Commit 9014a7ee authored by Simon Peyton Jones's avatar Simon Peyton Jones

Fix addDataConStrictness

See Note [Add demands for strict constructors].

The new bit is the test for isAbsDmd in addDataConStrictness.
There was a cryptic note that said that this function
should add a seqDmd even for Absent arguments, but that
is definitely a bad thing (as the Note now says), causing
unused arguments to be passed to the worker.

Easy fix!
parent cc0dba1e
......@@ -431,10 +431,7 @@ in this case.
In other words, for locally-bound lambdas we can infer
one-shotness.
-}
{-
Note [Add demands for strict constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this program (due to Roman):
......@@ -462,17 +459,23 @@ because X is strict, so its argument must be evaluated. And if we
because the seq is discarded (very early) since X is strict!
We achieve the effect using addDataConStrictness. It is called at a
case expression, such as the pattern match on (X a) in the example
above. After computing how 'a' is used in the alternatives, we add an
extra 'seqDmd' to it. The case alternative isn't itself strict in the
sub-components, but simply evaluating the scrutinee to HNF does force
those sub-components.
If the argument is not used at all in the alternative (i.e. it is
Absent), then *don't* add a 'seqDmd'. If we do, it makes it look used
and hence it'll be passed to the worker when it doesn't need to be.
Hence the isAbsDmd test in addDataConStrictness.
There is the usual danger of reboxing, which as usual we ignore. But
if X is monomorphic, and has an UNPACK pragma, then this optimisation
is even more important. We don't want the wrapper to rebox an unboxed
argument, and pass an Int to $wfoo!
We add these extra strict demands to the demand on the *scrutinee* of
the case expression; hence the use of addDataConStrictness when
forming scrut_dmd. The case alternatives aren't strict in their
sub-components, but simply evaluating the scrutinee to HNF does force
those sub-components.
************************************************************************
* *
......@@ -1101,9 +1104,9 @@ addDataConStrictness con ds
zipWith add ds strs
where
strs = dataConRepStrictness con
add dmd str | isMarkedStrict str = dmd `bothDmd` seqDmd
add dmd str | isMarkedStrict str
, not (isAbsDmd dmd) = dmd `bothDmd` seqDmd
| otherwise = dmd
-- Yes, even if 'dmd' is Absent!
findBndrsDmds :: AnalEnv -> DmdType -> [Var] -> (DmdType, [Demand])
-- Return the demands on the Ids in the [Var]
......
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