diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/Simplify/Utils.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/Simplify/Utils.hs | 33 |
1 files changed, 22 insertions, 11 deletions
diff --git a/compiler/GHC/Core/Opt/Simplify/Utils.hs b/compiler/GHC/Core/Opt/Simplify/Utils.hs index 6ecfce1442..cef65eb2b1 100644 --- a/compiler/GHC/Core/Opt/Simplify/Utils.hs +++ b/compiler/GHC/Core/Opt/Simplify/Utils.hs @@ -2266,26 +2266,37 @@ h y = case y of If we inline h into f, the default case of the inlined h can't happen. If we don't notice this, we may end up filtering out *all* the cases of the inner case y, which give us nowhere to go! + +Note [Shadowing in prepareAlts] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note that we pass case_bndr::InId to prepareAlts; an /InId/, not an +/OutId/. This is vital, because `refineDefaultAlt` uses `tys` to build +a new /InAlt/. If you pass an OutId, we'll end up appling the +substitution twice: disaster (#23012). + +However this does mean that filling in the default alt might be +delayed by a simplifier cycle, because an InId has less info than an +OutId. Test simplCore/should_compile/simpl013 apparently shows this +up, although I'm not sure exactly how.. -} -prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt]) +prepareAlts :: OutExpr -> InId -> [InAlt] -> SimplM ([AltCon], [InAlt]) -- The returned alternatives can be empty, none are possible -prepareAlts scrut case_bndr' alts - | Just (tc, tys) <- splitTyConApp_maybe (varType case_bndr') - -- Case binder is needed just for its type. Note that as an - -- OutId, it has maximum information; this is important. - -- Test simpl013 is an example +-- +-- Note that case_bndr is an InId; see Note [Shadowing in prepareAlts] +prepareAlts scrut case_bndr alts + | Just (tc, tys) <- splitTyConApp_maybe (idType case_bndr) = do { us <- getUniquesM - ; let (idcs1, alts1) = filterAlts tc tys imposs_cons alts - (yes2, alts2) = refineDefaultAlt us (idMult case_bndr') tc tys idcs1 alts1 - -- the multiplicity on case_bndr's is the multiplicity of the + ; let (idcs1, alts1) = filterAlts tc tys imposs_cons alts + (yes2, alts2) = refineDefaultAlt us (idMult case_bndr) tc tys idcs1 alts1 + -- The multiplicity on case_bndr's is the multiplicity of the -- case expression The newly introduced patterns in -- refineDefaultAlt must be scaled by this multiplicity (yes3, idcs3, alts3) = combineIdenticalAlts idcs1 alts2 -- "idcs" stands for "impossible default data constructors" -- i.e. the constructors that can't match the default case - ; when yes2 $ tick (FillInCaseDefault case_bndr') - ; when yes3 $ tick (AltMerge case_bndr') + ; when yes2 $ tick (FillInCaseDefault case_bndr) + ; when yes3 $ tick (AltMerge case_bndr) ; return (idcs3, alts3) } | otherwise -- Not a data type, so nothing interesting happens |