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Simon Peyton Jones pushed to branch wip/andreask/spec_tyfams at Glasgow Haskell Compiler / GHC

Commits:

39a685d7

by Simon Peyton Jones at 2025-07-04T13:52:43+01:00

Treat tuple dicationaries uniformly; don't unbox them

See the explanation in
https://gitlab.haskell.org/ghc/ghc/-/issues/19747#note_626297

ToDo: needs better Notes etc. This is for CI

e9bce3a3

by Simon Peyton Jones at 2025-07-04T13:55:01+01:00

Comment out the ad-hoc specCase stuff

See #26158.  Needs more documentation

2 changed files:

compiler/GHC/Core/Opt/DmdAnal.hs
compiler/GHC/Core/Opt/Specialise.hs

Changes:

compiler/GHC/Core/Opt/DmdAnal.hs

@@ -23,7 +23,7 @@ import GHC.Core.DataCon
  import GHC.Core.Utils
  import GHC.Core.TyCon
  import GHC.Core.Type
 -import GHC.Core.Predicate( isEqualityClass, isCTupleClass )
 +import GHC.Core.Predicate( isEqualityClass {- , isCTupleClass -} )
  import GHC.Core.FVs      ( rulesRhsFreeIds, bndrRuleAndUnfoldingIds )
  import GHC.Core.Coercion ( Coercion )
  import GHC.Core.TyCo.FVs     ( coVarsOfCos )
@@ -2194,8 +2194,10 @@ doNotUnbox :: Type -> Bool
  doNotUnbox arg_ty
    = case tyConAppTyCon_maybe arg_ty of
        Just tc | Just cls <- tyConClass_maybe tc
 -              -> not (isEqualityClass cls || isCTupleClass cls)
 +              -> not (isEqualityClass cls {- || isCTupleClass cls -})
         -- See (DNB2) and (DNB1) in Note [Do not unbox class dictionaries]
 +       --
 +       --   *** TODO *** document the removal of isCTupleClass!
        _ -> False
@@ -2243,7 +2245,7 @@ Wrinkle (DNB1): we /do/ want to unbox tuple dictionaries (#23398)
       f = /\a. \d:(% Eq a, Show a %). ... f @a (% sel1 d, sel2 d %)...
    where there is a recurive call to `f`, or to another function that takes the
    same tuple dictionary, but where the tuple is built from the components of
 -  `d`.  The Simplier does not fix this.  But if we unpacked the dictionary
 +  `d`.  The Simplifier does not fix this.  But if we unpacked the dictionary
    we'd get
       $wf = /\a. \(d1:Eq a) (d2:Show a). let d = (% d1, d2 %)
               in ...f @a (% sel1 d, sel2 d %)

compiler/GHC/Core/Opt/Specialise.hs

@@ -1283,6 +1283,7 @@ specCase :: SpecEnv
                    , OutId
                    , [OutAlt]
                    , UsageDetails)
 +{-
  specCase env scrut' case_bndr [Alt con args rhs]
    | -- See Note [Floating dictionaries out of cases]
      isDictTy (idType case_bndr)
@@ -1343,6 +1344,7 @@ specCase env scrut' case_bndr [Alt con args rhs]
                        && tyCoVarsOfType var_ty `disjointVarSet` arg_set
         where
           var_ty = idType var
 +-}
  specCase env scrut case_bndr alts
    = do { (alts', uds_alts) <- mapAndCombineSM spec_alt alts