[Git][ghc/ghc][wip/T23083] CorePrep: Eta expand arguments (#23083)
Sebastian Graf (@sgraf812)
gitlab at gitlab.haskell.org
Mon Mar 13 10:21:04 UTC 2023
Sebastian Graf pushed to branch wip/T23083 at Glasgow Haskell Compiler / GHC
Commits:
0492a3a7 by Sebastian Graf at 2023-03-13T11:20:57+01:00
CorePrep: Eta expand arguments (#23083)
Previously, we'd only eta expand let bindings and lambdas,
now we'll also eta expand arguments such as in T23083:
```hs
g f h = f (h `seq` (h $))
```
Unless `-fpedantic-bottoms` is set, we'll now transform to
```hs
g f h = f (\eta -> h eta)
```
in CorePrep.
See the new `Note [Eta expansion of arguments in CorePrep]` for the details.
Fixes #23083.
- - - - -
5 changed files:
- compiler/GHC/CoreToStg/Prep.hs
- compiler/GHC/Driver/Config/CoreToStg/Prep.hs
- + testsuite/tests/simplCore/should_compile/T23083.hs
- + testsuite/tests/simplCore/should_compile/T23083.stderr
- testsuite/tests/simplCore/should_compile/all.T
Changes:
=====================================
compiler/GHC/CoreToStg/Prep.hs
=====================================
@@ -1491,7 +1491,12 @@ cpeArg env dmd arg
; if okCpeArg arg2
then do { v <- newVar arg_ty
- ; let arg3 = cpeEtaExpand (exprArity arg2) arg2
+ ; let ao = cp_arityOpts (cpe_config env)
+ -- See Note [Eta expansion of arguments in CorePrep]
+ ; let arg3 | Just at <- exprEtaExpandArity ao arg2
+ = cpeEtaExpand (arityTypeArity at) arg2
+ | otherwise
+ = arg2
arg_float = mkFloat env dmd is_unlifted v arg3
; return (addFloat floats2 arg_float, varToCoreExpr v) }
else return (floats2, arg2)
@@ -1614,6 +1619,36 @@ and now we do NOT want eta expansion to give
Instead GHC.Core.Opt.Arity.etaExpand gives
f = /\a -> \y -> let s = h 3 in g s y
+Note [Eta expansion of arguments in CorePrep]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We eta expand arguments in here, in CorePrep, rather than in the Simplifier, and
+do so based on 'exprEtaExpandArity' rather than the cheaper 'exprArity' analysis
+we do on let RHSs and lambdas. The reason for the latter is that the Simplifier
+has already run the more costly analysis on lambdas and let RHSs and eta
+expanded accordingly, while it does not try to eta expand arguments at all.
+
+So why eta expand arguments in CorePrep rather than in the Simplifier?
+There are two reasons why eta expansion of arguments is useful
+
+ 1. In expressions like @f (h `seq` (g $))@ (from T23083) eta expanding the
+ argument to @f (\x -> h `seq` (g $ x))@ allows us to save allocation of a
+ closure and have a faster call sequence; a code-gen matter.
+
+ 2. The eta expansion to @f (\x -> h `seq` (g $ x))@ gives rise to another
+ opportunity: We could inline ($), saving call overhead and perhaps turning
+ an unknown call into a known call. In general, there could be further
+ simplification based on the structure of the concrete argument `x`.
+ Whether we should inline in the PAP `(g $)` (thus solving this problem
+ independently of (1)) is discussed in #22886.
+
+To profit from (1), it is enough to eta expand in CorePrep, while (2) shows
+that in some rare cases as above, eta expansion of arguments may enable
+further simplification. CorePrep would not allow to exploit (2), while eta
+expansion in the Simplifier would.
+
+Alas, trying to eta expand arguments in every round of the Simplifier is costly
+(!10088 measured a geom. mean of +2.0% regression in ghc/alloc perf, regressing
+as much as 27.2%), so we only exploit (1) for now.
-}
cpeEtaExpand :: Arity -> CpeRhs -> CpeRhs
@@ -1977,6 +2012,9 @@ data CorePrepConfig = CorePrepConfig
, cp_convertNumLit :: !(LitNumType -> Integer -> Maybe CoreExpr)
-- ^ Convert some numeric literals (Integer, Natural) into their final
-- Core form.
+
+ , cp_arityOpts :: !ArityOpts
+ -- ^ Configuration for arity analysis ('exprEtaExpandArity').
