Commit ee56dc56 authored by Simon Peyton Jones's avatar Simon Peyton Jones

Tidy up and improve comments about one-shot info

(Triggered by investigating Trac #10102 etc.)
parent e673b840
......@@ -163,7 +163,7 @@ type Alignment = Int -- align to next N-byte boundary (N must be a power of 2).
data OneShotInfo
= NoOneShotInfo -- ^ No information
| ProbOneShot -- ^ The lambda is probably applied at most once
-- See Note [Computing one-shot info, and ProbOneShot] in OccurAnl
-- See Note [Computing one-shot info, and ProbOneShot] in Demand
| OneShotLam -- ^ The lambda is applied at most once.
deriving (Eq)
......
......@@ -1618,9 +1618,8 @@ argOneShots one_shot_info (JD { absd = usg })
go (UCall Many u) = NoOneShotInfo : go u
go _ = []
{-
Note [Computing one-shot info, and ProbOneShot]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{- Note [Computing one-shot info, and ProbOneShot]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider a call
f (\pqr. e1) (\xyz. e2) e3
where f has usage signature
......
......@@ -1250,7 +1250,6 @@ appears un-applied, we'll end up just calling it.
Note [The oneShot function]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the context of making left-folds fuse somewhat okish (see ticket #7994
and Note [Left folds via right fold]) it was determined that it would be useful
if library authors could explicitly tell the compiler that a certain lambda is
......@@ -1270,9 +1269,11 @@ after unfolding the definition `oneShot = \f \x[oneshot]. f x` we get
--> \x[oneshot] e[x/y]
which is what we want.
It is only effective if this bits survives as long as possible and makes it into
the interface in unfoldings (See Note [Preserve OneShotInfo]). Also see
https://ghc.haskell.org/trac/ghc/wiki/OneShot.
It is only effective if the one-shot info survives as long as possible; in
particular it must make it into the interface in unfoldings. See Note [Preserve
OneShotInfo] in CoreTidy.
Also see https://ghc.haskell.org/trac/ghc/wiki/OneShot.
Note [magicDictId magic]
......
......@@ -258,12 +258,11 @@ but that seems more indirect and surprising.)
Note [Preserve OneShotInfo]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We keep the OneShotInfo because we want it to propagate into the interface.
Not all OneShotInfo is determined by a compiler analysis; some is added by a
call of GHC.Exts.oneShot, which is then discarded before the end of the
optimisation pipeline, leaving only the OneShotInfo on the lambda. Hence we
must preserve this info in inlinings.
must preserve this info in inlinings. See Note [The oneShot function] in MkId.
This applies to lambda binders only, hence it is stored in IfaceLamBndr.
-}
......
......@@ -87,8 +87,8 @@ type IfaceIdBndr = (IfLclName, IfaceType)
type IfaceTvBndr = (IfLclName, IfaceKind)
data IfaceOneShot -- see Note [Preserve OneShotInfo]
= IfaceNoOneShot
data IfaceOneShot -- See Note [Preserve OneShotInfo] in CoreTicy
= IfaceNoOneShot -- and Note [The oneShot function] in MkId
| IfaceOneShot
type IfaceLamBndr
......
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