Safe Haskell  None 

Language  GHC2021 
Implements a selective lambda lifter, running late in the optimisation pipeline.
If you are interested in the cost model that is employed to decide whether to lift a binding or not, look at GHC.Stg.Lift.Analysis. GHC.Stg.Lift.Monad contains the transformation monad that hides away some plumbing of the transformation.
Synopsis
 data StgLiftConfig = StgLiftConfig {
 c_targetProfile :: !Profile
 c_liftLamsRecArgs :: !(Maybe Int)
 c_liftLamsNonRecArgs :: !(Maybe Int)
 c_liftLamsKnown :: !Bool
 stgLiftLams :: Module > StgLiftConfig > UniqSupply > [InStgTopBinding] > [OutStgTopBinding]
Late lambda lifting in STG
See also the wiki page and #9476.
The basic idea behind lambda lifting is to turn locally defined functions into toplevel functions. Free variables are then passed as additional arguments at *call sites* instead of having a closure allocated for them at *definition site*. Example:
let x = ...; y = ... in let f = {x y} a > a + x + y in let g = {f x} b > f b + x in g 5
Lambda lifting f
would
 Turn
f
's free variables into formal parameters  Update
f
's call site withing
tof x y b
 Update
g
's closure: Addy
as an additional free variable, while removingf
, becausef
no longer allocates and can be floated to toplevel.  Actually float the binding of
f
to toplevel, eliminating thelet
in the process.
This results in the following program (with free var annotations):
f x y a = a + x + y; let x = ...; y = ... in let g = {x y} b > f x y b + x in g 5
This optimisation is all about lifting only when it is beneficial to do so.
The above seems like a worthwhile lift, judging from heap allocation:
We eliminate f
's closure, saving to allocate a closure with 2 words, while
not changing the size of g
's closure.
You can probably sense that there's some kind of cost model at play here. And you are right! But we also employ a couple of other heuristics for the lifting decision which are outlined in GHC.Stg.Lift.Analysis.
The transformation is done in GHC.Stg.Lift, which calls out to
goodToLift
for its lifting decision. It relies on
GHC.Stg.Lift.Monad, which abstracts some subtle STG invariants into a
monadic substrate.
Suffice to say: We trade heap allocation for stack allocation. The additional arguments have to passed on the stack (or in registers, depending on architecture) every time we call the function to save a single heap allocation when entering the let binding. Nofib suggests a mean improvement of about 1% for this pass, so it seems like a worthwhile thing to do. Compiletimes went up by 0.6%, so all in all a very modest change.
For a concrete example, look at spectral/atom
. There's a call to zipWith
that is ultimately compiled to something like this
(module desugaring/lowering to actual STG):
propagate dt = ...; runExperiment ... = let xs = ... in let ys = ... in let go = {dt go} xs ys > case (xs, ys) of ([], []) > [] (x:xs', y:ys') > propagate dt x y : go xs' ys' in go xs ys
This will lambda lift go
to toplevel, speeding up the resulting program
by roughly one percent:
propagate dt = ...; go dt xs ys = case (xs, ys) of ([], []) > [] (x:xs', y:ys') > propagate dt x y : go dt xs' ys' runExperiment ... = let xs = ... in let ys = ... in in go dt xs ys
data StgLiftConfig Source #
StgLiftConfig  

Instances
Read StgLiftConfig Source #  
Defined in GHC.Stg.Lift.Config readsPrec :: Int > ReadS StgLiftConfig # readList :: ReadS [StgLiftConfig] #  
Show StgLiftConfig Source #  
Defined in GHC.Stg.Lift.Config showsPrec :: Int > StgLiftConfig > ShowS # show :: StgLiftConfig > String # showList :: [StgLiftConfig] > ShowS #  
Eq StgLiftConfig Source #  
Defined in GHC.Stg.Lift.Config (==) :: StgLiftConfig > StgLiftConfig > Bool # (/=) :: StgLiftConfig > StgLiftConfig > Bool #  
Ord StgLiftConfig Source #  
Defined in GHC.Stg.Lift.Config compare :: StgLiftConfig > StgLiftConfig > Ordering # (<) :: StgLiftConfig > StgLiftConfig > Bool # (<=) :: StgLiftConfig > StgLiftConfig > Bool # (>) :: StgLiftConfig > StgLiftConfig > Bool # (>=) :: StgLiftConfig > StgLiftConfig > Bool # max :: StgLiftConfig > StgLiftConfig > StgLiftConfig # min :: StgLiftConfig > StgLiftConfig > StgLiftConfig # 
stgLiftLams :: Module > StgLiftConfig > UniqSupply > [InStgTopBinding] > [OutStgTopBinding] Source #
Lambda lifts bindings to toplevel deemed worth lifting (see goodToLift
).
(Mostly) textbook instance of the lambda lifting transformation, selecting
which bindings to lambda lift by consulting goodToLift
.