[Git][ghc/ghc][wip/andreask/untangle-inline] Separate core inlining logic from `Unfolding` type.
Andreas Klebinger (@AndreasK)
gitlab at gitlab.haskell.org
Mon Oct 17 23:44:11 UTC 2022
Andreas Klebinger pushed to branch wip/andreask/untangle-inline at Glasgow Haskell Compiler / GHC
Commits:
98c19f81 by Andreas Klebinger at 2022-10-18T01:36:42+02:00
Separate core inlining logic from `Unfolding` type.
This seems like a good idea either way, but is mostly motivated by a
patch where this avoids a module loop.
- - - - -
7 changed files:
- compiler/GHC/Core/Opt/LiberateCase.hs
- + compiler/GHC/Core/Opt/Simplify/Inline.hs
- compiler/GHC/Core/Opt/Simplify/Iteration.hs
- compiler/GHC/Core/Opt/Simplify/Utils.hs
- compiler/GHC/Core/Opt/SpecConstr.hs
- compiler/GHC/Core/Unfold.hs
- compiler/ghc.cabal.in
Changes:
=====================================
compiler/GHC/Core/Opt/LiberateCase.hs
=====================================
@@ -14,6 +14,7 @@ import GHC.Prelude
import GHC.Core
import GHC.Core.Unfold
+import GHC.Core.Opt.Simplify.Inline
import GHC.Builtin.Types ( unitDataConId )
import GHC.Types.Id
import GHC.Types.Var.Env
=====================================
compiler/GHC/Core/Opt/Simplify/Inline.hs
=====================================
@@ -0,0 +1,595 @@
+{-
+(c) The University of Glasgow 2006
+(c) The AQUA Project, Glasgow University, 1994-1998
+
+This module contains inlining logic used by the simplifier.
+-}
+
+
+{-# LANGUAGE BangPatterns #-}
+
+module GHC.Core.Opt.Simplify.Inline (
+ -- * Cheap and cheerful inlining checks.
+ couldBeSmallEnoughToInline,
+ smallEnoughToInline,
+
+ -- * The smart inlining decisions are made by callSiteInline
+ callSiteInline, CallCtxt(..),
+ ) where
+
+import GHC.Prelude
+
+import GHC.Driver.Flags
+
+import GHC.Core
+import GHC.Core.Unfold
+import GHC.Types.Id
+import GHC.Types.Basic ( Arity, RecFlag(..) )
+import GHC.Utils.Logger
+import GHC.Utils.Misc
+import GHC.Utils.Outputable
+import GHC.Types.Name
+
+import Data.List (isPrefixOf)
+
+{-
+************************************************************************
+* *
+\subsection[considerUnfolding]{Given all the info, do (not) do the unfolding}
+* *
+************************************************************************
+
+We use 'couldBeSmallEnoughToInline' to avoid exporting inlinings that
+we ``couldn't possibly use'' on the other side. Can be overridden w/
+flaggery. Just the same as smallEnoughToInline, except that it has no
+actual arguments.
+-}
+
+couldBeSmallEnoughToInline :: UnfoldingOpts -> Int -> CoreExpr -> Bool
+couldBeSmallEnoughToInline opts threshold rhs
+ = case sizeExpr opts threshold [] body of
+ TooBig -> False
+ _ -> True
+ where
+ (_, body) = collectBinders rhs
+
+----------------
+smallEnoughToInline :: UnfoldingOpts -> Unfolding -> Bool
+smallEnoughToInline opts (CoreUnfolding {uf_guidance = guidance})
+ = case guidance of
+ UnfIfGoodArgs {ug_size = size} -> size <= unfoldingUseThreshold opts
+ UnfWhen {} -> True
+ UnfNever -> False
+smallEnoughToInline _ _
+ = False
+
+
+{-
+************************************************************************
+* *
+\subsection{callSiteInline}
+* *
+************************************************************************
+
+This is the key function. It decides whether to inline a variable at a call site
+
+callSiteInline is used at call sites, so it is a bit more generous.
+It's a very important function that embodies lots of heuristics.
+A non-WHNF can be inlined if it doesn't occur inside a lambda,
+and occurs exactly once or
+ occurs once in each branch of a case and is small
+
+If the thing is in WHNF, there's no danger of duplicating work,
+so we can inline if it occurs once, or is small
+
+NOTE: we don't want to inline top-level functions that always diverge.
+It just makes the code bigger. Tt turns out that the convenient way to prevent
+them inlining is to give them a NOINLINE pragma, which we do in
+StrictAnal.addStrictnessInfoToTopId
+-}
+
+{-
+************************************************************************
+* *
+\subsection{callSiteInline}
+* *
+************************************************************************
+
+This is the key function. It decides whether to inline a variable at a call site
+
+callSiteInline is used at call sites, so it is a bit more generous.
+It's a very important function that embodies lots of heuristics.
+A non-WHNF can be inlined if it doesn't occur inside a lambda,
+and occurs exactly once or
+ occurs once in each branch of a case and is small
+
+If the thing is in WHNF, there's no danger of duplicating work,
+so we can inline if it occurs once, or is small
+
+NOTE: we don't want to inline top-level functions that always diverge.
+It just makes the code bigger. Tt turns out that the convenient way to prevent
+them inlining is to give them a NOINLINE pragma, which we do in
+StrictAnal.addStrictnessInfoToTopId
+-}
+
+{-
+************************************************************************
+* *
+\subsection{callSiteInline}
+* *
+************************************************************************
+
+This is the key function. It decides whether to inline a variable at a call site
+
+callSiteInline is used at call sites, so it is a bit more generous.
+It's a very important function that embodies lots of heuristics.
+A non-WHNF can be inlined if it doesn't occur inside a lambda,
+and occurs exactly once or
+ occurs once in each branch of a case and is small
+
+If the thing is in WHNF, there's no danger of duplicating work,
+so we can inline if it occurs once, or is small
+
+NOTE: we don't want to inline top-level functions that always diverge.
