Andreas Klebinger pushed to branch wip/andreask/bomb_out at Glasgow Haskell Compiler / GHC Commits: 66c56b5f by Andreas Klebinger at 2025-09-26T13:12:52+02:00 exprSize: Accumulate size as we go to allow early bomb out. When dealing with branches in the AST we now accumulate expr size across branches, rather than computing both branches before adding them up. This way we can abort early when it's clear an expression is too large to be useful. This fixes an issue I observed in #26425 where we sometimes spent a significant amount of time computing unfolding sizes in deeply nested but branching rhss. Speedup is on the order of ~1%-4% depending on the program we are compiling. - - - - - 1 changed file: - compiler/GHC/Core/Unfold.hs Changes: ===================================== compiler/GHC/Core/Unfold.hs ===================================== @@ -566,44 +566,48 @@ sizeExpr :: UnfoldingOpts -- Forcing bOMB_OUT_SIZE early prevents repeated -- unboxing of the Int argument. +-- {-# NOINLINE sizeExpr #-} sizeExpr opts !bOMB_OUT_SIZE top_args expr - = size_up expr + = size_up sizeZero expr where - size_up (Cast e _) = size_up e - size_up (Tick _ e) = size_up e - size_up (Type _) = sizeZero -- Types cost nothing - size_up (Coercion _) = sizeZero - size_up (Lit lit) = sizeN (litSize lit) - size_up (Var f) | isZeroBitId f = sizeZero - -- Make sure we get constructor discounts even - -- on nullary constructors - | otherwise = size_up_call f [] 0 - - size_up (App fun arg) - | isTyCoArg arg = size_up fun - | otherwise = size_up arg `addSizeNSD` - size_up_app fun [arg] (if isZeroBitExpr arg then 1 else 0) - - size_up (Lam b e) - | isId b && not (isZeroBitId b) = lamScrutDiscount opts (size_up e `addSizeN` 10) - | otherwise = size_up e - - size_up (Let (NonRec binder rhs) body) - = size_up_rhs (binder, rhs) `addSizeNSD` - size_up body `addSizeN` - size_up_alloc binder - - size_up (Let (Rec pairs) body) - = foldr (addSizeNSD . size_up_rhs) - (size_up body `addSizeN` sum (map (size_up_alloc . fst) pairs)) - pairs - - size_up (Case e _ _ alts) = case nonEmpty alts of - Nothing -> size_up e -- case e of {} never returns, so take size of scrutinee + size_up :: ExprSize -> CoreExpr -> ExprSize + size_up (TooBig) !_ = TooBig + size_up (SizeIs !s _ _) _ + | s > bOMB_OUT_SIZE = TooBig + size_up s (Cast e _) = size_up s e + size_up s (Tick _ e) = size_up s e + size_up s (Type _) = s -- Types cost nothing + size_up s (Coercion _) = s + size_up s (Lit lit) = addSizeNSD (sizeN (litSize lit)) s + size_up s (Var f) | isZeroBitId f = s + -- Make sure we get constructor discounts even + -- on nullary constructors + | otherwise = size_up_call s f [] 0 + + size_up s (App fun arg) + | isTyCoArg arg = size_up s fun + | otherwise = size_up_app (stripDiscounts $ size_up s arg) + fun [arg] (if isZeroBitExpr arg then 1 else 0) + + size_up s (Lam b e) + | isId b && not (isZeroBitId b) = lamScrutDiscount opts (size_up (addSizeN s 10) e) + | otherwise = size_up s e + + size_up s (Let (NonRec binder rhs) body) + = let rhs_s = size_up_let s (binder, rhs) + in size_up (stripDiscounts $ rhs_s) body + + + size_up s (Let (Rec pairs) body) + = size_up (stripDiscounts (foldr (flip size_up_let) s pairs)) + body + + size_up s (Case e _ _ alts) = case nonEmpty alts of + Nothing -> size_up s e -- case e of {} never returns, so take size of scrutinee Just alts | Just v <- is_top_arg e -> -- We are scrutinising an argument variable let - alt_sizes = NE.map size_up_alt alts + alt_sizes = NE.map (size_up_alt s) alts -- alts_size tries to compute a good discount for -- the case when we are scrutinising an argument variable @@ -625,21 +629,24 @@ sizeExpr opts !bOMB_OUT_SIZE top_args expr alts_size tot_size _ = tot_size in - alts_size (foldr1 addAltSize alt_sizes) -- alts is non-empty - (foldr1 maxSize alt_sizes) + s `addSizeNSD` alts_size (foldr1 addAltSize alt_sizes) -- alts is non-empty + (foldr1 maxSize alt_sizes) -- Good to inline if an arg is scrutinised, because -- that may eliminate allocation in the caller -- And it eliminates the case itself - | otherwise -> size_up e `addSizeNSD` - foldr (addAltSize . size_up_alt) case_size alts + | otherwise -> foldr (addAltSize . size_up_alt s) + (size_up s e `addSizeNSD` case_size) + alts where is_top_arg (Var v) | v `elem` top_args = Just v is_top_arg (Cast e _) = is_top_arg e + is_top_arg (Tick _t e) = is_top_arg e is_top_arg _ = Nothing where + case_size | is_inline_scrut e, lengthAtMost alts 1 = sizeN (-10) | otherwise = sizeZero @@ -675,48 +682,61 @@ sizeExpr opts !bOMB_OUT_SIZE