I get a 404 for https://gitlab.haskell.org/ghc/ghc/-/issues/18372
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From: Andrzej Rybczak
Sent: 21 June 2020 15:44
To: Simon Peyton Jones
Subject: GHC/ | GHC doesn't inline small type class method marked as INLINE with profiling enabled (#18372)
Andrzej Rybczakhttps://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Farybczak&data=02%7C01%7Csimonpj%40microsoft.com%7C69826edc95e94f123d4208d815f18406%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637283474331207255&sdata=oSLeWvPZsSFhqr07wt3RveuWf9F%2Bs052fjkuTKEVzqE%3D&reserved=0 created an issue:
Summary
When profiling is enabled, GHC doesn't inline small type class methods which prevents further optimizations and specialization from happening.
Steps to reproduce
When the following module:
module Main where
import Optics.Core
data T = T { _t1 :: Int
, _t2 :: Int
}
t1 :: Lens' T Int
t1 = lensVL $ \f s -> (\n -> s { _t1 = n}) <$> f (_t1 s)
{-# INLINE t1 #-}
t2 :: Lens' T Int
t2 = lensVL $ \f s -> (\n -> s { _t2 = n}) <$> f (_t2 s)
{-# INLINE t2 #-}
t_val :: T
t_val = T 1 2
{-# NOINLINE t_val #-}
main :: IO ()
main = putStrLn . show $ view t1 t_val + view t2 t_val
is compiled with profiling enabled, here's how core of main looks:
main1
= case ((($fStrongForget_$clinear main_l1) (id `cast` Co:41))
`cast` Co:42)
t_val
of
{ I# x_a2c8 ->
case ((($fStrongForget_$clinear main_l2) (id `cast` Co:41))
`cast` Co:42)
t_val
of
{ I# y_a2cb ->
case $wshowSignedInt 0# (+# x_a2c8 y_a2cb) [] of
{ (# ww5_a277, ww6_a278 #) ->
: ww5_a277 ww6_a278
}
}
}
So linear doesn't get inlined even though it's a small function that is even explicitly marked INLINEhttps://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fwell-typed%2Foptics%2Fblob%2F5758cd6a18162cd0a69f4a044026d1bd878cfaf9%2Findexed-profunctors%2Fsrc%2FData%2FProfunctor%2FIndexed.hs%23L355&data=02%7C01%7Csimonpj%40microsoft.com%7C69826edc95e94f123d4208d815f18406%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637283474331207255&sdata=v6agV%2FTB5%2BpzLDcmEdTI8NNf281F39KhfLinCEi7vI0%3D&reserved=0.
This prevents further optimizations from happening and leaves optics-related code in semi-optimized state, making looking at cost centers unreliable (see well-typed/optics#324https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fwell-typed%2Foptics%2Fissues%2F324&data=02%7C01%7Csimonpj%40microsoft.com%7C69826edc95e94f123d4208d815f18406%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637283474331217249&sdata=ThSQiMn4GzrEWaEg9uFycOCkka32wf9wjXAhznlZ%2Bjo%3D&reserved=0).
When profiling is disabled, everything inlines and optimizes away as expected:
main1
= case t_val of { T ds_d25F ds1_d25G ->
case ds_d25F of { I# x_a2c7 ->
case ds1_d25G of { I# y_a2ca ->
case $wshowSignedInt 0# (+# x_a2c7 y_a2ca) [] of
{ (# ww5_a276, ww6_a277 #) ->
: ww5_a276 ww6_a277
}
}
}
}
I'm attaching archive with cabal project that contains above module for easy reproduction: prof_test.tar.gzhttps://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fuploads%2Fd3c051396724bcac1eeeb17ccb4bc779%2Fprof_test.tar.gz&data=02%7C01%7Csimonpj%40microsoft.com%7C69826edc95e94f123d4208d815f18406%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637283474331217249&sdata=2kBWLGNzNNoLhiU3GF4QsLoL%2BnWEzmLVxmtPoaQ9UfI%3D&reserved=0
Profiling build was tested with profiling: True in cabal.project.local.
Expected behavior
GHC should inline class methods marked as INLINE when profiling is enabled.
Environment
* GHC version used: 8.10.1
Optional:
* Operating System: Arch Linux
* System Architecture: x86_64
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Simon Peyton Jones