+It just makes the code bigger. Tt turns out that the convenient way to prevent
+them inlining is to give them a NOINLINE pragma, which we do in
+StrictAnal.addStrictnessInfoToTopId
+-}
+
+callSiteInline :: Logger
+ -> UnfoldingOpts
+ -> Int -- Case depth
+ -> Id -- The Id
+ -> Bool -- True <=> unfolding is active
+ -> Bool -- True if there are no arguments at all (incl type args)
+ -> [ArgSummary] -- One for each value arg; True if it is interesting
+ -> CallCtxt -- True <=> continuation is interesting
+ -> Maybe CoreExpr -- Unfolding, if any
+callSiteInline logger opts !case_depth id active_unfolding lone_variable arg_infos cont_info
+ = case idUnfolding id of
+ -- idUnfolding checks for loop-breakers, returning NoUnfolding
+ -- Things with an INLINE pragma may have an unfolding *and*
+ -- be a loop breaker (maybe the knot is not yet untied)
+ CoreUnfolding { uf_tmpl = unf_template
+ , uf_is_work_free = is_wf
+ , uf_guidance = guidance, uf_expandable = is_exp }
+ | active_unfolding -> tryUnfolding logger opts case_depth id lone_variable
+ arg_infos cont_info unf_template
+ is_wf is_exp guidance
+ | otherwise -> traceInline logger opts id "Inactive unfolding:" (ppr id) Nothing
+ NoUnfolding -> Nothing
+ BootUnfolding -> Nothing
+ OtherCon {} -> Nothing
+ DFunUnfolding {} -> Nothing -- Never unfold a DFun
+
+-- | Report the inlining of an identifier's RHS to the user, if requested.
+traceInline :: Logger -> UnfoldingOpts -> Id -> String -> SDoc -> a -> a
+traceInline logger opts inline_id str doc result
+ -- We take care to ensure that doc is used in only one branch, ensuring that
+ -- the simplifier can push its allocation into the branch. See Note [INLINE
+ -- conditional tracing utilities].
+ | enable = logTraceMsg logger str doc result
+ | otherwise = result
+ where
+ enable
+ | logHasDumpFlag logger Opt_D_dump_verbose_inlinings
+ = True
+ | Just prefix <- unfoldingReportPrefix opts
+ = prefix `isPrefixOf` occNameString (getOccName inline_id)
+ | otherwise
+ = False
+{-# INLINE traceInline #-} -- see Note [INLINE conditional tracing utilities]
+
+{- Note [Avoid inlining into deeply nested cases]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Consider a function f like this:
+
+ f arg1 arg2 =
+ case ...
+ ... -> g arg1
+ ... -> g arg2
+
+This function is small. So should be safe to inline.
+However sometimes this doesn't quite work out like that.
+Consider this code:
+
+f1 arg1 arg2 ... = ...
+ case _foo of
+ alt1 -> ... f2 arg1 ...
+ alt2 -> ... f2 arg2 ...
+
+f2 arg1 arg2 ... = ...
+ case _foo of
+ alt1 -> ... f3 arg1 ...
+ alt2 -> ... f3 arg2 ...
+
+f3 arg1 arg2 ... = ...
+
+... repeats up to n times. And then f1 is
+applied to some arguments:
+
+foo = ... f1 <interestingArgs> ...
+
+Initially f2..fn are not interesting to inline so we don't.
+However we see that f1 is applied to interesting args.
+So it's an obvious choice to inline those:
+
+foo =
+ ...
+ case _foo of
+ alt1 -> ... f2 <interestingArg> ...
+ alt2 -> ... f2 <interestingArg> ...
+
+As a result we go and inline f2 both mentions of f2 in turn are now applied to interesting
+arguments and f2 is small:
+
+foo =
+ ...
+ case _foo of
+ alt1 -> ... case _foo of
+ alt1 -> ... f3 <interestingArg> ...
+ alt2 -> ... f3 <interestingArg> ...
+
+ alt2 -> ... case _foo of
+ alt1 -> ... f3 <interestingArg> ...
+ alt2 -> ... f3 <interestingArg> ...
+
+The same thing happens for each binding up to f_n, duplicating the amount of inlining
+done in each step. Until at some point we are either done or run out of simplifier
+ticks/RAM. This pattern happened #18730.
+
+To combat this we introduce one more heuristic when weighing inlining decision.
+We keep track of a "case-depth". Which increases each time we look inside a case
+expression with more than one alternative.
+
+We then apply a penalty to inlinings based on the case-depth at which they would
+be inlined. Bounding the number of inlinings in such a scenario.
+
+The heuristic can be tuned in two ways:
+
+* We can ignore the first n levels of case nestings for inlining decisions using
+ -funfolding-case-threshold.
+* The penalty grows linear with the depth. It's computed as size*(depth-threshold)/scaling.
+ Scaling can be set with -funfolding-case-scaling.
+
+Some guidance on setting these defaults:
+
+* A low treshold (<= 2) is needed to prevent exponential cases from spiraling out of
+ control. We picked 2 for no particular reason.
+* Scaling the penalty by any more than 30 means the reproducer from
+ T18730 won't compile even with reasonably small values of n. Instead
+ it will run out of runs/ticks. This means to positively affect the reproducer
+ a scaling <= 30 is required.
+* A scaling of >= 15 still causes a few very large regressions on some nofib benchmarks.
+ (+80% for gc/fulsom, +90% for real/ben-raytrace, +20% for spectral/fibheaps)
+* A scaling of >= 25 showed no regressions on nofib. However it showed a number of
+ (small) regression for compiler perf benchmarks.
+
+The end result is that we are settling for a scaling of 30, with a threshold of 2.
+This gives us minimal compiler perf regressions. No nofib runtime regressions and
+will still avoid this pattern sometimes. This is a "safe" default, where we err on
+the side of compiler blowup instead of risking runtime regressions.
+
+For cases where the default falls short the flag can be changed to allow more/less inlining as
+needed on a per-module basis.