top_args expr | otherwise = False - size_up_rhs (bndr, rhs) + size_up_let :: ExprSize -> (Id, CoreExpr) -> ExprSize + size_up_let s (bndr, rhs) | JoinPoint join_arity <- idJoinPointHood bndr -- Skip arguments to join point , (_bndrs, body) <- collectNBinders join_arity rhs - = size_up body + = size_up s body | otherwise - = size_up rhs + = size_up s rhs `addSizeN` size_up_alloc bndr ------------ -- size_up_app is used when there's ONE OR MORE value args - size_up_app (App fun arg) args voids - | isTyCoArg arg = size_up_app fun args voids - | isZeroBitExpr arg = size_up_app fun (arg:args) (voids + 1) - | otherwise = size_up arg `addSizeNSD` - size_up_app fun (arg:args) voids - size_up_app (Var fun) args voids = size_up_call fun args voids - size_up_app (Tick _ expr) args voids = size_up_app expr args voids - size_up_app (Cast expr _) args voids = size_up_app expr args voids - size_up_app other args voids = size_up other `addSizeN` - callSize (length args) voids + size_up_app :: ExprSize -> CoreExpr -> [CoreExpr] -> Int -> ExprSize + size_up_app s (App fun arg) args voids + | isTyCoArg arg = size_up_app s fun args voids + | isZeroBitExpr arg = size_up_app s fun (arg:args) (voids + 1) + | otherwise = let arg_size = stripDiscounts $ size_up s arg + in size_up_app arg_size fun (arg:args) voids + size_up_app s (Var fun) args voids = size_up_call s fun args voids + size_up_app s (Tick _ expr) args voids = size_up_app s expr args voids + size_up_app s (Cast expr _) args voids = size_up_app s expr args voids + size_up_app s other args voids = size_up (s `addSizeN` callSize (length args) voids) other + -- if the lhs is not an App or a Var, or an invisible thing like a -- Tick or Cast, then we should charge for a complete call plus the -- size of the lhs itself. ------------ - size_up_call :: Id -> [CoreExpr] -> Int -> ExprSize - size_up_call fun val_args voids - = case idDetails fun of - FCallId _ -> sizeN (callSize (length val_args) voids) - DataConWorkId dc -> conSize dc (length val_args) - PrimOpId op _ -> primOpSize op (length val_args) - ClassOpId cls _ -> classOpSize opts cls top_args val_args - _ | fun `hasKey` buildIdKey -> buildSize - | fun `hasKey` augmentIdKey -> augmentSize - | otherwise -> funSize opts top_args fun (length val_args) voids + size_up_call :: ExprSize -> Id -> [CoreExpr] -> Int -> ExprSize + size_up_call !s fun val_args voids + = let !n_args = length val_args + call_size = case idDetails fun of + FCallId _ -> sizeN (callSize n_args voids) + DataConWorkId dc -> conSize dc n_args + PrimOpId op _ -> primOpSize op n_args + ClassOpId cls _ -> classOpSize opts cls top_args val_args + _ | fun `hasKey` buildIdKey -> buildSize + | fun `hasKey` augmentIdKey -> augmentSize + | otherwise -> funSize opts top_args fun n_args voids + in s `addSizeNSD` call_size ------------ - size_up_alt (Alt _con _bndrs rhs) = size_up rhs `addSizeN` 10 + -- size_up_alt returns on the alternatives size, not counting the accumulated + -- size passed in unless we reach TooBig. This is to facility better discount + -- calculation based on the size of only the alternative. + size_up_alt TooBig _ = TooBig + size_up_alt (SizeIs {_es_size_is=s}) (Alt _con _bndrs rhs) = + size_up (sizeN s) rhs + -- Why add and then subtract s? + -- If the expression large enough this will ensure we bomb out early. + `addSizeN` (10 -s) + -- Don't charge for args, so that wrappers look cheap -- (See comments about wrappers with Case) -- - -- IMPORTANT: *do* charge 1 for the alternative, else we + -- IMPORTANT: *do* charge 10 for the alternative, else we -- find that giant case nests are treated as practically free -- A good example is Foreign.C.Error.errnoToIOError @@ -753,6 +773,14 @@ sizeExpr opts !bOMB_OUT_SIZE top_args expr (xs `unionBags` ys) d2 -- Ignore d1 + -- Throw away the discount for scrutinizing the expression. + -- Used for things like `let x = rhs in body` where we only consider + -- this benefit for the body. + -- Why? `x` is visible to `body` either way, so it really should not + -- affect our inlining decision either way. + stripDiscounts TooBig = TooBig + stripDiscounts (SizeIs n xs _) = (SizeIs n xs 0) + -- don't count expressions such as State# RealWorld -- exclude join points, because they can be rep-polymorphic -- and typePrimRep will crash View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/66c56b5fcd8bfb9eafce80f204ce093a... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/66c56b5fcd8bfb9eafce80f204ce093a... 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