Commit 4d8a7206 authored by Ben Gamari's avatar Ben Gamari 🐢

Don't eta-expand unsaturated primops

Previously, as described in Note [Primop wrappers], `hasNoBinding` would
return False in the case of `PrimOpId`s. This would result in eta
expansion of unsaturated primop applications during CorePrep. Not only
did this expansion result in unnecessary allocations, but it also meant
lead to rather nasty inconsistencies between the CAFfy-ness
determinations made by TidyPgm and CorePrep.

This fixes #16846.

(cherry picked from commit 4ae71eba)
parent 9781903c
......@@ -511,9 +511,17 @@ hasNoBinding :: Id -> Bool
-- Data constructor workers used to be things of this kind, but
-- they aren't any more. Instead, we inject a binding for
-- them at the CorePrep stage.
-- 'PrimOpId's also used to be of this kind. See Note [Primop wrappers] in PrimOp.hs.
-- for the history of this.
-- Note that CorePrep currently eta expands things no-binding things and this
-- can cause quite subtle bugs. See Note [Eta expansion of hasNoBinding things
-- in CorePrep] in CorePrep for details.
-- EXCEPT: unboxed tuples, which definitely have no binding
hasNoBinding id = case Var.idDetails id of
PrimOpId _ -> True -- See Note [Primop wrappers]
PrimOpId _ -> False -- See Note [Primop wrappers] in PrimOp.hs
FCallId _ -> True
DataConWorkId dc -> isUnboxedTupleCon dc || isUnboxedSumCon dc
_ -> isCompulsoryUnfolding (idUnfolding id)
......@@ -557,19 +565,6 @@ The easiest way to do this is for hasNoBinding to return True of all things
that have compulsory unfolding. A very Ids with a compulsory unfolding also
have a binding, but it does not harm to say they don't here, and its a very
simple way to fix Trac #14561.
Note [Primop wrappers]
Currently hasNoBinding claims that PrimOpIds don't have a curried
function definition. But actually they do, in GHC.PrimopWrappers,
which is auto-generated from prelude/primops.txt.pp. So actually, hasNoBinding
could return 'False' for PrimOpIds.
But we'd need to add something in CoreToStg to swizzle any unsaturated
applications of GHC.Prim.plusInt# to GHC.PrimopWrappers.plusInt#.
Nota Bene: GHC.PrimopWrappers is needed *regardless*, because it's
used by GHCi, which does not implement primops direct at all.
isDeadBinder :: Id -> Bool
......@@ -46,7 +46,7 @@ module Unique (
-- now all the built-in Uniques (and functions to make them)
-- [the Oh-So-Wonderful Haskell module system wins again...]
mkPrimOpIdUnique, mkPrimOpWrapperUnique,
mkPreludeMiscIdUnique, mkPreludeDataConUnique,
mkPreludeTyConUnique, mkPreludeClassUnique,
......@@ -368,6 +368,8 @@ mkPreludeClassUnique :: Int -> Unique
mkPreludeTyConUnique :: Int -> Unique
mkPreludeDataConUnique :: Arity -> Unique
mkPrimOpIdUnique :: Int -> Unique
-- See Note [Primop wrappers] in PrimOp.hs.
mkPrimOpWrapperUnique :: Int -> Unique
mkPreludeMiscIdUnique :: Int -> Unique
mkCoVarUnique :: Int -> Unique
......@@ -405,7 +407,8 @@ dataConWorkerUnique u = incrUnique u
dataConTyRepNameUnique u = stepUnique u 2
mkPrimOpIdUnique op = mkUnique '9' op
mkPrimOpIdUnique op = mkUnique '9' (2*op)
mkPrimOpWrapperUnique op = mkUnique '9' (2*op+1)
mkPreludeMiscIdUnique i = mkUnique '0' i
-- The "tyvar uniques" print specially nicely: a, b, c, etc.
......@@ -72,7 +72,7 @@ import qualified Data.Set as S
The goal of this pass is to prepare for code generation.
1. Saturate constructor and primop applications.
1. Saturate constructor applications.
2. Convert to A-normal form; that is, function arguments
are always variables.
......@@ -1064,8 +1064,21 @@ because that has different strictness. Hence the use of 'allLazy'.
-- Building the saturated syntax
-- ---------------------------------------------------------------------------
maybeSaturate deals with saturating primops and constructors
The type is the type of the entire application
Note [Eta expansion of hasNoBinding things in CorePrep]
maybeSaturate deals with eta expanding to saturate things that can't deal with
unsaturated applications (identified by 'hasNoBinding', currently just
foreign calls and unboxed tuple/sum constructors).
