Commit e923340f authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Run simplifier before SpecConstr

Arrange to run the simplifier before SpecConstr, to (almost entirely) 
eliminate shadowing.

Reason: otherwise SpecConstr can generate a RULE that never
files; and LiberateCase specifically *does* generate lots of shadowing.

See Note [Shadowing] in SpecConstr.lhs
parent 1375c0a7
......@@ -722,19 +722,27 @@ getCoreToDo dflags
-- Case-liberation for -O2. This should be after
-- strictness analysis and the simplification which follows it.
case rule_check of { Just pat -> CoreDoRuleCheck 0 pat; Nothing -> CoreDoNothing },
case rule_check of { Just pat -> CoreDoRuleCheck 0 pat; Nothing -> CoreDoNothing }
]
++
if opt_level >= 2 then
CoreLiberateCase
else
CoreDoNothing,
if opt_level >= 2 then
CoreDoSpecConstr
else
CoreDoNothing,
if opt_level >= 2 then
[ CoreLiberateCase,
CoreDoSimplify (SimplPhase 0) [
MaxSimplifierIterations max_iter
], -- Run the simplifier after LiberateCase to vastly
-- reduce the possiblility of shadowing
-- Reason: see Note [Shadowing] in SpecConstr.lhs
CoreDoSpecConstr
]
else
[]
++
-- Final clean-up simplification:
CoreDoSimplify (SimplPhase 0) [
[ CoreDoSimplify (SimplPhase 0) [
MaxSimplifierIterations max_iter
]
]
......
......@@ -122,6 +122,8 @@ So we look for
Those are the only uses of the parameter
What to abstract over
~~~~~~~~~~~~~~~~~~~~~
There's a bit of a complication with type arguments. If the call
site looks like
......@@ -157,7 +159,7 @@ So the grand plan is:
* Find the free variables of the abstracted pattern
* Pass these variables, less any that are in scope at
the fn defn.
the fn defn. But see Note [Shadowing] below.
NOTICE that we only abstract over variables that are not in scope,
......@@ -165,6 +167,30 @@ so we're in no danger of shadowing variables used in "higher up"
in f_spec's RHS.
Note [Shadowing]
~~~~~~~~~~~~~~~~
In this pass we gather up usage information that may mention variables
that are bound between the usage site and the definition site; or (more
seriously) may be bound to something different at the definition site.
For example:
f x = letrec g y v = let x = ...
in ...(g (a,b) x)...
Since 'x' is in scope at the call site, we may make a rewrite rule that
looks like
RULE forall a,b. g (a,b) x = ...
But this rule will never match, because it's really a different 'x' at
the call site -- and that difference will be manifest by the time the
simplifier gets to it. [A worry: the simplifier doesn't *guarantee*
no-shadowing, so perhaps it may not be distinct?]
Anyway, the rule isn't actually wrong, it's just not useful. One possibility
is to run deShadowBinds before running SpecConstr, but instead we run the
simplifier. That gives the simplest possible program for SpecConstr to
chew on; and it virtually guarantees no shadowing.
%************************************************************************
%* *
\subsection{Top level wrapper stuff}
......@@ -535,6 +561,8 @@ spec_one env fn rhs (pats, rule_number)
spec_occ = mkSpecOcc (nameOccName fn_name)
pat_fvs = varSetElems (exprsFreeVars pats)
vars_to_bind = filter not_avail pat_fvs
-- See Note [Shadowing] at the top
not_avail v = not (v `elemVarEnv` scope env)
-- Put the type variables first; the type of a term
-- variable may mention a type variable
......
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