+
+-}
+
+tryUnfolding :: Logger -> UnfoldingOpts -> Int -> Id -> Bool -> [ArgSummary] -> CallCtxt
+ -> CoreExpr -> Bool -> Bool -> UnfoldingGuidance
+ -> Maybe CoreExpr
+tryUnfolding logger opts !case_depth id lone_variable
+ arg_infos cont_info unf_template
+ is_wf is_exp guidance
+ = case guidance of
+ UnfNever -> traceInline logger opts id str (text "UnfNever") Nothing
+
+ UnfWhen { ug_arity = uf_arity, ug_unsat_ok = unsat_ok, ug_boring_ok = boring_ok }
+ | enough_args && (boring_ok || some_benefit || unfoldingVeryAggressive opts)
+ -- See Note [INLINE for small functions] (3)
+ -> traceInline logger opts id str (mk_doc some_benefit empty True) (Just unf_template)
+ | otherwise
+ -> traceInline logger opts id str (mk_doc some_benefit empty False) Nothing
+ where
+ some_benefit = calc_some_benefit uf_arity
+ enough_args = (n_val_args >= uf_arity) || (unsat_ok && n_val_args > 0)
+
+ UnfIfGoodArgs { ug_args = arg_discounts, ug_res = res_discount, ug_size = size }
+ | unfoldingVeryAggressive opts
+ -> traceInline logger opts id str (mk_doc some_benefit extra_doc True) (Just unf_template)
+ | is_wf && some_benefit && small_enough
+ -> traceInline logger opts id str (mk_doc some_benefit extra_doc True) (Just unf_template)
+ | otherwise
+ -> traceInline logger opts id str (mk_doc some_benefit extra_doc False) Nothing
+ where
+ some_benefit = calc_some_benefit (length arg_discounts)
+ extra_doc = vcat [ text "case depth =" <+> int case_depth
+ , text "depth based penalty =" <+> int depth_penalty
+ , text "discounted size =" <+> int adjusted_size ]
+ -- See Note [Avoid inlining into deeply nested cases]
+ depth_treshold = unfoldingCaseThreshold opts
+ depth_scaling = unfoldingCaseScaling opts
+ depth_penalty | case_depth <= depth_treshold = 0
+ | otherwise = (size * (case_depth - depth_treshold)) `div` depth_scaling
+ adjusted_size = size + depth_penalty - discount
+ small_enough = adjusted_size <= unfoldingUseThreshold opts
+ discount = computeDiscount arg_discounts res_discount arg_infos cont_info
+
+ where
+ mk_doc some_benefit extra_doc yes_or_no
+ = vcat [ text "arg infos" <+> ppr arg_infos
+ , text "interesting continuation" <+> ppr cont_info
+ , text "some_benefit" <+> ppr some_benefit
+ , text "is exp:" <+> ppr is_exp
+ , text "is work-free:" <+> ppr is_wf
+ , text "guidance" <+> ppr guidance
+ , extra_doc
+ , text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"]
+
+ ctx = log_default_dump_context (logFlags logger)
+ str = "Considering inlining: " ++ showSDocOneLine ctx (ppr id)
+ n_val_args = length arg_infos
+
+ -- some_benefit is used when the RHS is small enough
+ -- and the call has enough (or too many) value
+ -- arguments (ie n_val_args >= arity). But there must
+ -- be *something* interesting about some argument, or the
+ -- result context, to make it worth inlining
+ calc_some_benefit :: Arity -> Bool -- The Arity is the number of args
+ -- expected by the unfolding
+ calc_some_benefit uf_arity
+ | not saturated = interesting_args -- Under-saturated
+ -- Note [Unsaturated applications]
+ | otherwise = interesting_args -- Saturated or over-saturated
+ || interesting_call
+ where
+ saturated = n_val_args >= uf_arity
+ over_saturated = n_val_args > uf_arity
+ interesting_args = any nonTriv arg_infos
+ -- NB: (any nonTriv arg_infos) looks at the
+ -- over-saturated args too which is "wrong";
+ -- but if over-saturated we inline anyway.
+
+ interesting_call
+ | over_saturated
+ = True
+ | otherwise
+ = case cont_info of
+ CaseCtxt -> not (lone_variable && is_exp) -- Note [Lone variables]
+ ValAppCtxt -> True -- Note [Cast then apply]
+ RuleArgCtxt -> uf_arity > 0 -- See Note [RHS of lets]
+ DiscArgCtxt -> uf_arity > 0 -- Note [Inlining in ArgCtxt]
+ RhsCtxt NonRecursive
+ -> uf_arity > 0 -- See Note [RHS of lets]
+ _other -> False -- See Note [Nested functions]
+
+
+{- Note [RHS of lets]
+~~~~~~~~~~~~~~~~~~~~~
+When the call is the argument of a function with a RULE, or the RHS of a let,
+we are a little bit keener to inline (in tryUnfolding). For example
+ f y = (y,y,y)
+ g y = let x = f y in ...(case x of (a,b,c) -> ...) ...
+We'd inline 'f' if the call was in a case context, and it kind-of-is,
+only we can't see it. Also
+ x = f v
+could be expensive whereas
+ x = case v of (a,b) -> a
+is patently cheap and may allow more eta expansion.
+
+So, in `interesting_call` in `tryUnfolding`, we treat the RHS of a
+/non-recursive/ let as not-totally-boring. A /recursive/ let isn't
+going be inlined so there is much less point. Hence the (only reason
+for the) RecFlag in RhsCtxt
+
+Note [Unsaturated applications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When a call is not saturated, we *still* inline if one of the
+arguments has interesting structure. That's sometimes very important.
+A good example is the Ord instance for Bool in Base:
+
+ Rec {
+ $fOrdBool =GHC.Classes.D:Ord
+ @ Bool
+ ...
+ $cmin_ajX
+
+ $cmin_ajX [Occ=LoopBreaker] :: Bool -> Bool -> Bool
+ $cmin_ajX = GHC.Classes.$dmmin @ Bool $fOrdBool
+ }
+
+But the defn of GHC.Classes.$dmmin is:
+
+ $dmmin :: forall a. GHC.Classes.Ord a => a -> a -> a
+ {- Arity: 3, HasNoCafRefs, Strictness: SLL,
+ Unfolding: (\ @ a $dOrd :: GHC.Classes.Ord a x :: a y :: a ->
+ case @ a GHC.Classes.<= @ a $dOrd x y of wild {
+ GHC.Types.False -> y GHC.Types.True -> x }) -}
+
+We *really* want to inline $dmmin, even though it has arity 3, in
+order to unravel the recursion.
+
+
+Note [Things to watch]
+~~~~~~~~~~~~~~~~~~~~~~
+* { y = I# 3; x = y `cast` co; ...case (x `cast` co) of ... }
+ Assume x is exported, so not inlined unconditionally.
+ Then we want x to inline unconditionally; no reason for it
+ not to, and doing so avoids an indirection.
+
+* { x = I# 3; ....f x.... }
+ Make sure that x does not inline unconditionally!
+ Lest we get extra allocation.
+
+Note [Nested functions]
+~~~~~~~~~~~~~~~~~~~~~~~
+At one time we treated a call of a non-top-level function as
+"interesting" (regardless of how boring the context) in the hope
+that inlining it would eliminate the binding, and its allocation.
+Specifically, in the default case of interesting_call we had
+ _other -> not is_top && uf_arity > 0
+
+But actually postInlineUnconditionally does some of this and overall
+it makes virtually no difference to nofib. So I simplified away this
+special case
+
+Note [Cast then apply]
+~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ myIndex = __inline_me ( (/\a. <blah>) |> co )
+ co :: (forall a. a -> a) ~ (forall a. T a)
+ ... /\a.\x. case ((myIndex a) |> sym co) x of { ... } ...
