Commit 30c17e70 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Substitution should just substitute, not optimise

This was causing Trac #4524, by optimising
     (e |> co)  to   e
on the LHS of a rule.  Result, the template variable
'co' wasn't bound any more.

Now that substition doesn't optimise, it seems sensible to call
simpleOptExpr rather than substExpr when substituting in the
RHS of rules.  Not a big deal either way.
parent 45a61df3
......@@ -28,7 +28,7 @@ module CoreSubst (
cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs,
-- ** Simple expression optimiser
simpleOptPgm, simpleOptExpr
simpleOptPgm, simpleOptExpr, simpleOptExprWith
) where
#include "HsVersions.h"
......@@ -307,13 +307,12 @@ subst_expr subst expr
go (Lit lit) = Lit lit
go (App fun arg) = App (go fun) (go arg)
go (Note note e) = Note (go_note note) (go e)
go (Cast e co)
| isIdentityCoercion co' = go e
| otherwise = Cast (go e) co'
where
co' = optCoercion (getTvSubst subst) co
-- Optimise coercions as we go; this is good, for example
-- in the RHS of rules, which are only substituted in
go (Cast e co) = Cast (go e) (optCoercion (getTvSubst subst) co)
-- Do not optimise even identity coercions
-- Reason: substitution applies to the LHS of RULES, and
-- if you "optimise" an identity coercion, you may
-- lose a binder. We optimise the LHS of rules at
-- construction time
go (Lam bndr body) = Lam bndr' (subst_expr subst' body)
where
......@@ -633,7 +632,9 @@ substRule subst subst_ru_fn rule@(Rule { ru_bndrs = bndrs, ru_args = args
then subst_ru_fn fn_name
else fn_name,
ru_args = map (substExpr (text "subst-rule" <+> ppr fn_name) subst') args,
ru_rhs = substExpr (text "subst-rule" <+> ppr fn_name) subst' rhs }
ru_rhs = simpleOptExprWith subst' rhs }
-- Do simple optimisation on RHS, in case substitution lets
-- you improve it. The real simplifier never gets to look at it.
where
(subst', bndrs') = substBndrs subst bndrs
......@@ -678,7 +679,7 @@ simpleOptExpr :: CoreExpr -> CoreExpr
simpleOptExpr expr
= -- pprTrace "simpleOptExpr" (ppr init_subst $$ ppr expr)
simple_opt_expr init_subst (occurAnalyseExpr expr)
simpleOptExprWith init_subst expr
where
init_subst = mkEmptySubst (mkInScopeSet (exprFreeVars expr))
-- It's potentially important to make a proper in-scope set
......@@ -691,6 +692,9 @@ simpleOptExpr expr
-- It's a bit painful to call exprFreeVars, because it makes
-- three passes instead of two (occ-anal, and go)
simpleOptExprWith :: Subst -> InExpr -> OutExpr
simpleOptExprWith subst expr = simple_opt_expr subst (occurAnalyseExpr expr)
----------------------
simpleOptPgm :: DynFlags -> [CoreBind] -> [CoreRule] -> IO ([CoreBind], [CoreRule])
simpleOptPgm dflags binds rules
......
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