From f53e3de5fac3c5eb12f94ee6b26488b1d07d361e Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Wed, 10 Jan 2007 11:13:44 +0000 Subject: [PATCH] Fix apparently-long-standing bug in FloatIn The float-in pass wasn't doing the right thing when you have let x{rule mentions y} = rhs in body It allowed a binding mentioning y to float into the body, which is obviously wrong. I think this bug has been there a long time; I don't really know why it has not come up before. It showed up when compiling Text.Regex.Base.Context with WAY=p in package regex-base. --- compiler/simplCore/FloatIn.lhs | 81 ++++++++++++++++++++++++---------------- 1 file changed, 49 insertions(+), 32 deletions(-) diff --git a/compiler/simplCore/FloatIn.lhs b/compiler/simplCore/FloatIn.lhs index e32a8ea..f84a64e 100644 --- a/compiler/simplCore/FloatIn.lhs +++ b/compiler/simplCore/FloatIn.lhs @@ -20,7 +20,7 @@ import DynFlags ( DynFlags, DynFlag(..) ) import CoreSyn import CoreUtils ( exprIsHNF, exprIsDupable ) import CoreLint ( showPass, endPass ) -import CoreFVs ( CoreExprWithFVs, freeVars, freeVarsOf ) +import CoreFVs ( CoreExprWithFVs, freeVars, freeVarsOf, idRuleVars ) import Id ( isOneShotBndr ) import Var ( Id, idType ) import Type ( isUnLiftedType ) @@ -124,7 +124,7 @@ the closure for a is not built. type FreeVarsSet = IdSet type FloatingBinds = [(CoreBind, FreeVarsSet)] - -- In reverse dependency order (innermost bindiner first) + -- In reverse dependency order (innermost binder first) -- The FreeVarsSet is the free variables of the binding. In the case -- of recursive bindings, the set doesn't include the bound @@ -240,23 +240,52 @@ So: rather than drop \tr{w}'s binding here, we add it onto the list of things to drop in the outer let's body, and let nature take its course. +Note [extra_fvs (1): avoid floating into RHS] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consdider let x=\y....t... in body. We do not necessarily want to float +a binding for t into the RHS, because it'll immediately be floated out +again. (It won't go inside the lambda else we risk losing work.) +In letrec, we need to be more careful still. We don't want to transform + let x# = y# +# 1# + in + letrec f = \z. ...x#...f... + in ... +into + letrec f = let x# = y# +# 1# in \z. ...x#...f... in ... +because now we can't float the let out again, because a letrec +can't have unboxed bindings. + +So we make "extra_fvs" which is the rhs_fvs of such bindings, and +arrange to dump bindings that bind extra_fvs before the entire let. + +Note [extra_fvs (s): free variables of rules] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider let x{rule mentioning y} = rhs in body +Here y is not free in rhs or body; but we still want to dump bindings +that bind y outside the let. So we augment extra_fvs with the +idRuleVars of x. + + \begin{code} fiExpr to_drop (_,AnnLet (AnnNonRec id rhs@(rhs_fvs, ann_rhs)) body) = fiExpr new_to_drop body where body_fvs = freeVarsOf body - final_body_fvs | noFloatIntoRhs ann_rhs - || isUnLiftedType (idType id) = body_fvs `unionVarSet` rhs_fvs - | otherwise = body_fvs - -- See commments with letrec below + rule_fvs = idRuleVars id -- See Note [extra_fvs (2): free variables of rules] + extra_fvs | noFloatIntoRhs ann_rhs + || isUnLiftedType (idType id) = rule_fvs `unionVarSet` rhs_fvs + | otherwise = rule_fvs + -- See Note [extra_fvs (2): avoid floating into RHS] -- No point in floating in only to float straight out again -- Ditto ok-for-speculation unlifted RHSs - [shared_binds, rhs_binds, body_binds] = sepBindsByDropPoint False [rhs_fvs, final_body_fvs] to_drop + [shared_binds, extra_binds, rhs_binds, body_binds] + = sepBindsByDropPoint False [extra_fvs, rhs_fvs, body_fvs] to_drop new_to_drop = body_binds ++ -- the bindings used only in the body [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself + extra_binds ++ -- bindings from extra_fvs shared_binds -- the bindings used both in rhs and body -- Push rhs_binds into the right hand side of the binding @@ -271,32 +300,20 @@ fiExpr to_drop (_,AnnLet (AnnRec bindings) body) rhss_fvs = map freeVarsOf rhss body_fvs = freeVarsOf body - -- Add to body_fvs the free vars of any RHS that has - -- a lambda at the top. This has the effect of making it seem - -- that such things are used in the body as well, and hence prevents - -- them getting floated in. The big idea is to avoid turning: - -- let x# = y# +# 1# - -- in - -- letrec f = \z. ...x#...f... - -- in ... - -- into - -- letrec f = let x# = y# +# 1# in \z. ...x#...f... in ... - -- - -- Because now we can't float the let out again, because a letrec - -- can't have unboxed bindings. - - final_body_fvs = foldr (unionVarSet . get_extras) body_fvs rhss - get_extras (rhs_fvs, rhs) | noFloatIntoRhs rhs = rhs_fvs - | otherwise = emptyVarSet - - (shared_binds:body_binds:rhss_binds) = sepBindsByDropPoint False (final_body_fvs:rhss_fvs) to_drop - - new_to_drop = -- the bindings used only in the body - body_binds ++ - -- the new binding itself + -- See Note [extra_fvs (1,2)] + extra_fvs = foldr (unionVarSet . get_extras) emptyVarSet bindings + get_extras (id, (rhs_fvs, rhs)) + | noFloatIntoRhs rhs = idRuleVars id `unionVarSet` rhs_fvs + | otherwise = idRuleVars id + + (shared_binds:extra_binds:body_binds:rhss_binds) + = sepBindsByDropPoint False (extra_fvs:body_fvs:rhss_fvs) to_drop + + new_to_drop = body_binds ++ -- the bindings used only in the body [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++ - -- the bindings used both in rhs and body or in more than one rhs - shared_binds + -- The new binding itself + extra_binds ++ -- Note [extra_fvs (1,2)] + shared_binds -- Used in more than one place rhs_fvs' = unionVarSet (unionVarSets rhss_fvs) (unionVarSets (map floatedBindsFVs rhss_binds)) -- 1.7.10.4