+
+We need to inline myIndex to unravel this; but the actual call (myIndex a) has
+no value arguments. The ValAppCtxt gives it enough incentive to inline.
+
+Note [Inlining in ArgCtxt]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+The condition (arity > 0) here is very important, because otherwise
+we end up inlining top-level stuff into useless places; eg
+ x = I# 3#
+ f = \y. g x
+This can make a very big difference: it adds 16% to nofib 'integer' allocs,
+and 20% to 'power'.
+
+At one stage I replaced this condition by 'True' (leading to the above
+slow-down). The motivation was test eyeball/inline1.hs; but that seems
+to work ok now.
+
+NOTE: arguably, we should inline in ArgCtxt only if the result of the
+call is at least CONLIKE. At least for the cases where we use ArgCtxt
+for the RHS of a 'let', we only profit from the inlining if we get a
+CONLIKE thing (modulo lets).
+
+Note [Lone variables] See also Note [Interaction of exprIsWorkFree and lone variables]
+~~~~~~~~~~~~~~~~~~~~~ which appears below
+The "lone-variable" case is important. I spent ages messing about
+with unsatisfactory variants, but this is nice. The idea is that if a
+variable appears all alone
+
+ as an arg of lazy fn, or rhs BoringCtxt
+ as scrutinee of a case CaseCtxt
+ as arg of a fn ArgCtxt
+AND
+ it is bound to a cheap expression
+
+then we should not inline it (unless there is some other reason,
+e.g. it is the sole occurrence). That is what is happening at
+the use of 'lone_variable' in 'interesting_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, provided
+the RHS is "expandable" (see Note [exprIsExpandable] in GHC.Core.Utils) the
+context can ``see'' the unfolding of the variable (e.g. case or a
+RULE) so there's no gain.
+
+However, watch out:
+
+ * Consider this:
+ foo = \n. [n]) {-# INLINE foo #-}
+ bar = foo 20 {-# INLINE bar #-}
+ 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.
+
+ So the non-inlining of lone_variables should only apply if the
+ unfolding is regarded as expandable; because that is when
+ exprIsConApp_maybe looks through the unfolding. Hence the "&&
+ is_exp" in the CaseCtxt branch of interesting_call
+
+ * 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
+
+Note [Interaction of exprIsWorkFree and lone variables]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The lone-variable test says "don't inline if a case expression
+scrutinises a lone variable whose unfolding is cheap". It's very
+important that, under these circumstances, exprIsConApp_maybe
+can spot a constructor application. So, for example, we don't
+consider
+ let x = e in (x,x)
+to be cheap, and that's good because exprIsConApp_maybe doesn't
+think that expression is a constructor application.
+
+In the 'not (lone_variable && is_wf)' test, I used to test is_value
+rather than is_wf, which was utterly wrong, because the above
+expression responds True to exprIsHNF, which is what sets is_value.
+
+This kind of thing can occur if you have
+
+ {-# INLINE foo #-}
+ foo = let x = e in (x,x)
+
+which Roman did.
+
+
+-}
+
+computeDiscount :: [Int] -> Int -> [ArgSummary] -> CallCtxt
+ -> Int
+computeDiscount arg_discounts res_discount arg_infos cont_info
+
+ = 10 -- Discount of 10 because the result replaces the call
+ -- so we count 10 for the function itself
+
+ + 10 * length actual_arg_discounts
+ -- Discount of 10 for each arg supplied,
+ -- because the result replaces the call
+
+ + total_arg_discount + res_discount'
+ where
+ actual_arg_discounts = zipWith mk_arg_discount arg_discounts arg_infos
+ total_arg_discount = sum actual_arg_discounts
+
+ mk_arg_discount _ TrivArg = 0
+ mk_arg_discount _ NonTrivArg = 10
+ mk_arg_discount discount ValueArg = discount
+
+ res_discount'
+ | LT <- arg_discounts `compareLength` arg_infos
+ = res_discount -- Over-saturated
+ | otherwise
+ = case cont_info of
+ BoringCtxt -> 0
+ CaseCtxt -> res_discount -- Presumably a constructor
+ ValAppCtxt -> res_discount -- Presumably a function
+ _ -> 40 `min` res_discount
+ -- ToDo: this 40 `min` res_discount doesn't seem right
+ -- for DiscArgCtxt it shouldn't matter because the function will
+ -- get the arg discount for any non-triv arg
+ -- for RuleArgCtxt we do want to be keener to inline; but not only
+ -- constructor results
+ -- for RhsCtxt I suppose that exposing a data con is good in general
+ -- And 40 seems very arbitrary
+ --
+ -- res_discount can be very large when a function returns
+ -- constructors; 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 avoid inlining large functions that return
+ -- constructors into contexts that are simply "interesting"
\ No newline at end of file
=====================================
compiler/GHC/Core/Opt/Simplify/Iteration.hs
=====================================
@@ -21,6 +21,7 @@ import GHC.Core
import GHC.Core.Opt.Simplify.Monad
import GHC.Core.Type hiding ( substTy, substTyVar, extendTvSubst, extendCvSubst )
import GHC.Core.Opt.Simplify.Env
+import GHC.Core.Opt.Simplify.Inline
import GHC.Core.Opt.Simplify.Utils
import GHC.Core.Opt.OccurAnal ( occurAnalyseExpr, zapLambdaBndrs )
import GHC.Core.Make ( FloatBind, mkImpossibleExpr, castBottomExpr )
=====================================
compiler/GHC/Core/Opt/Simplify/Utils.hs
=====================================
@@ -47,6 +47,7 @@ import GHC.Prelude
import GHC.Core
import GHC.Types.Literal ( isLitRubbish )
import GHC.Core.Opt.Simplify.Env
+import GHC.Core.Opt.Simplify.Inline
import GHC.Core.Opt.Stats ( Tick(..) )
import qualified GHC.Core.Subst
import GHC.Core.Ppr
=====================================
compiler/GHC/Core/Opt/SpecConstr.hs
=====================================
@@ -32,6 +32,7 @@ import GHC.Core
import GHC.Core.Subst
import GHC.Core.Utils
import GHC.Core.Unfold
+import GHC.Core.Opt.Simplify.Inline
import GHC.Core.FVs ( exprsFreeVarsList, exprFreeVars )
import GHC.Core.Opt.Monad
import GHC.Core.Opt.WorkWrap.Utils
=====================================
compiler/GHC/Core/Unfold.hs
=====================================
@@ -21,25 +21,22 @@ find, unsurprisingly, a Core expression.