Note that eta expansion in CorePrep is very fragile due to the "prediction" of
CAFfyness made by TidyPgm (see Note [CAFfyness inconsistencies due to eta
expansion in CorePrep] in TidyPgm for details. We previously saturated primop
applications here as well but due to this fragility (see #16846) we now deal
with this another way, as described in Note [Primop wrappers] in PrimOp.
It's quite likely that eta expansion of constructor applications will
eventually break in a similar way to how primops did. We really should
eliminate this case as well.
maybeSaturate :: Id -> CpeApp -> Int -> UniqSM CpeRhs
......@@ -1285,7 +1285,48 @@ So we have to *predict* the result here, which is revolting.
In particular CorePrep expands Integer and Natural literals. So in the
prediction code here we resort to applying the same expansion (cvt_literal).
There are also numberous other ways in which we can introduce inconsistencies
between CorePrep and TidyPgm. See Note [CAFfyness inconsistencies due to eta
expansion in TidyPgm] for one such example.
Ugh! What ugliness we hath wrought.
Note [CAFfyness inconsistencies due to eta expansion in TidyPgm]
Eta expansion during CorePrep can have non-obvious negative consequences on
the CAFfyness computation done by TidyPgm (see Note [Disgusting computation of
CafRefs] in TidyPgm). This late expansion happens/happened for a few reasons:
* CorePrep previously eta expanded unsaturated primop applications, as
described in Note [Primop wrappers]).
* CorePrep still does eta expand unsaturated data constructor applications.
In particular, consider the program:
data Ty = Ty (RealWorld# -> (# RealWorld#, Int #))
-- Is this CAFfy?
x :: STM Int
x = Ty (retry# @Int)
Consider whether x is CAFfy. One might be tempted to answer "no".
Afterall, f obviously has no CAF references and the application (retry#
@Int) is essentially just a variable reference at runtime.
However, when CorePrep expanded the unsaturated application of 'retry#'
it would rewrite this to
x = \u []
let sat = retry# @Int
in Ty sat
This is now a CAF. Failing to handle this properly was the cause of
#16846. We fixed this by eliminating the need to eta expand primops, as
described in Note [Primop wrappers]), However we have not yet done the same for
data constructor applications.
type CafRefEnv = (VarEnv Id, LitNumType -> Integer -> Maybe CoreExpr)
......@@ -131,6 +131,7 @@ knownKeyNames
, map idName wiredInIds
, map (idName . primOpId) allThePrimOps
, map (idName . primOpWrapperId) allThePrimOps
, basicKnownKeyNames
, templateHaskellNames
......@@ -498,7 +498,8 @@ pRELUDE :: Module
pRELUDE = mkBaseModule_ pRELUDE_NAME
......@@ -516,6 +517,7 @@ gHC_TYPES = mkPrimModule (fsLit "GHC.Types")
gHC_MAGIC = mkPrimModule (fsLit "GHC.Magic")
gHC_CSTRING = mkPrimModule (fsLit "GHC.CString")
gHC_CLASSES = mkPrimModule (fsLit "GHC.Classes")
gHC_PRIMOPWRAPPERS = mkPrimModule (fsLit "GHC.PrimopWrappers")
gHC_BASE = mkBaseModule (fsLit "GHC.Base")
gHC_ENUM = mkBaseModule (fsLit "GHC.Enum")
......@@ -13,6 +13,7 @@ module PrimOp (
PrimOp(..), PrimOpVecCat(..), allThePrimOps,
primOpType, primOpSig,
primOpTag, maxPrimOpTag, primOpOcc,
......@@ -34,14 +35,18 @@ import TysWiredIn
import CmmType
import Demand
import OccName ( OccName, pprOccName, mkVarOccFS )
import Id ( Id, mkVanillaGlobalWithInfo )
import IdInfo ( vanillaIdInfo, setCafInfo, CafInfo(NoCafRefs) )
import Name
import PrelNames ( gHC_PRIMOPWRAPPERS )
import TyCon ( TyCon, isPrimTyCon, PrimRep(..) )
import Type
import RepType ( typePrimRep1, tyConPrimRep1 )
import BasicTypes ( Arity, Fixity(..), FixityDirection(..), Boxity(..),
SourceText(..) )
import SrcLoc ( wiredInSrcSpan )
import ForeignCall ( CLabelString )
import Unique ( Unique, mkPrimOpIdUnique )
import Unique ( Unique, mkPrimOpIdUnique, mkPrimOpWrapperUnique )
import Outputable
import FastString
import Module ( UnitId )
......@@ -572,6 +577,47 @@ primOpOcc op = case primOpInfo op of
Compare occ _ -> occ
GenPrimOp occ _ _ _ -> occ
{- Note [Primop wrappers]
Previously hasNoBinding would claim that PrimOpIds didn't have a curried
function definition. This caused quite some trouble as we would be forced to
eta expand unsaturated primop applications very late in the Core pipeline. Not
only would this produce unnecessary thunks, but it would also result in nasty
inconsistencies in CAFfy-ness determinations (see #16846 and
Note [CAFfyness inconsistencies due to late eta expansion] in TidyPgm).
