GitLab

Andreas Klebinger pushed to branch wip/andreask/fold_tag_to_enum at Glasgow Haskell Compiler / GHC

Commits:

e7d74372

by Andreas Klebinger at 2025-04-19T14:08:46+02:00

Simplifier: Constant fold invald tagToEnum# calls to bottom expr.

When applying tagToEnum# to a out-of-range value it's best to simply
constant fold it to a bottom expression. That potentially allows more
dead code elimination and makes debugging easier.

Fixes #25976

3 changed files:

compiler/GHC/Core/Opt/ConstantFold.hs
+ testsuite/tests/simplCore/should_compile/T25976.hs
testsuite/tests/simplCore/should_compile/all.T

Changes:

compiler/GHC/Core/Opt/ConstantFold.hs

@@ -69,7 +69,6 @@ import GHC.Cmm.MachOp ( FMASign(..) )
  import GHC.Cmm.Type ( Width(..) )
  import GHC.Data.FastString
 -import GHC.Data.Maybe      ( orElse )
  import GHC.Utils.Outputable
  import GHC.Utils.Misc
@@ -2008,9 +2007,13 @@ tagToEnumRule = do
      Just (tycon, tc_args) | isEnumerationTyCon tycon -> do
        let tag = fromInteger i
            correct_tag dc = (dataConTagZ dc) == tag
 -      (dc:rest) <- return $ filter correct_tag (tyConDataCons_maybe tycon `orElse` [])
 -      massert (null rest)
 -      return $ mkTyApps (Var (dataConWorkId dc)) tc_args
 +      Just dataCons <- pure $ tyConDataCons_maybe tycon
 +      case filter correct_tag dataCons of
 +        (dc:rest) -> do
 +          massert (null rest)
 +          pure $ mkTyApps (Var (dataConWorkId dc)) tc_args
 +        -- Literal is out of range, e.g. tagToEnum @Bool #4
 +        [] -> pure $ mkImpossibleExpr ty "tagToEnum: Argument out of range"
      -- See Note [tagToEnum#]
      _ -> warnPprTrace True "tagToEnum# on non-enumeration type" (ppr ty) $

testsuite/tests/simplCore/should_compile/T25976.hs

 +{-# LANGUAGE MagicHash #-}
++
 +module T25976 where
++
 +import GHC.PrimOps (tagToEnum#)
++
 +-- Spoiler - it's all dead code since tagToEnum 3# is undefined
 +main = case (tagToEnum# 4# :: Bool) of
 +  True -> print "Dead Code"
 +  False -> print "Dead Code"
 \ No newline at end of file

testsuite/tests/simplCore/should_compile/all.T

@@ -535,3 +535,4 @@ test('T25197', [req_th, extra_files(["T25197_TH.hs"]), only_ways(['optasm'])], m
  test('T25389', normal, compile, ['-O -ddump-simpl -dsuppress-uniques -dno-typeable-binds'])
  test('T24359a', normal, compile, ['-O -ddump-rules'])
  test('T25713', [grep_errmsg('W:::')], compile, ['-O -ddump-simpl'])
 +test('T25976', [grep_errmsg('Dead Code')], compile, ['-O -ddump-simpl -dsuppress-uniques -dno-typeable-binds'])