Commit 71c7067b authored by Simon Peyton Jones's avatar Simon Peyton Jones

Make the demand analyser take account of lambda-bound unfoldings

This is a long-standing lurking bug. See Note [Lamba-bound unfoldings]
in DmdAnal.

I'm still not really happy with this lambda-bound-unfolding stuff.
parent 979c1134
......@@ -2082,12 +2082,22 @@ An alternative plan is this:
but that is bad if 'c' is *not* later scrutinised.
So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
that it's really I# c#, thus
(an InlineRule) that it's really I# c#, thus
$j = \c# -> \c[=I# c#] -> ...c....
Absence analysis may later discard 'c'.
NB: take great care when doing strictness analysis;
see Note [Lamba-bound unfoldings] in DmdAnal.
Also note that we can still end up passing stuff that isn't used. Before
strictness analysis we have
let $j x y c{=(x,y)} = (h c, ...)
in ...
After strictness analysis we see that h is strict, we end up with
let $j x y c{=(x,y)} = ($wh x y, ...)
and c is unused.
Note [Duplicated env]
......@@ -32,7 +32,7 @@ import TyCon ( isProductTyCon, isRecursiveTyCon )
import Id ( Id, idType, idInlineActivation,
isDataConWorkId, isGlobalId, idArity,
idStrictness, idStrictness_maybe,
setIdStrictness, idDemandInfo,
setIdStrictness, idDemandInfo, idUnfolding,
......@@ -205,14 +205,14 @@ dmdAnal sigs dmd (Lam var body)
= let
sigs' = extendSigsWithLam sigs var
(body_ty, body') = dmdAnal sigs' body_dmd body
(lam_ty, var') = annotateLamIdBndr body_ty var
(lam_ty, var') = annotateLamIdBndr sigs body_ty var
(lam_ty, Lam var' body')
| otherwise -- Not enough demand on the lambda; but do the body
= let -- anyway to annotate it and gather free var info
(body_ty, body') = dmdAnal sigs evalDmd body
(lam_ty, var') = annotateLamIdBndr body_ty var
(lam_ty, var') = annotateLamIdBndr sigs body_ty var
(deferType lam_ty, Lam var' body')
......@@ -728,17 +728,27 @@ annotateBndr dmd_ty@(DmdType fv ds res) var
annotateBndrs = mapAccumR annotateBndr
annotateLamIdBndr :: DmdType -- Demand type of body
annotateLamIdBndr :: SigEnv
-> DmdType -- Demand type of body
-> Id -- Lambda binder
-> (DmdType, -- Demand type of lambda
Id) -- and binder annotated with demand
annotateLamIdBndr dmd_ty@(DmdType fv ds res) id
annotateLamIdBndr sigs dmd_ty@(DmdType fv ds res) id
-- For lambdas we add the demand to the argument demands
-- Only called for Ids
= ASSERT( isId id )
(DmdType fv' (hacked_dmd:ds) res, setIdDemandInfo id hacked_dmd)
(final_ty, setIdDemandInfo id hacked_dmd)
-- Watch out! See note [Lambda-bound unfoldings]
final_ty = case maybeUnfoldingTemplate (idUnfolding id) of
Nothing -> main_ty
Just unf -> main_ty `bothType` unf_ty
(unf_ty, _) = dmdAnal sigs dmd unf
main_ty = DmdType fv' (hacked_dmd:ds) res
(fv', dmd) = removeFV fv id res
hacked_dmd = argDemand dmd
-- This call to argDemand is vital, because otherwise we label
......@@ -764,6 +774,16 @@ zapUnlifted id dmd | isUnLiftedType (idType id) = lazyDmd
| otherwise = dmd
Note [Lamba-bound unfoldings]
We allow a lambda-bound variable to carry an unfolding, a facility that is used
exclusively for join points; see Note [Case binders and join points]. If so,
we must be careful to demand-analyse the RHS of the unfolding! Example
\x. \y{=Just x}. <body>
Then if <body> uses 'y', then transitively it uses 'x', and we must not
forget that fact, otherwise we might make 'x' absent when it isn't.
%* *
\subsection{Strictness signatures}
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