couldBeSmallEnoughToInline,
certainlyWillInline, smallEnoughToInline,
- callSiteInline
+ callSiteInline, CallContInfo(..)
+
) where
#include "HsVersions.h"
callSiteInline :: DynFlags
-> Bool -- True <=> the Id can be inlined
-> Id -- The Id
+ -> Bool -- True if there are are no arguments at all (incl type args)
-> [Bool] -- One for each value arg; True if it is interesting
- -> Bool -- True <=> continuation is interesting
+ -> CallContInfo -- True <=> continuation is interesting
-> Maybe CoreExpr -- Unfolding, if any
-callSiteInline dflags active_inline id arg_infos interesting_cont
+data CallContInfo = BoringCont
+ | InterestingCont -- Somewhat interesting
+ | CaseCont -- Very interesting; the argument of a case
+ -- that decomposes its scrutinee
+
+instance Outputable CallContInfo where
+ ppr BoringCont = ptext SLIT("BoringCont")
+ ppr InterestingCont = ptext SLIT("InterestingCont")
+ ppr CaseCont = ptext SLIT("CaseCont")
+
+callSiteInline dflags active_inline id lone_variable arg_infos cont_info
= case idUnfolding id of {
NoUnfolding -> Nothing ;
OtherCon cs -> Nothing ;
n_val_args = length arg_infos
- yes_or_no
- | not active_inline = False
- | otherwise = is_cheap && consider_safe False
+ yes_or_no = active_inline && is_cheap && consider_safe
-- We consider even the once-in-one-branch
-- occurrences, because they won't all have been
-- caught by preInlineUnconditionally. In particular,
-- pre-inline will not have inlined it for fear of
-- invalidating the occurrence info in the rhs.
- consider_safe once
+ consider_safe
-- consider_safe decides whether it's a good idea to
-- inline something, given that there's no
-- work-duplication issue (the caller checks that).
= case guidance of
UnfoldNever -> False
UnfoldIfGoodArgs n_vals_wanted arg_discounts size res_discount
-
| enough_args && size <= (n_vals_wanted + 1)
-- Inline unconditionally if there no size increase
-- Size of call is n_vals_wanted (+1 for the function)
-> some_benefit && small_enough
where
- some_benefit = or arg_infos || really_interesting_cont ||
- (not is_top && ({- once || -} (n_vals_wanted > 0 && enough_args)))
- -- [was (once && not in_lam)]
- -- If it occurs more than once, there must be
- -- something interesting about some argument, or the
- -- result context, to make it worth inlining
- --
- -- If a function has a nested defn we also record
- -- some-benefit, on the grounds that we are often able
- -- to eliminate the binding, and hence the allocation,
- -- for the function altogether; this is good for join
- -- points. But this only makes sense for *functions*;
- -- inlining a constructor doesn't help allocation
- -- unless the result is scrutinised. UNLESS the
- -- constructor occurs just once, albeit possibly in
- -- multiple case branches. Then inlining it doesn't
- -- increase allocation, but it does increase the
- -- chance that the constructor won't be allocated at
- -- all in the branches that don't use it.
-
- enough_args = n_val_args >= n_vals_wanted
- really_interesting_cont | n_val_args < n_vals_wanted = False -- Too few args
- | n_val_args == n_vals_wanted = interesting_cont
- | otherwise = True -- Extra args
+ enough_args = n_val_args >= n_vals_wanted
+
+ some_benefit = or arg_infos || really_interesting_cont
+ -- There must be something interesting
+ -- about some argument, or the result
+ -- context, to make it worth inlining
+
+ really_interesting_cont
+ | n_val_args < n_vals_wanted = False -- Too few args
+ | n_val_args == n_vals_wanted = interesting_saturated_call
+ | otherwise = True -- Extra args
-- really_interesting_cont tells if the result of the
-- call is in an interesting context.
+ interesting_saturated_call
+ = case cont_info of
+ BoringCont -> not is_top && n_vals_wanted > 0 -- Note [Nested functions]
+ CaseCont -> not lone_variable || not is_value -- Note [Lone variables]
+ InterestingCont -> n_vals_wanted > 0
+
small_enough = (size - discount) <= opt_UF_UseThreshold
- discount = computeDiscount n_vals_wanted arg_discounts res_discount
- arg_infos really_interesting_cont
+ discount = computeDiscount n_vals_wanted arg_discounts
+ res_discount' arg_infos
+ res_discount' = case cont_info of
+ BoringCont -> 0
+ CaseCont -> res_discount
+ InterestingCont -> 4 `min` res_discount
+ -- res_discount can be very large when a function returns
+ -- construtors; but we only want to invoke that large discount
+ -- when there's a case continuation.
