Zubin pushed to branch wip/25924 at Glasgow Haskell Compiler / GHC Commits: 9fe864da by Zubin Duggal at 2026-06-23T15:26:30+05:30 Revert "Do not use an error thunk for an absent dictionary" This reverts commit b09571e2055c24daffa170e74fe8ef424285307e. - - - - - 1 changed file: - compiler/GHC/Core/Opt/WorkWrap/Utils.hs Changes: ===================================== compiler/GHC/Core/Opt/WorkWrap/Utils.hs ===================================== @@ -30,7 +30,6 @@ import GHC.Core.Subst import GHC.Core.Type import GHC.Core.Multiplicity import GHC.Core.Coercion -import GHC.Core.Predicate( isDictTy ) import GHC.Core.Reduction import GHC.Core.FamInstEnv import GHC.Core.Predicate( isEqualityClass ) @@ -1069,25 +1068,21 @@ unbox_one_arg opts arg_var -- same type as @id@. Otherwise, no suitable filler could be found. mkAbsentFiller :: WwOpts -> Id -> StrictnessMark -> Maybe CoreExpr mkAbsentFiller opts arg str - -- The lifted case: bind 'absentError'. See (AF1) in Note [Absent fillers] - -- We want to use this case if possible, because we get a nice runtime panic message - -- if we are wrong (like we were in #11126). Otherwise we fall through to the - -- less-desirable mkLitRubbish case. + -- The lifted case: Bind 'absentError' for a nice panic message if we are + -- wrong (like we were in #11126). See (1) in Note [Absent fillers] | mightBeLiftedType arg_ty - , not (isDictTy arg_ty) -- See (AF4) in Note [Absent fillers] - , not (isStrictDmd (idDemandInfo arg)) -- See (AF2) - , not (isMarkedStrict str) -- in Note [Absent fillers] + , not is_strict + , not (isMarkedStrict str) -- See (2) in Note [Absent fillers] = Just (mkAbsentErrorApp arg_ty msg) -- The default case for mono rep: Bind `RUBBISH[rr] arg_ty` - -- See Note [Absent fillers] - -- (AF3): mkLitRubbish returns Nothing if the representation is not - -- monomorphic, in which case we can't make a filler + -- See Note [Absent fillers], the main part | otherwise = mkLitRubbish arg_ty where - arg_ty = idType arg + arg_ty = idType arg + is_strict = isStrictDmd (idDemandInfo arg) msg = renderWithContext (defaultSDocContext { sdocSuppressUniques = True }) @@ -1250,7 +1245,7 @@ conjure filler values at any type (and any representation or levity!). Needless to say, there are some wrinkles: -(AF1) In case we have a absent, /lazy/, and /lifted/ arg, we use an error-thunk + 1. In case we have a absent, /lazy/, and /lifted/ arg, we use an error-thunk instead. If absence analysis was wrong (e.g., #11126) and the binding in fact is used, then we get a nice panic message instead of undefined runtime behavior (See Modes of failure from Note [Rubbish literals]). @@ -1258,7 +1253,7 @@ Needless to say, there are some wrinkles: Obviously, we can't use an error-thunk if the value is of unlifted rep (like 'Int#' or 'MutVar#'), because we'd immediately evaluate the panic. -(AF2) We also mustn't put an error-thunk (that fills in for an absent value of + 2. We also mustn't put an error-thunk (that fills in for an absent value of lifted rep) in a strict field, because #16970 establishes the invariant that strict fields are always evaluated, by possibly (re-)evaluating what is put in a strict field. That's the reason why 'zs' binds a rubbish literal instead @@ -1284,8 +1279,8 @@ Needless to say, there are some wrinkles: strictness check in place on top of threading through the marks from the constructor. It's a *really* cheap and easy check to make anyway. -(AF3) We can only emit a LitRubbish if the arg's type `arg_ty` is mono-rep, e.g. - of the form `TYPE rep` where `rep` is not (and doesn't contain) a variable. + 3. We can only emit a LitRubbish if the arg's type @arg_ty@ is mono-rep, e.g. + of the form @TYPE rep@ where @rep@ is not (and doesn't contain) a variable. Why? Because if we don't know its representation (e.g. size in memory, register class), we don't know what or how much rubbish to emit in codegen. 'mkLitRubbish' returns 'Nothing' in this case and we simply fall @@ -1295,30 +1290,8 @@ Needless to say, there are some wrinkles: have to be representation monomorphic. But in the future, we might allow levity polymorphism, e.g. a polymorphic levity variable in 'BoxedRep'. -(AF4) Consider (#24934) - f :: (a~b) => blah {-# INLINE f #-} - f d x = case eq_sel d of co -> body - In #24934 it turned out that `co` was unused; and we discarded the - entire case-scrutinisation via the `exprOkToDiscard` test in - `GHC.Core.Opt.Simplify.Iteration.rebuildCase`. So now `d` is absent. - But in the /unfolding/ for some reason we did not discard the `case`; - so when we inline `f` we end up evaluating that `d` argument. So we had - better not replace it with an error thunk! - - The root of it is this: `exprOkToDiscard` assumes that a dictionary is - non-bottom (Note [exprOkForSpeculation and type classes]); but then we replace - the (a~b) dictionary with an error thunk, breaking the invariant that every - dictionary is non-bottom. (If -XDictsStrict is on, the invariant is even - more important.) - - Simple solution: never use an error thunk for a dictionary; instead fall - through to mkRubbishLit. (The only downside is that we lose the compiler - debugging advantages of (AF1).) - - This is quite delicate. - -While (AF1) and (AF2) are simply an optimisation in terms of compiler debugging -experience, (AF3) should be irrelevant in most programs, if not all. +While (1) and (2) are simply an optimisation in terms of compiler debugging +experience, (3) should be irrelevant in most programs, if not all. Historical note: I did try the experiment of using an error thunk for unlifted things too, relying on the simplifier to drop it as dead code. But this is View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/9fe864daa77a968b9f7e996e93d38834... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/9fe864daa77a968b9f7e996e93d38834... 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