}
data CorePrepEnv
@@ -1987,6 +2025,7 @@ data CorePrepEnv
-- enabled we instead produce an 'error' expression to catch
-- the case where a function we think should bottom
-- unexpectedly returns.
+
, cpe_env :: IdEnv CoreExpr -- Clone local Ids
-- ^ This environment is used for three operations:
--
=====================================
compiler/GHC/Driver/Config/CoreToStg/Prep.hs
=====================================
@@ -9,6 +9,7 @@ import GHC.Core.Opt.Pipeline.Types ( CoreToDo(..) )
import GHC.Driver.Env
import GHC.Driver.Session
import GHC.Driver.Config.Core.Lint
+import GHC.Driver.Config.Core.Opt.Arity
import GHC.Tc.Utils.Env
import GHC.Types.Var
import GHC.Utils.Outputable ( alwaysQualify )
@@ -17,14 +18,16 @@ import GHC.CoreToStg.Prep
initCorePrepConfig :: HscEnv -> IO CorePrepConfig
initCorePrepConfig hsc_env = do
+ let dflags = hsc_dflags hsc_env
convertNumLit <- do
- let platform = targetPlatform $ hsc_dflags hsc_env
+ let platform = targetPlatform dflags
home_unit = hsc_home_unit hsc_env
lookup_global = lookupGlobal hsc_env
mkConvertNumLiteral platform home_unit lookup_global
return $ CorePrepConfig
{ cp_catchNonexhaustiveCases = gopt Opt_CatchNonexhaustiveCases $ hsc_dflags hsc_env
, cp_convertNumLit = convertNumLit
+ , cp_arityOpts = initArityOpts dflags
}
initCorePrepPgmConfig :: DynFlags -> [Var] -> CorePrepPgmConfig
=====================================
testsuite/tests/simplCore/should_compile/T23083.hs
=====================================
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -O2 -fforce-recomp #-}
+
+module T23083 where
+
+g :: ((Integer -> Integer) -> Integer) -> (Integer -> Integer) -> Integer
+g f h = f (h `seq` (h $))
=====================================
testsuite/tests/simplCore/should_compile/T23083.stderr
=====================================
@@ -0,0 +1,36 @@
+
+==================== Tidy Core ====================
+Result size of Tidy Core = {terms: 21, types: 17, coercions: 0, joins: 0/0}
+
+-- RHS size: {terms: 6, types: 6, coercions: 0, joins: 0/0}
+g :: ((Integer -> Integer) -> Integer) -> (Integer -> Integer) -> Integer
+[GblId, Arity=2, Str=<1C(1,L)><LC(S,L)>, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [60 60] 50 0}]
+g = \ (f :: (Integer -> Integer) -> Integer) (h :: Integer -> Integer) -> f (\ (eta :: Integer) -> h eta)
+
+-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
+T23083.$trModule4 :: GHC.Prim.Addr#
+[GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 20 0}]
+T23083.$trModule4 = "main"#
+
+-- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
+T23083.$trModule3 :: GHC.Types.TrName
+[GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
+T23083.$trModule3 = GHC.Types.TrNameS T23083.$trModule4
+
+-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
+T23083.$trModule2 :: GHC.Prim.Addr#
+[GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 30 0}]
+T23083.$trModule2 = "T23083"#
+
+-- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
+T23083.$trModule1 :: GHC.Types.TrName
+[GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
+T23083.$trModule1 = GHC.Types.TrNameS T23083.$trModule2
+
+-- RHS size: {terms: 3, types: 0, coercions: 0, joins: 0/0}
+T23083.$trModule :: GHC.Types.Module
+[GblId, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 10}]
+T23083.$trModule = GHC.Types.Module T23083.$trModule3 T23083.$trModule1
+
+
+
=====================================
testsuite/tests/simplCore/should_compile/all.T
=====================================
@@ -477,3 +477,4 @@ test('T23012', normal, compile, ['-O'])
test('RewriteHigherOrderPatterns', normal, compile, ['-O -ddump-rule-rewrites -dsuppress-all -dsuppress-uniques'])
test('T23024', normal, multimod_compile, ['T23024', '-O -v0'])
test('T23026', normal, compile, ['-O'])
+test('T23083', [ grep_errmsg(r'f.*eta') ], compile, ['-O -ddump-simpl -dsuppress-uniques -dppr-cols=99999'])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0492a3a73e990adb93cc93b19e4f964161a0528f
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0492a3a73e990adb93cc93b19e4f964161a0528f
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