Commit aff23274 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Tidy up computation of result discounts in CoreUnfold

Mostly this patch is a tidy-up, but it did reveal one inconsistency
that I fixed.  When computing result discounts for case expressions,
we were *adding* result-discounts for cases on non-arguments, but
*picking the one for the max-size branch* for arguments. I think you
could argue the toss, but it seems neater (and the code is nicer)
to be consistent (ie always add).  See Note [addAltSize result discounts].

The nofib results seem fine

        Program           Size    Allocs   Runtime   Elapsed
--------------------------------------------------------------------------------
          boyer          -0.8%     -4.8%      0.06      0.07
         sphere          -0.7%     -2.5%      0.15      0.16
--------------------------------------------------------------------------------
            Min          -0.8%     -4.8%    -19.1%    -24.8%
            Max          -0.5%     +0.0%     +3.4%   +127.1%
 Geometric Mean          -0.7%     -0.1%     -4.3%     -1.3%

The +127% elapsed is a timing error; I re-ran the same binary and it's
unchanged from the baseline.
parent 21eeb926
......@@ -283,29 +283,27 @@ sizeExpr bOMB_OUT_SIZE top_args expr
-- discounts even on nullary constructors
size_up (App fun (Type _)) = size_up fun
size_up (App fun arg) = size_up_app fun [arg]
`addSize` nukeScrutDiscount (size_up arg)
size_up (App fun arg) = size_up arg `addSizeNSD`
size_up_app fun [arg]
size_up (Lam b e) | isId b = lamScrutDiscount (size_up e `addSizeN` 1)
| otherwise = size_up e
size_up (Let (NonRec binder rhs) body)
= nukeScrutDiscount (size_up rhs) `addSize`
size_up body `addSizeN`
= size_up rhs `addSizeNSD`
size_up body `addSizeN`
(if isUnLiftedType (idType binder) then 0 else 1)
-- For the allocation
-- If the binder has an unlifted type there is no allocation
size_up (Let (Rec pairs) body)
= nukeScrutDiscount rhs_size `addSize`
size_up body `addSizeN`
length pairs -- For the allocation
where
rhs_size = foldr (addSize . size_up . snd) sizeZero pairs
= foldr (addSizeNSD . size_up . snd)
(size_up body `addSizeN` length pairs) -- (length pairs) for the allocation
pairs
size_up (Case (Var v) _ _ alts)
| v `elem` top_args -- We are scrutinising an argument variable
= alts_size (foldr1 addSize alt_sizes) -- The 1 is for the case itself
= alts_size (foldr1 addAltSize alt_sizes)
(foldr1 maxSize alt_sizes)
-- Good to inline if an arg is scrutinised, because
-- that may eliminate allocation in the caller
......@@ -315,9 +313,9 @@ sizeExpr bOMB_OUT_SIZE top_args expr
-- alts_size tries to compute a good discount for
-- the case when we are scrutinising an argument variable
alts_size (SizeIs tot tot_disc _tot_scrut) -- Size of all alternatives
(SizeIs max _max_disc max_scrut) -- Size of biggest alternative
= SizeIs tot (unitBag (v, iBox (_ILIT(2) +# tot -# max)) `unionBags` tot_disc) max_scrut
alts_size (SizeIs tot tot_disc tot_scrut) -- Size of all alternatives
(SizeIs max _ _) -- Size of biggest alternative
= SizeIs tot (unitBag (v, iBox (_ILIT(2) +# tot -# max)) `unionBags` tot_disc) tot_scrut
-- If the variable is known, we produce a discount that
-- will take us back to 'max', the size of the largest alternative
-- The 1+ is a little discount for reduced allocation in the caller
......@@ -327,9 +325,8 @@ sizeExpr bOMB_OUT_SIZE top_args expr
alts_size tot_size _ = tot_size
size_up (Case e _ _ alts) = foldr (addSize . size_up_alt)
(nukeScrutDiscount (size_up e))
alts
size_up (Case e _ _ alts) = size_up e `addSizeNSD`
foldr (addAltSize . size_up_alt) sizeZero alts
-- We don't charge for the case itself
-- It's a strict thing, and the price of the call
-- is paid by scrut. Also consider
......@@ -342,8 +339,8 @@ sizeExpr bOMB_OUT_SIZE top_args expr
-- size_up_app is used when there's ONE OR MORE value args
size_up_app (App fun arg) args
| isTypeArg arg = size_up_app fun args
| otherwise = size_up_app fun (arg:args)
`addSize` nukeScrutDiscount (size_up arg)
| otherwise = size_up arg `addSizeNSD`
size_up_app fun (arg:args)
size_up_app (Var fun) args = size_up_call fun args
size_up_app other args = size_up other `addSizeN` length args
......@@ -372,10 +369,22 @@ sizeExpr bOMB_OUT_SIZE top_args expr
addSizeN TooBig _ = TooBig
addSizeN (SizeIs n xs d) m = mkSizeIs bOMB_OUT_SIZE (n +# iUnbox m) xs d
addSize TooBig _ = TooBig
addSize _ TooBig = TooBig
addSize (SizeIs n1 xs d1) (SizeIs n2 ys d2)
= mkSizeIs bOMB_OUT_SIZE (n1 +# n2) (xs `unionBags` ys) (d1 +# d2)
-- addAltSize is used to add the sizes of case alternatives
addAltSize TooBig _ = TooBig
addAltSize _ TooBig = TooBig
addAltSize (SizeIs n1 xs d1) (SizeIs n2 ys d2)
= mkSizeIs bOMB_OUT_SIZE (n1 +# n2)
(xs `unionBags` ys)
(d1 +# d2) -- Note [addAltSize result discounts]
-- This variant ignores the result discount from its LEFT argument
-- It's used when the second argument isn't part of the result
addSizeNSD TooBig _ = TooBig
addSizeNSD _ TooBig = TooBig
addSizeNSD (SizeIs n1 xs _) (SizeIs n2 ys d2)
= mkSizeIs bOMB_OUT_SIZE (n1 +# n2)
(xs `unionBags` ys)
d2 -- Ignore d1
\end{code}
\begin{code}
......@@ -481,16 +490,21 @@ augmentSize = SizeIs (_ILIT(0)) emptyBag (_ILIT(4))
-- Ditto (augment t (\cn -> e) ys) should cost only the cost of
-- e plus ys. The -2 accounts for the \cn
nukeScrutDiscount :: ExprSize -> ExprSize
nukeScrutDiscount (SizeIs n vs _) = SizeIs n vs (_ILIT(0))
nukeScrutDiscount TooBig = TooBig
-- When we return a lambda, give a discount if it's used (applied)
lamScrutDiscount :: ExprSize -> ExprSize
lamScrutDiscount (SizeIs n vs _) = SizeIs n vs (iUnbox opt_UF_FunAppDiscount)
lamScrutDiscount TooBig = TooBig
\end{code}
Note [addAltSize result discounts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When adding the size of alternatives, we *add* the result discounts
too, rather than take the *maximum*. For a multi-branch case, this
gives a discount for each branch that returns a constructor, making us
keener to inline. I did try using 'max' instead, but it makes nofib
'rewrite' and 'puzzle' allocate significantly more, and didn't make
binary sizes shrink significantly either.
Note [Discounts and thresholds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Constants for discounts and thesholds are defined in main/StaticFlags,
......
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