Commit facfbf28 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Comments only, relating to Roman's new built-in rule for seq

parent fb9f8859
......@@ -942,13 +942,15 @@ seqId = pcMiscPrelId seqName ty info
[x,y] = mkTemplateLocals [alphaTy, openBetaTy]
rhs = mkLams [alphaTyVar,openBetaTyVar,x,y] (Case (Var x) x openBetaTy [(DEFAULT, [], Var y)])
-- See Note [Built-in RULES for seq]
seq_cast_rule = BuiltinRule { ru_name = fsLit "seq of cast"
, ru_fn = seqName
, ru_nargs = 4
, ru_try = match_seq_of_cast
}
match_seq_of_cast :: [CoreExpr] -> Maybe CoreExpr -- Note [RULES for seq]
match_seq_of_cast :: [CoreExpr] -> Maybe CoreExpr
-- See Note [Built-in RULES for seq]
match_seq_of_cast [Type _, Type res_ty, Cast scrut co, expr]
= Just (Var seqId `mkApps` [Type (fst (coercionKind co)), Type res_ty,
scrut, expr])
......@@ -974,10 +976,10 @@ b) Its fixity is set in LoadIface.ghcPrimIface
c) It has quite a bit of desugaring magic.
See DsUtils.lhs Note [Desugaring seq (1)] and (2) and (3)
d) There is some special rule handing: Note [RULES for seq]
d) There is some special rule handing: Note [User-defined RULES for seq]
Note [RULES for seq]
~~~~~~~~~~~~~~~~~~~~
Note [User-defined RULES for seq]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Roman found situations where he had
case (f n) of _ -> e
where he knew that f (which was strict in n) would terminate if n did.
......@@ -999,12 +1001,20 @@ To make this work, we need to be careful that the magical desugaring
done in Note [seqId magic] item (c) is *not* done on the LHS of a rule.
Or rather, we arrange to un-do it, in DsBinds.decomposeRuleLhs.
We also have the following builtin rule:
Note [Built-in RULES for seq]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We also have the following built-in rule for seq
seq (x `cast` co) y = seq x y
This eliminates unnecessary casts and also allows other seq rules to
match more often.
match more often. Notably,
seq (f x `cast` co) y --> seq (f x) y
and now a user-defined rule for seq (see Note [User-defined RULES for seq])
may fire.
Note [lazyId magic]
~~~~~~~~~~~~~~~~~~~
......
......@@ -1450,7 +1450,7 @@ rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
| all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
= -- For this case, see Note [RULES for seq] in MkId
= -- For this case, see Note [User-defined RULES for seq] in MkId
do { let rhs' = substExpr env rhs
out_args = [Type (substTy env (idType case_bndr)),
Type (exprType rhs'), scrut, rhs']
......@@ -1539,13 +1539,31 @@ where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
I# x# -> let x = x' `cast` sym co
in rhs
so that 'rhs' can take advantage of the form of x'. Notice that Note
[Case of cast] may then apply to the result. We only do this if x is actually
used in the rhs. There is no point in adding the cast if this is really just a
seq and doing so would interfere with seq rules (Note [RULES for seq]), in
particular with the one that removes casts.
This showed up in Roman's experiments. Example:
so that 'rhs' can take advantage of the form of x'.
Notice that Note [Case of cast] may then apply to the result.
Nota Bene: We only do the [Improving seq] transformation if the
case binder 'x' is actually used in the rhs; that is, if the case
is *not* a *pure* seq.
a) There is no point in adding the cast to a pure seq.
b) There is a good reason not to: doing so would interfere
with seq rules (Note [Built-in RULES for seq] in MkId).
In particular, this [Improving seq] thing *adds* a cast
while [Built-in RULES for seq] *removes* one, so they
just flip-flop.
You might worry about
case v of x { __DEFAULT ->
... case (v `cast` co) of y { I# -> ... }}
This is a pure seq (since x is unused), so [Improving seq] won't happen.
But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
case v of x { __DEFAULT ->
... case (x `cast` co) of y { I# -> ... }}
Now the outer case is not a pure seq, so [Improving seq] will happen,
and then the inner case will disappear.
The need for [Improving seq] showed up in Roman's experiments. Example:
foo :: F Int -> Int -> Int
foo t n = t `seq` bar n
where
......@@ -1554,11 +1572,9 @@ This showed up in Roman's experiments. Example:
Here we'd like to avoid repeated evaluating t inside the loop, by
taking advantage of the `seq`.
At one point I did transformation in LiberateCase, but it's more robust here.
(Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
LiberateCase gets to see it.)
At one point I did transformation in LiberateCase, but it's more
robust here. (Otherwise, there's a danger that we'll simply drop the
'seq' altogether, before LiberateCase gets to see it.)
\begin{code}
......@@ -1567,7 +1583,7 @@ improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
-> SimplM (SimplEnv, OutExpr, OutId)
-- Note [Improving seq]
improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
| not (isDeadBinder case_bndr)
| not (isDeadBinder case_bndr) -- Not a pure seq! See the Note!
, Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
= do { case_bndr2 <- newId (fsLit "nt") ty2
; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
......
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