module GHC.Core.Unfold (
Unfolding, UnfoldingGuidance, -- Abstract types
+ ExprSize(..), sizeExpr,
+
+ ArgSummary(..), nonTriv,
+ CallCtxt(..),
+
UnfoldingOpts (..), defaultUnfoldingOpts,
updateCreationThreshold, updateUseThreshold,
updateFunAppDiscount, updateDictDiscount,
updateVeryAggressive, updateCaseScaling,
updateCaseThreshold, updateReportPrefix,
- ArgSummary(..),
-
- couldBeSmallEnoughToInline, inlineBoringOk,
- smallEnoughToInline,
-
- callSiteInline, CallCtxt(..),
- calcUnfoldingGuidance
+ inlineBoringOk, calcUnfoldingGuidance
) where
import GHC.Prelude
-import GHC.Driver.Flags
-
import GHC.Core
import GHC.Core.Utils
import GHC.Types.Id
@@ -48,20 +45,16 @@ import GHC.Types.Literal
import GHC.Builtin.PrimOps
import GHC.Types.Id.Info
import GHC.Types.RepType ( isZeroBitTy )
-import GHC.Types.Basic ( Arity, RecFlag(..) )
+import GHC.Types.Basic ( Arity, RecFlag )
import GHC.Core.Type
import GHC.Builtin.Names
import GHC.Data.Bag
-import GHC.Utils.Logger
import GHC.Utils.Misc
import GHC.Utils.Outputable
import GHC.Types.ForeignCall
-import GHC.Types.Name
import GHC.Types.Tickish
import qualified Data.ByteString as BS
-import Data.List (isPrefixOf)
-
-- | Unfolding options
data UnfoldingOpts = UnfoldingOpts
@@ -151,6 +144,41 @@ updateCaseScaling n opts = opts { unfoldingCaseScaling = n }
updateReportPrefix :: Maybe String -> UnfoldingOpts -> UnfoldingOpts
updateReportPrefix n opts = opts { unfoldingReportPrefix = n }
+data ArgSummary = TrivArg -- Nothing interesting
+ | NonTrivArg -- Arg has structure
+ | ValueArg -- Arg is a con-app or PAP
+ -- ..or con-like. Note [Conlike is interesting]
+
+instance Outputable ArgSummary where
+ ppr TrivArg = text "TrivArg"
+ ppr NonTrivArg = text "NonTrivArg"
+ ppr ValueArg = text "ValueArg"
+
+nonTriv :: ArgSummary -> Bool
+nonTriv TrivArg = False
+nonTriv _ = True
+
+data CallCtxt
+ = BoringCtxt
+ | RhsCtxt RecFlag -- Rhs of a let-binding; see Note [RHS of lets]
+ | DiscArgCtxt -- Argument of a function with non-zero arg discount
+ | RuleArgCtxt -- We are somewhere in the argument of a function with rules
+
+ | ValAppCtxt -- We're applied to at least one value arg
+ -- This arises when we have ((f x |> co) y)
+ -- Then the (f x) has argument 'x' but in a ValAppCtxt
+
+ | CaseCtxt -- We're the scrutinee of a case
+ -- that decomposes its scrutinee
+
+instance Outputable CallCtxt where
+ ppr CaseCtxt = text "CaseCtxt"
+ ppr ValAppCtxt = text "ValAppCtxt"
+ ppr BoringCtxt = text "BoringCtxt"
+ ppr (RhsCtxt ir)= text "RhsCtxt" <> parens (ppr ir)
+ ppr DiscArgCtxt = text "DiscArgCtxt"
+ ppr RuleArgCtxt = text "RuleArgCtxt"
+
{-
Note [Occurrence analysis of unfoldings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -931,550 +959,3 @@ sizeN :: Int -> ExprSize
sizeZero = SizeIs 0 emptyBag 0
sizeN n = SizeIs n emptyBag 0
-{-
-************************************************************************
-* *
-\subsection[considerUnfolding]{Given all the info, do (not) do the unfolding}
-* *
-************************************************************************
-
-We use 'couldBeSmallEnoughToInline' to avoid exporting inlinings that
-we ``couldn't possibly use'' on the other side. Can be overridden w/
-flaggery. Just the same as smallEnoughToInline, except that it has no
-actual arguments.
--}
-
-couldBeSmallEnoughToInline :: UnfoldingOpts -> Int -> CoreExpr -> Bool
-couldBeSmallEnoughToInline opts threshold rhs
- = case sizeExpr opts threshold [] body of
- TooBig -> False
- _ -> True
- where
- (_, body) = collectBinders rhs
-
-----------------
-smallEnoughToInline :: UnfoldingOpts -> Unfolding -> Bool
-smallEnoughToInline opts (CoreUnfolding {uf_guidance = guidance})
- = case guidance of
- UnfIfGoodArgs {ug_size = size} -> size <= unfoldingUseThreshold opts
- UnfWhen {} -> True
- UnfNever -> False
-smallEnoughToInline _ _
- = False
-
-{-
-************************************************************************
-* *
-\subsection{callSiteInline}
-* *
-************************************************************************
-
-This is the key function. It decides whether to inline a variable at a call site
-
-callSiteInline is used at call sites, so it is a bit more generous.
-It's a very important function that embodies lots of heuristics.
-A non-WHNF can be inlined if it doesn't occur inside a lambda,
-and occurs exactly once or
- occurs once in each branch of a case and is small
-
-If the thing is in WHNF, there's no danger of duplicating work,
-so we can inline if it occurs once, or is small
-
-NOTE: we don't want to inline top-level functions that always diverge.
