Commit 8d34ae39 authored by Joachim Breitner's avatar Joachim Breitner
Browse files

Some polishing of the demand analyser.

I did some refactoring of the demand analyser, because I was smelling
some minor code smell. Most of my changes I had to undo, though,
adding notes and testcases on why the existing code was correct after

Especially the semantics of the DmdResult is confusing, as it differs in
a DmdType and a StrictSig.

I got to imrpove the readability of the code for lubDmdType, though.

Also, dmdAnalRhs was a bit fishy in how it removed the demand on
further arguments of the body, but used the DmdResult. This would be
wrong if a body would return a demand type of "<L>m" (which currently
does not happen).  This is now treated better in removeDmdTyArgs.
parent 26acb498
......@@ -20,7 +20,7 @@ module Demand (
DmdType(..), dmdTypeDepth, lubDmdType, bothDmdType,
nopDmdType, botDmdType, mkDmdType,
addDemand, removeDmdTyArgs,
BothDmdArg, mkBothDmdArg, toBothDmdArg,
DmdEnv, emptyDmdEnv,
......@@ -931,35 +931,48 @@ data DmdType = DmdType
DmdEnv -- Demand on explicitly-mentioned
-- free variables
[Demand] -- Demand on arguments
DmdResult -- Nature of result
DmdResult -- See [Nature of result demand]
Note [Nature of result demand]
We assume the result in a demand type to be either a top or bottom
in order to represent the information about demand on the function
result, imposed by its definition. There are not so many things we
can say, though.
For instance, one can consider a function
h = \v -> error "urk"
Taking the definition of strictness, we can easily see that
h undefined = undefined
that is, h is strict in v. However, v is not used somehow in the body
of h How can we know that h is strict in v? In fact, we know it by
considering a result demand of error and bottom and unleashing it on
all the variables in scope at a call site (in this case, this is only
v). We can also consider a case
h = \v -> f x
where we know that the result of f is not hyper-strict (i.e, it is
lazy, or top). So, we put the same demand on v, which allow us to
infer a lazy demand that h puts on v.
A DmdResult contains information about termination (currently distinguishing
definite divergence and no information; it is possible to include definite
convergence here), and CPR information about the result.
The semantics of this depends on whether we are looking at a DmdType, i.e. the
demand put on by an expression _under a specific incoming demand_ on its
environment, or at a StrictSig describing a demand transformer.
For a
* DmdType, the termination information is true given the demand it was
generated with, while for
* a StrictSig it is olds after applying enough arguments.
The CPR information, though, is valid after the number of arguments mentioned
in the type is given. Therefore, when forgetting the demand on arguments, as in
dmdAnalRhs, this needs to be considere (via removeDmdTyArgs).
b2 x y = x `seq` y `seq` error (show x)
this has a strictness signature of
meaning that "b2 `seq` ()" and "b2 1 `seq` ()" might well terminate, but
for "b2 1 2 `seq` ()" we get definite divergence.
For comparision,
b1 x = x `seq` error (show x)
has a strictness signature of
and "b1 1 `seq` ()" is known to terminate.
Now consider a function h with signature "<C(S)>", and the expression
e1 = h b1
now h puts a demand of <C(S)> onto its argument, and the demand transformer
turns it into
Now the DmdResult "b" does apply to us, even though "b1 `seq` ()" does not
diverge, and we do not anything being passed to b.
Note [Asymmetry of 'both' for DmdType and DmdResult]
......@@ -988,17 +1001,28 @@ instance Eq DmdType where
&& ds1 == ds2 && res1 == res2
lubDmdType :: DmdType -> DmdType -> DmdType
lubDmdType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
= DmdType lub_fv (lub_ds ds1 ds2) (r1 `lubDmdResult` r2)
lubDmdType d1 d2
= DmdType lub_fv lub_ds lub_res
n = max (dmdTypeDepth d1) (dmdTypeDepth d2)
(DmdType fv1 ds1 r1) = ensureArgs n d1
(DmdType fv2 ds2 r2) = ensureArgs n d2
lub_fv = plusVarEnv_CD lubDmd fv1 (defaultDmd r1) fv2 (defaultDmd r2)
lub_ds = zipWithEqual "lubDmdType" lubDmd ds1 ds2
lub_res = lubDmdResult r1 r2
Note [The need for BothDmdArg]
Previously, the right argument to bothDmdType, as well as the return value of
dmdAnalStar via postProcessDmdTypeM, was a DmdType. But bothDmdType only needs
to know about the free variables and termination information, but nothing about
the demand put on arguments, nor cpr information. So we make that explicit by
only passing the relevant information.
-- Extend the shorter argument list to match the longer, using resTypeArgDmd
lub_ds (d1:ds1) (d2:ds2) = lubDmd d1 d2 : lub_ds ds1 ds2
lub_ds (d1:ds1) [] = (d1 `lubDmd` resTypeArgDmd r2) : lub_ds ds1 []
lub_ds [] (d2:ds2) = (resTypeArgDmd r1 `lubDmd` d2) : lub_ds [] ds2
lub_ds [] [] = []
type BothDmdArg = (DmdEnv, Termination ())
mkBothDmdArg :: DmdEnv -> BothDmdArg
......@@ -1054,6 +1078,25 @@ mkDmdType fv ds res = DmdType fv ds res
dmdTypeDepth :: DmdType -> Arity
dmdTypeDepth (DmdType _ ds _) = length ds
-- Remove any demand on arguments. This is used in dmdAnalRhs on the body
removeDmdTyArgs :: DmdType -> DmdType
removeDmdTyArgs = ensureArgs 0
-- This makes sure we can use the demand type with n arguments,
