Commit 3794b597 authored by Simon Peyton Jones's avatar Simon Peyton Jones Committed by Ben Gamari
Browse files

Do not optimise RULE lhs in substRule

This was causing Trac #10627.
See Note [Substitute lazily] in CoreSubst.

The bug was introduced by
   commit 30c17e70
    Author: <unknown>
    Date:   Thu Nov 25 17:23:56 2010 +0000
    Substitution should just substitute, not optimise

The fix is not to optimise the RHS as well as not-optimising the LHS!
The simplifier does the right thing in Simplify.simplRule
parent 358e0a8d
......@@ -780,15 +780,16 @@ substRule _ _ rule@(BuiltinRule {}) = rule
substRule subst subst_ru_fn rule@(Rule { ru_bndrs = bndrs, ru_args = args
, ru_fn = fn_name, ru_rhs = rhs
, ru_local = is_local })
= rule { ru_bndrs = bndrs',
ru_fn = if is_local
= rule { ru_bndrs = bndrs'
, ru_fn = if is_local
then subst_ru_fn fn_name
else fn_name,
ru_args = map (substExpr (text "subst-rule" <+> ppr fn_name) subst') args,
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.
else fn_name
, ru_args = map (substExpr doc subst') args
, ru_rhs = substExpr (text "foo") subst' rhs }
-- Do NOT optimise the RHS (previously we did simplOptExpr here)
-- See Note [Substitute lazily]
doc = ptext (sLit "subst-rule") <+> ppr fn_name
(subst', bndrs') = substBndrs subst bndrs
......@@ -818,8 +819,22 @@ substTickish subst (Breakpoint n ids) = Breakpoint n (map do_one ids)
where do_one = getIdFromTrivialExpr . lookupIdSubst (text "subst_tickish") subst
substTickish _subst other = other
{- Note [substTickish]
{- Note [Substitute lazily]
The functions that substitute over IdInfo must be pretty lazy, becuause
they are knot-tied by substRecBndrs.
One case in point was Trac #10627 in which a rule for a function 'f'
referred to 'f' (at a differnet type) on the RHS. But instead of just
substituting in the rhs of the rule, we were calling simpleOptExpr, which
looked at the idInfo for 'f'; result <<loop>>.
In any case we don't need to optimise the RHS of rules, or unfoldings,
because the simplifier will do that.
Note [substTickish]
A Breakpoint contains a list of Ids. What happens if we ever want to
substitute an expression for one of these Ids?
-- Made GHC 6.10.2 go into a loop in substRecBndrs
{-# OPTIONS_GHC -w #-}
module T10627 where
import Data.Word
class C a where
splitFraction :: a -> (b,a)
roundSimple :: (C a) => a -> b
roundSimple x = error "rik"
"rs" roundSimple = (fromIntegral :: Int -> Word) . roundSimple;
......@@ -211,3 +211,4 @@ test('T9583', only_ways(['optasm']), compile, [''])
test('T9565', only_ways(['optasm']), compile, [''])
test('T10176', only_ways(['optasm']), compile, [''])
test('T10602', only_ways(['optasm']), compile, ['-O2'])
test('T10627', only_ways(['optasm']), compile, [''])
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