-It just makes the code bigger. Tt turns out that the convenient way to prevent
-them inlining is to give them a NOINLINE pragma, which we do in
-StrictAnal.addStrictnessInfoToTopId
--}
-
-data ArgSummary = TrivArg -- Nothing interesting
- | NonTrivArg -- Arg has structure
- | ValueArg -- Arg is a con-app or PAP
- -- ..or con-like. Note [Conlike is interesting]
-
-instance Outputable ArgSummary where
- ppr TrivArg = text "TrivArg"
- ppr NonTrivArg = text "NonTrivArg"
- ppr ValueArg = text "ValueArg"
-
-nonTriv :: ArgSummary -> Bool
-nonTriv TrivArg = False
-nonTriv _ = True
-
-data CallCtxt
- = BoringCtxt
- | RhsCtxt RecFlag -- Rhs of a let-binding; see Note [RHS of lets]
- | DiscArgCtxt -- Argument of a function with non-zero arg discount
- | RuleArgCtxt -- We are somewhere in the argument of a function with rules
-
- | ValAppCtxt -- We're applied to at least one value arg
- -- This arises when we have ((f x |> co) y)
- -- Then the (f x) has argument 'x' but in a ValAppCtxt
-
- | CaseCtxt -- We're the scrutinee of a case
- -- that decomposes its scrutinee
-
-instance Outputable CallCtxt where
- ppr CaseCtxt = text "CaseCtxt"
- ppr ValAppCtxt = text "ValAppCtxt"
- ppr BoringCtxt = text "BoringCtxt"
- ppr (RhsCtxt ir)= text "RhsCtxt" <> parens (ppr ir)
- ppr DiscArgCtxt = text "DiscArgCtxt"
- ppr RuleArgCtxt = text "RuleArgCtxt"
-
-callSiteInline :: Logger
- -> UnfoldingOpts
- -> Int -- Case depth
- -> Id -- The Id
- -> Bool -- True <=> unfolding is active
- -> Bool -- True if there are no arguments at all (incl type args)
- -> [ArgSummary] -- One for each value arg; True if it is interesting
- -> CallCtxt -- True <=> continuation is interesting
- -> Maybe CoreExpr -- Unfolding, if any
-callSiteInline logger opts !case_depth id active_unfolding lone_variable arg_infos cont_info
- = case idUnfolding id of
- -- idUnfolding checks for loop-breakers, returning NoUnfolding
- -- Things with an INLINE pragma may have an unfolding *and*
- -- be a loop breaker (maybe the knot is not yet untied)
- CoreUnfolding { uf_tmpl = unf_template
- , uf_is_work_free = is_wf
- , uf_guidance = guidance, uf_expandable = is_exp }
- | active_unfolding -> tryUnfolding logger opts case_depth id lone_variable
- arg_infos cont_info unf_template
- is_wf is_exp guidance
- | otherwise -> traceInline logger opts id "Inactive unfolding:" (ppr id) Nothing
- NoUnfolding -> Nothing
- BootUnfolding -> Nothing
- OtherCon {} -> Nothing
- DFunUnfolding {} -> Nothing -- Never unfold a DFun
-
--- | Report the inlining of an identifier's RHS to the user, if requested.
-traceInline :: Logger -> UnfoldingOpts -> Id -> String -> SDoc -> a -> a
-traceInline logger opts inline_id str doc result
- -- We take care to ensure that doc is used in only one branch, ensuring that
- -- the simplifier can push its allocation into the branch. See Note [INLINE
- -- conditional tracing utilities].
- | enable = logTraceMsg logger str doc result
- | otherwise = result
- where
- enable
- | logHasDumpFlag logger Opt_D_dump_verbose_inlinings
- = True
- | Just prefix <- unfoldingReportPrefix opts
- = prefix `isPrefixOf` occNameString (getOccName inline_id)
- | otherwise
- = False
-{-# INLINE traceInline #-} -- see Note [INLINE conditional tracing utilities]
-
-{- Note [Avoid inlining into deeply nested cases]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Consider a function f like this:
-
- f arg1 arg2 =
- case ...
- ... -> g arg1
- ... -> g arg2
-
-This function is small. So should be safe to inline.
-However sometimes this doesn't quite work out like that.
-Consider this code:
-
-f1 arg1 arg2 ... = ...
- case _foo of
- alt1 -> ... f2 arg1 ...
- alt2 -> ... f2 arg2 ...
-
-f2 arg1 arg2 ... = ...
- case _foo of
- alt1 -> ... f3 arg1 ...
- alt2 -> ... f3 arg2 ...
-
-f3 arg1 arg2 ... = ...
-
-... repeats up to n times. And then f1 is
-applied to some arguments:
-
-foo = ... f1 <interestingArgs> ...
-
-Initially f2..fn are not interesting to inline so we don't.
-However we see that f1 is applied to interesting args.
-So it's an obvious choice to inline those:
-
-foo =
- ...
- case _foo of
- alt1 -> ... f2 <interestingArg> ...
- alt2 -> ... f2 <interestingArg> ...
-
-As a result we go and inline f2 both mentions of f2 in turn are now applied to interesting
-arguments and f2 is small:
-
-foo =
- ...
- case _foo of
- alt1 -> ... case _foo of
- alt1 -> ... f3 <interestingArg> ...
- alt2 -> ... f3 <interestingArg> ...
-
- alt2 -> ... case _foo of
- alt1 -> ... f3 <interestingArg> ...
- alt2 -> ... f3 <interestingArg> ...
-
-The same thing happens for each binding up to f_n, duplicating the amount of inlining
-done in each step. Until at some point we are either done or run out of simplifier
-ticks/RAM. This pattern happened #18730.
-
-To combat this we introduce one more heuristic when weighing inlining decision.
-We keep track of a "case-depth". Which increases each time we look inside a case
-expression with more than one alternative.
-
-We then apply a penalty to inlinings based on the case-depth at which they would
-be inlined. Bounding the number of inlinings in such a scenario.
-
-The heuristic can be tuned in two ways:
-
-* We can ignore the first n levels of case nestings for inlining decisions using
- -funfolding-case-threshold.
-* The penalty grows linear with the depth. It's computed as size*(depth-threshold)/scaling.
- Scaling can be set with -funfolding-case-scaling.
-
-Some guidance on setting these defaults:
-
-* A low treshold (<= 2) is needed to prevent exponential cases from spiraling out of
- control. We picked 2 for no particular reason.
-* Scaling the penalty by any more than 30 means the reproducer from
- T18730 won't compile even with reasonably small values of n. Instead
- it will run out of runs/ticks. This means to positively affect the reproducer
- a scaling <= 30 is required.
-* A scaling of >= 15 still causes a few very large regressions on some nofib benchmarks.
- (+80% for gc/fulsom, +90% for real/ben-raytrace, +20% for spectral/fibheaps)
-* A scaling of >= 25 showed no regressions on nofib. However it showed a number of
- (small) regression for compiler perf benchmarks.
-
-The end result is that we are settling for a scaling of 30, with a threshold of 2.
-This gives us minimal compiler perf regressions. No nofib runtime regressions and
-will still avoid this pattern sometimes. This is a "safe" default, where we err on
-the side of compiler blowup instead of risking runtime regressions.