However, it was quite unnecessary for hasNoBinding to claim this; primops in
fact *do* have curried definitions which are found in GHC.PrimopWrappers, which
is auto-generated by utils/genprimops from prelude/primops.txt.pp. These wrappers
are standard Haskell functions mirroring the types of the primops they wrap.
For instance, in the case of plusInt# we would have:
module GHC.PrimopWrappers where
import GHC.Prim as P
plusInt# a b = P.plusInt# a b
We now take advantage of these curried definitions by letting hasNoBinding
claim that PrimOpIds have a curried definition and then rewrite any unsaturated
PrimOpId applications that we find during CoreToStg as applications of the
associated wrapper (e.g. `GHC.Prim.plusInt# 3#` will get rewritten to
`GHC.PrimopWrappers.plusInt# 3#`).` The Id of the wrapper for a primop can be
found using 'PrimOp.primOpWrapperId'.
Nota Bene: GHC.PrimopWrappers is needed *regardless*, because it's
used by GHCi, which does not implement primops direct at all.
-- | Returns the 'Id' of the wrapper associated with the given 'PrimOp'.
-- See Note [Primop wrappers].
primOpWrapperId :: PrimOp -> Id
primOpWrapperId op = mkVanillaGlobalWithInfo name ty info
info = setCafInfo vanillaIdInfo NoCafRefs
name = mkExternalName uniq gHC_PRIMOPWRAPPERS (primOpOcc op) wiredInSrcSpan
uniq = mkPrimOpWrapperUnique (primOpTag op)
ty = primOpType op
isComparisonPrimOp :: PrimOp -> Bool
isComparisonPrimOp op = case primOpInfo op of
Compare {} -> True
......@@ -45,7 +45,7 @@ import Util
import DynFlags
import ForeignCall
import Demand ( isUsedOnce )
import PrimOp ( PrimCall(..) )
import PrimOp ( PrimCall(..), primOpWrapperId )
import SrcLoc ( mkGeneralSrcSpan )
import Data.List.NonEmpty (nonEmpty, toList)
......@@ -537,8 +537,12 @@ coreToStgApp _ f args ticks = do
(dropRuntimeRepArgs (fromMaybe [] (tyConAppArgs_maybe res_ty)))
-- Some primitive operator that might be implemented as a library call.
PrimOpId op -> ASSERT( saturated )
StgOpApp (StgPrimOp op) args' res_ty
-- As described in Note [Primop wrappers] in PrimOp.hs, here we
-- turn unsaturated primop applications into applications of
-- the primop's wrapper.
PrimOpId op
| saturated -> StgOpApp (StgPrimOp op) args' res_ty
| otherwise -> StgApp (primOpWrapperId op) args'
-- A call to some primitive Cmm function.
FCallId (CCall (CCallSpec (StaticTarget _ lbl (Just pkgId) True)
......@@ -21,6 +21,9 @@ obscure _ = ()
quux :: ()
quux = obscure (#,#)
-- It used to be that primops has no binding. However, as described in
-- Note [Primop wrappers] in PrimOp we now rewrite unsaturated primop
-- applications to their wrapper, which allows safe use of levity polymorphism.
primop :: forall (rep :: RuntimeRep) (a :: TYPE rep) b c.
a -> b -> (State# RealWorld -> (# State# RealWorld, c #))
-> State# RealWorld -> (# State# RealWorld, Weak# b #)
......@@ -15,12 +15,3 @@ T13233.hs:22:16: error:
Levity-polymorphic arguments:
a :: TYPE rep1
b :: TYPE rep2
T13233.hs:27:10: error:
Cannot use primitive with levity-polymorphic arguments:
mkWeak# :: a
-> b
-> (State# RealWorld -> (# State# RealWorld, c #))
-> State# RealWorld
-> (# State# RealWorld, Weak# b #)
Levity-polymorphic arguments: a :: TYPE rep
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