+ -- Otherwise we, rather arbitrarily, threshold it. Yuk.
+ -- But we want to aovid inlining large functions that return
+ -- constructors into contexts that are simply "interesting"
in
if dopt Opt_D_dump_inlinings dflags then
pprTrace "Considering inlining"
(ppr id <+> vcat [text "active:" <+> ppr active_inline,
text "arg infos" <+> ppr arg_infos,
- text "interesting continuation" <+> ppr interesting_cont,
+ text "interesting continuation" <+> ppr cont_info,
text "is value:" <+> ppr is_value,
text "is cheap:" <+> ppr is_cheap,
text "guidance" <+> ppr guidance,
else
result
}
+\end{code}
+
+Note [Nested functions]
+~~~~~~~~~~~~~~~~~~~~~~~
+If a function has a nested defn we also record some-benefit, on the
+grounds that we are often able to eliminate the binding, and hence the
+allocation, for the function altogether; this is good for join points.
+But this only makes sense for *functions*; inlining a constructor
+doesn't help allocation unless the result is scrutinised. UNLESS the
+constructor occurs just once, albeit possibly in multiple case
+branches. Then inlining it doesn't increase allocation, but it does
+increase the chance that the constructor won't be allocated at all in
+the branches that don't use it.
+
+Note [Lone variables]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The "lone-variable" case is important. I spent ages messing about
+with unsatisfactory varaints, but this is nice. The idea is that if a
+variable appears all alone
+ as an arg of lazy fn, or rhs Stop
+ as scrutinee of a case Select
+ as arg of a strict fn ArgOf
+AND
+ it is bound to a value
+then we should not inline it (unless there is some other reason,
+e.g. is is the sole occurrence). That is what is happening at
+the use of 'lone_variable' in 'interesting_saturated_call'.
+
+Why? At least in the case-scrutinee situation, turning
+ let x = (a,b) in case x of y -> ...
+into
+ let x = (a,b) in case (a,b) of y -> ...
+and thence to
+ let x = (a,b) in let y = (a,b) in ...
+is bad if the binding for x will remain.
+
+Another example: I discovered that strings
+were getting inlined straight back into applications of 'error'
+because the latter is strict.
+ s = "foo"
+ f = \x -> ...(error s)...
+
+Fundamentally such contexts should not encourage inlining because the
+context can ``see'' the unfolding of the variable (e.g. case or a
+RULE) so there's no gain. If the thing is bound to a value.
+
+However, watch out:
+
+ * Consider this:
+ foo = _inline_ (\n. [n])
+ bar = _inline_ (foo 20)
+ baz = \n. case bar of { (m:_) -> m + n }
+ Here we really want to inline 'bar' so that we can inline 'foo'
+ and the whole thing unravels as it should obviously do. This is
+ important: in the NDP project, 'bar' generates a closure data
+ structure rather than a list.
+
+ * Even a type application or coercion isn't a lone variable.
+ Consider
+ case $fMonadST @ RealWorld of { :DMonad a b c -> c }
+ We had better inline that sucker! The case won't see through it.
+
+ For now, I'm treating treating a variable applied to types
+ in a *lazy* context "lone". The motivating example was
+ f = /\a. \x. BIG
+ g = /\a. \y. h (f a)
+ There's no advantage in inlining f here, and perhaps
+ a significant disadvantage. Hence some_val_args in the Stop case
-computeDiscount :: Int -> [Int] -> Int -> [Bool] -> Bool -> Int
-computeDiscount n_vals_wanted arg_discounts res_discount arg_infos result_used
+\begin{code}
+computeDiscount :: Int -> [Int] -> Int -> [Bool] -> Int
+computeDiscount n_vals_wanted arg_discounts result_discount arg_infos
-- We multiple the raw discounts (args_discount and result_discount)
-- ty opt_UnfoldingKeenessFactor because the former have to do with
-- *size* whereas the discounts imply that there's some extra
mk_arg_discount discount is_evald | is_evald = discount
| otherwise = 0
-
- -- Don't give a result discount unless there are enough args
- result_discount | result_used = res_discount -- Over-applied, or case scrut
- | otherwise = 0
\end{code}
projects / ghc-hetmet.git / blobdiff