-
-For cases where the default falls short the flag can be changed to allow more/less inlining as
-needed on a per-module basis.
-
--}
-
-tryUnfolding :: Logger -> UnfoldingOpts -> Int -> Id -> Bool -> [ArgSummary] -> CallCtxt
- -> CoreExpr -> Bool -> Bool -> UnfoldingGuidance
- -> Maybe CoreExpr
-tryUnfolding logger opts !case_depth id lone_variable
- arg_infos cont_info unf_template
- is_wf is_exp guidance
- = case guidance of
- UnfNever -> traceInline logger opts id str (text "UnfNever") Nothing
-
- UnfWhen { ug_arity = uf_arity, ug_unsat_ok = unsat_ok, ug_boring_ok = boring_ok }
- | enough_args && (boring_ok || some_benefit || unfoldingVeryAggressive opts)
- -- See Note [INLINE for small functions] (3)
- -> traceInline logger opts id str (mk_doc some_benefit empty True) (Just unf_template)
- | otherwise
- -> traceInline logger opts id str (mk_doc some_benefit empty False) Nothing
- where
- some_benefit = calc_some_benefit uf_arity
- enough_args = (n_val_args >= uf_arity) || (unsat_ok && n_val_args > 0)
-
- UnfIfGoodArgs { ug_args = arg_discounts, ug_res = res_discount, ug_size = size }
- | unfoldingVeryAggressive opts
- -> traceInline logger opts id str (mk_doc some_benefit extra_doc True) (Just unf_template)
- | is_wf && some_benefit && small_enough
- -> traceInline logger opts id str (mk_doc some_benefit extra_doc True) (Just unf_template)
- | otherwise
- -> traceInline logger opts id str (mk_doc some_benefit extra_doc False) Nothing
- where
- some_benefit = calc_some_benefit (length arg_discounts)
- extra_doc = vcat [ text "case depth =" <+> int case_depth
- , text "depth based penalty =" <+> int depth_penalty
- , text "discounted size =" <+> int adjusted_size ]
- -- See Note [Avoid inlining into deeply nested cases]
- depth_treshold = unfoldingCaseThreshold opts
- depth_scaling = unfoldingCaseScaling opts
- depth_penalty | case_depth <= depth_treshold = 0
- | otherwise = (size * (case_depth - depth_treshold)) `div` depth_scaling
- adjusted_size = size + depth_penalty - discount
- small_enough = adjusted_size <= unfoldingUseThreshold opts
- discount = computeDiscount arg_discounts res_discount arg_infos cont_info
-
- where
- mk_doc some_benefit extra_doc yes_or_no
- = vcat [ text "arg infos" <+> ppr arg_infos
- , text "interesting continuation" <+> ppr cont_info
- , text "some_benefit" <+> ppr some_benefit
- , text "is exp:" <+> ppr is_exp
- , text "is work-free:" <+> ppr is_wf
- , text "guidance" <+> ppr guidance
- , extra_doc
- , text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"]
-
- ctx = log_default_dump_context (logFlags logger)
- str = "Considering inlining: " ++ showSDocOneLine ctx (ppr id)
- n_val_args = length arg_infos
-
- -- some_benefit is used when the RHS is small enough
- -- and the call has enough (or too many) value
- -- arguments (ie n_val_args >= arity). But there must
- -- be *something* interesting about some argument, or the
- -- result context, to make it worth inlining
- calc_some_benefit :: Arity -> Bool -- The Arity is the number of args
- -- expected by the unfolding
- calc_some_benefit uf_arity
- | not saturated = interesting_args -- Under-saturated
- -- Note [Unsaturated applications]
- | otherwise = interesting_args -- Saturated or over-saturated
- || interesting_call
- where
- saturated = n_val_args >= uf_arity
- over_saturated = n_val_args > uf_arity
- interesting_args = any nonTriv arg_infos
- -- NB: (any nonTriv arg_infos) looks at the
- -- over-saturated args too which is "wrong";
- -- but if over-saturated we inline anyway.
-
- interesting_call
- | over_saturated
- = True
- | otherwise
- = case cont_info of
- CaseCtxt -> not (lone_variable && is_exp) -- Note [Lone variables]
- ValAppCtxt -> True -- Note [Cast then apply]
- RuleArgCtxt -> uf_arity > 0 -- See Note [RHS of lets]
- DiscArgCtxt -> uf_arity > 0 -- Note [Inlining in ArgCtxt]
- RhsCtxt NonRecursive
- -> uf_arity > 0 -- See Note [RHS of lets]
- _other -> False -- See Note [Nested functions]
-
-
-{- Note [RHS of lets]
-~~~~~~~~~~~~~~~~~~~~~
-When the call is the argument of a function with a RULE, or the RHS of a let,
-we are a little bit keener to inline (in tryUnfolding). For example
- f y = (y,y,y)
- g y = let x = f y in ...(case x of (a,b,c) -> ...) ...
-We'd inline 'f' if the call was in a case context, and it kind-of-is,
-only we can't see it. Also
- x = f v
-could be expensive whereas
- x = case v of (a,b) -> a
-is patently cheap and may allow more eta expansion.
-
-So, in `interesting_call` in `tryUnfolding`, we treat the RHS of a
-/non-recursive/ let as not-totally-boring. A /recursive/ let isn't
-going be inlined so there is much less point. Hence the (only reason
-for the) RecFlag in RhsCtxt
-
-Note [Unsaturated applications]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-When a call is not saturated, we *still* inline if one of the
-arguments has interesting structure. That's sometimes very important.
-A good example is the Ord instance for Bool in Base:
-
- Rec {
- $fOrdBool =GHC.Classes.D:Ord
- @ Bool
- ...
- $cmin_ajX
-
- $cmin_ajX [Occ=LoopBreaker] :: Bool -> Bool -> Bool
- $cmin_ajX = GHC.Classes.$dmmin @ Bool $fOrdBool
- }
-
-But the defn of GHC.Classes.$dmmin is:
-
- $dmmin :: forall a. GHC.Classes.Ord a => a -> a -> a
- {- Arity: 3, HasNoCafRefs, Strictness: SLL,
- Unfolding: (\ @ a $dOrd :: GHC.Classes.Ord a x :: a y :: a ->
- case @ a GHC.Classes.<= @ a $dOrd x y of wild {
- GHC.Types.False -> y GHC.Types.True -> x }) -}
-
-We *really* want to inline $dmmin, even though it has arity 3, in
-order to unravel the recursion.
