `hash` inlined at every call site.
Here is the code to repro with explanation below. I'm compiling like: `ghc --make -Wall -O2 -rtsopts -funbox-strict-fields -ddump-to-file -ddump-simpl -dsuppress-module-prefixes -dsuppress-uniques -ddump-core- stats -ddump-inlinings -ddump-asm -fforce-recomp Main.hs`, and we get the same bad behavior on GHC 7.10.3 and GHC 8.0.1:
Lib.hs:
{{{#!hs module Lib where
class H h where hash :: (S s)=> s -> h -> s
class S s where mix :: s -> Int -> s
instance H Int where {-# INLINABLE hash #-} hash s = \x -> s `mix` x -- make this look big: `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x }}}
S.hs:
{{{#!hs module S where
import Lib
newtype Foo = Foo Int deriving Show
instance S Foo where {-# INLINABLE mix #-} mix (Foo x) y = Foo (x+y) }}}
And the `Main` I'm using, though you can just call print; it's obvious dumping inlinings when the functions get specialized and unboxed (look for "Inlining done: $fNumInt_$c+"):
{{{#!hs module Main where
import Lib import S
import Criterion.Main
main = defaultMain [ bench "foo" $ whnf (hash (Foo 1)) (1::Int) ] }}}
If I use the `INLINABLE` pragmas above or omit them entirely we get bad code.
If I put an `INLINE` on the `hash` declaration in Lib (and no pragmas in S), we get good unboxed additions and things are fast.
Finally and most bizarrely, if I omit the `INLINE` pragma in `hash` (and similarly no pragmas in `S`) but make the body small enough (5 lines of the "`mix` x `mix` x..." junk) then we also get nice unboxed code.
New description: This is a boiled down version of a library I'm working on. It's possible this is the same issue as #8668 which seems to have stalled. Hopefully this example is simpler and useful in that case. Also likely the same as this https://github.com/jmoy/testing-specialize I have a library which defines the classes `H` and `S`; library consumers are likely to define their own `H` instances, and import `S` instances declared by ''other'' library authors (not me), who will depend on my `H`. Performance depends on all of it getting fully-specialized `hash` (i.e. for each combination of `H` and `S` that the consumer uses). But I don't really want `hash` inlined at every call site. Here is the code to repro with explanation below. I'm compiling like: `ghc --make -Wall -O2 -rtsopts -funbox-strict-fields -ddump-to-file -ddump- simpl -dsuppress-module-prefixes -dsuppress-uniques -ddump-core-stats -ddump-inlinings -ddump-asm -fforce-recomp Main.hs`, and we get the same bad behavior on GHC 7.10.3 and GHC 8.0.1: Lib.hs: {{{#!hs module Lib where class H h where hash :: (S s)=> s -> h -> s class S s where mix :: s -> Int -> s instance H Int where {-# INLINABLE hash #-} hash s = \x -> s `mix` x -- make this look big: `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x `mix` x }}} S.hs: {{{#!hs module S where import Lib newtype Foo = Foo Int deriving Show instance S Foo where {-# INLINABLE mix #-} mix (Foo x) y = Foo (x+y) }}} And the `Main` I'm using, though you can just call print; it's obvious dumping inlinings when the functions get specialized and unboxed (look for "Inlining done: $fNumInt_$c+"): {{{#!hs module Main where import Lib import S import Criterion.Main main = defaultMain [ bench "foo" $ whnf (hash (Foo 1)) (1::Int) ] }}} If I use the `INLINABLE` pragmas above or omit them entirely we get bad code. If I put an `INLINE` on the `hash` declaration in Lib (and no pragmas in S), we get good unboxed additions and things are fast. Finally and most bizarrely, if I omit the `INLINE` pragma in `hash` (and similarly no pragmas in `S`) but make the body small enough (5 lines of the "`mix` x `mix` x..." junk) then we also get nice unboxed code. **EDIT**: Also if I move the `S` constraint into the head of `H` then INLINABLE and stuff seem to work as expected: {{{#!hs -- lousy workaround; we can tell users to just not touch the `s` -- parameter in their own instances: class (S s)=> H s h where hash :: s -> h -> s }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13376#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler