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From: Andrzej Rybczak <gitlab@gitlab.haskell.org>
Sent: 21 June 2020 15:44
To: Simon Peyton Jones <simonpj@microsoft.com>
Subject: GHC/ | GHC doesn't inline small type class method marked as INLINE with profiling enabled (#18372)

Andrzej Rybczak 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 INLINE.

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#324).

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.gz

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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