-
-
-Note [Things to watch]
-~~~~~~~~~~~~~~~~~~~~~~
-* { y = I# 3; x = y `cast` co; ...case (x `cast` co) of ... }
- Assume x is exported, so not inlined unconditionally.
- Then we want x to inline unconditionally; no reason for it
- not to, and doing so avoids an indirection.
-
-* { x = I# 3; ....f x.... }
- Make sure that x does not inline unconditionally!
- Lest we get extra allocation.
-
-Note [Nested functions]
-~~~~~~~~~~~~~~~~~~~~~~~
-At one time we treated a call of a non-top-level function as
-"interesting" (regardless of how boring the context) in the hope
-that inlining it would eliminate the binding, and its allocation.
-Specifically, in the default case of interesting_call we had
- _other -> not is_top && uf_arity > 0
-
-But actually postInlineUnconditionally does some of this and overall
-it makes virtually no difference to nofib. So I simplified away this
-special case
-
-Note [Cast then apply]
-~~~~~~~~~~~~~~~~~~~~~~
-Consider
- myIndex = __inline_me ( (/\a. <blah>) |> co )
- co :: (forall a. a -> a) ~ (forall a. T a)
- ... /\a.\x. case ((myIndex a) |> sym co) x of { ... } ...
-
-We need to inline myIndex to unravel this; but the actual call (myIndex a) has
-no value arguments. The ValAppCtxt gives it enough incentive to inline.
-
-Note [Inlining in ArgCtxt]
-~~~~~~~~~~~~~~~~~~~~~~~~~~
-The condition (arity > 0) here is very important, because otherwise
-we end up inlining top-level stuff into useless places; eg
- x = I# 3#
- f = \y. g x
-This can make a very big difference: it adds 16% to nofib 'integer' allocs,
-and 20% to 'power'.
-
-At one stage I replaced this condition by 'True' (leading to the above
-slow-down). The motivation was test eyeball/inline1.hs; but that seems
-to work ok now.
-
-NOTE: arguably, we should inline in ArgCtxt only if the result of the
-call is at least CONLIKE. At least for the cases where we use ArgCtxt
-for the RHS of a 'let', we only profit from the inlining if we get a
-CONLIKE thing (modulo lets).
-
-Note [Lone variables] See also Note [Interaction of exprIsWorkFree and lone variables]
-~~~~~~~~~~~~~~~~~~~~~ which appears below
-The "lone-variable" case is important. I spent ages messing about
-with unsatisfactory variants, but this is nice. The idea is that if a
-variable appears all alone
-
- as an arg of lazy fn, or rhs BoringCtxt
- as scrutinee of a case CaseCtxt
- as arg of a fn ArgCtxt
-AND
- it is bound to a cheap expression
-
-then we should not inline it (unless there is some other reason,
-e.g. it is the sole occurrence). That is what is happening at
-the use of 'lone_variable' in 'interesting_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, provided
-the RHS is "expandable" (see Note [exprIsExpandable] in GHC.Core.Utils) the
-context can ``see'' the unfolding of the variable (e.g. case or a
-RULE) so there's no gain.
-
-However, watch out:
-
- * Consider this:
- foo = \n. [n]) {-# INLINE foo #-}
- bar = foo 20 {-# INLINE bar #-}
- 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.
-
- So the non-inlining of lone_variables should only apply if the
- unfolding is regarded as expandable; because that is when
- exprIsConApp_maybe looks through the unfolding. Hence the "&&
- is_exp" in the CaseCtxt branch of interesting_call
-
- * 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
-
-Note [Interaction of exprIsWorkFree and lone variables]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The lone-variable test says "don't inline if a case expression
-scrutinises a lone variable whose unfolding is cheap". It's very
-important that, under these circumstances, exprIsConApp_maybe
-can spot a constructor application. So, for example, we don't
-consider
- let x = e in (x,x)
-to be cheap, and that's good because exprIsConApp_maybe doesn't
-think that expression is a constructor application.
-
-In the 'not (lone_variable && is_wf)' test, I used to test is_value
-rather than is_wf, which was utterly wrong, because the above
-expression responds True to exprIsHNF, which is what sets is_value.
-
-This kind of thing can occur if you have
-
- {-# INLINE foo #-}
- foo = let x = e in (x,x)
-
-which Roman did.
-
-
--}
-
-computeDiscount :: [Int] -> Int -> [ArgSummary] -> CallCtxt
- -> Int
-computeDiscount arg_discounts res_discount arg_infos cont_info
-
- = 10 -- Discount of 10 because the result replaces the call
- -- so we count 10 for the function itself
-
- + 10 * length actual_arg_discounts
- -- Discount of 10 for each arg supplied,
- -- because the result replaces the call
-
- + total_arg_discount + res_discount'
- where
- actual_arg_discounts = zipWith mk_arg_discount arg_discounts arg_infos
- total_arg_discount = sum actual_arg_discounts
-
- mk_arg_discount _ TrivArg = 0
- mk_arg_discount _ NonTrivArg = 10
- mk_arg_discount discount ValueArg = discount
-
- res_discount'
- | LT <- arg_discounts `compareLength` arg_infos
- = res_discount -- Over-saturated
- | otherwise
- = case cont_info of
- BoringCtxt -> 0
- CaseCtxt -> res_discount -- Presumably a constructor
- ValAppCtxt -> res_discount -- Presumably a function
- _ -> 40 `min` res_discount
- -- ToDo: this 40 `min` res_discount doesn't seem right
- -- for DiscArgCtxt it shouldn't matter because the function will
- -- get the arg discount for any non-triv arg
- -- for RuleArgCtxt we do want to be keener to inline; but not only
- -- constructor results
- -- for RhsCtxt I suppose that exposing a data con is good in general
- -- And 40 seems very arbitrary
- --
- -- res_discount can be very large when a function returns
- -- constructors; 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 avoid inlining large functions that return
- -- constructors into contexts that are simply "interesting"
=====================================
compiler/ghc.cabal.in
=====================================
@@ -322,6 +322,7 @@ Library
GHC.Core.Opt.SetLevels
GHC.Core.Opt.Simplify
GHC.Core.Opt.Simplify.Env
+ GHC.Core.Opt.Simplify.Inline
GHC.Core.Opt.Simplify.Iteration
GHC.Core.Opt.Simplify.Monad
GHC.Core.Opt.Simplify.Utils
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/98c19f810d4ca7477ee4cde70ad5266d7dfa341e
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/98c19f810d4ca7477ee4cde70ad5266d7dfa341e
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