-- It extends the argument list with the correct resTypeArgDmd
-- It also adjusts the DmdResult: Divergence survives additional arguments,
-- CPR information does not (and definite converge also would not).
ensureArgs :: Arity -> DmdType -> DmdType
ensureArgs n d | n == depth = d
| otherwise = DmdType fv ds' r'
where depth = dmdTypeDepth d
DmdType fv ds r = d
ds' = take n (ds ++ repeat (resTypeArgDmd r))
r' | Diverges <- r = r
| otherwise = topRes
-- See [Nature of result demand]
seqDmdType :: DmdType -> ()
seqDmdType (DmdType _env ds res) =
{- ??? env `seq` -} seqDemandList ds `seq` seqDmdResult res `seq` ()
......@@ -1112,6 +1155,7 @@ toCleanDmd (JD { strd = s, absd = u })
-- This is used in dmdAnalStar when post-processing
-- a function's argument demand. So we only care about what
-- does to free variables, and whether it terminates.
-- see Note [The need for BothDmdArg]
postProcessDmdTypeM :: DeferAndUseM -> DmdType -> BothDmdArg
postProcessDmdTypeM Nothing _ = (emptyDmdEnv, Dunno ())
-- Incoming demand was Absent, so just discard all usage information
......@@ -1200,16 +1244,20 @@ then d1 and d2 is precisely the demand unleashed onto x1 and x2 (similar for
the free variable environment) and furthermore the result information r is the
one we want to use.
An anonymous lambda is also an unsaturated function all (needs one argument,
none given), so this applies to that case as well.
But the demand fed into f might be less than <C(C(S)), C1(C1(S))>. There are a few cases:
* Not enough demand on the strictness side:
- In that case, we need to zap all strictness in the demand on arguments and
free variables.
- Furthermore, we need to remove CPR information (after all, "f x1" surely
does not return a constructor).
- And finally, if r said that f would (possible or definitely) diverge when
called with two arguments, then "f x1" may diverge. So we use topRes here.
(We could return "Converges NoCPR" if f would converge for sure, but that
information would currently not be useful in any way.)
- Furthermore, we remove CPR information. It could be left, but given the incoming
demand is not enough to evaluate so far we just do not bother.
- And finally termination information: If r says that f diverges for sure,
then this holds when the demand guarantees that two arguments are going to
be passed. If the demand is lower, we may just as well converge.
If we were tracking definite convegence, than that would still hold under
a weaker demand than expected by the demand transformer.
* Not enough demand from the usage side: The missing usage can be expanded
using UCall Many, therefore this is subsumed by the third case:
* At least one of the uses has a cardinality of Many.
......@@ -1230,7 +1278,7 @@ peelFV (DmdType fv ds res) id = -- pprTrace "rfv" (ppr id <+> ppr dmd $$ ppr fv)
(DmdType fv' ds res, dmd)
fv' = fv `delVarEnv` id
-- See note [Default demand on free variables]
-- See Note [Default demand on free variables]
dmd = lookupVarEnv fv id `orElse` defaultDmd res
addDemand :: Demand -> DmdType -> DmdType
......@@ -1314,6 +1362,8 @@ transfomer, namely
This DmdType gives the demands unleashed by the Id when it is applied
to as many arguments as are given in by the arg demands in the DmdType.
Also see Note [Nature of result demand] for the meaning of a DmdResult in a
strictness signature.
If an Id is applied to less arguments than its arity, it means that
the demand on the function at a call site is weaker than the vanilla
......@@ -1321,11 +1371,11 @@ call demand, used for signature inference. Therefore we place a top
demand on all arguments. Otherwise, the demand is specified by Id's
For example, the demand transformer described by the DmdType
DmdType {x -> <S(LL),U(UU)>} [V,A] Top
For example, the demand transformer described by the demand signature
StrictSig (DmdType {x -> <S,1*U>} <L,A><L,U(U,U)>m)
says that when the function is applied to two arguments, it
unleashes demand <S(LL),U(UU)> on the free var x, V on the first arg,
and A on the second.
unleashes demand <S,1*U> on the free var x, <L,A> on the first arg,
and <L,U(U,U)> on the second, then returning a constructor.
If this same function is applied to one arg, all we can say is that it
uses x with <L,U>, and its arg with demand <L,U>.
......@@ -602,13 +602,14 @@ dmdAnalRhs top_lvl rec_flag env id rhs
| otherwise
= (sig_ty, lazy_fv, id', mkLams bndrs' body')
(bndrs, body) = collectBinders rhs
env_body = foldl extendSigsWithLam env bndrs
(DmdType body_fv _ body_res, body') = dmdAnal env_body body_dmd body
(DmdType rhs_fv rhs_dmds rhs_res, bndrs') = annotateLamBndrs env (isDFunId id)
(DmdType body_fv [] body_res) bndrs
sig_ty = mkStrictSig (mkDmdType sig_fv rhs_dmds rhs_res')
id' = set_idStrictness env id sig_ty
(bndrs, body) = collectBinders rhs
env_body = foldl extendSigsWithLam env bndrs
(body_ty, body') = dmdAnal env_body body_dmd body
body_ty' = removeDmdTyArgs body_ty -- zap possible deep CPR info
(DmdType rhs_fv rhs_dmds rhs_res, bndrs')
= annotateLamBndrs env (isDFunId id) body_ty' bndrs
sig_ty = mkStrictSig (mkDmdType sig_fv rhs_dmds rhs_res')
id' = set_idStrictness env id sig_ty
-- See Note [NOINLINE and strictness]
-- See Note [Product demands for function body]
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