Commit cad6d468 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Use eta-expansion to ensure that INLINE things have their expected arity

See Note [Eta-expanding INLINE things] in DsBinds

This is to fix a performance bug that Roman was encountering.
parent 1106d279
......@@ -29,6 +29,7 @@ import CoreSyn -- lots of things
import CoreSubst
import MkCore
import CoreUtils
import CoreArity ( etaExpand )
import CoreUnfold
import CoreFVs
......@@ -318,7 +319,13 @@ dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds)
------------------------
makeCorePair :: Id-> Arity -> CoreExpr -> (Id, CoreExpr)
makeCorePair gbl_id arity rhs
= (addInline gbl_id arity rhs, rhs)
| isInlinePragma (idInlinePragma gbl_id)
-- Add an Unfolding for an INLINE (but not for NOINLINE)
-- And eta-expand the RHS; see Note [Eta-expanding INLINE things]
= (gbl_id `setIdUnfolding` mkInlineRule InlSat rhs arity,
etaExpand arity rhs)
| otherwise
= (gbl_id, rhs)
------------------------
type AbsBindEnv = VarEnv ([TyVar], Id, Id, [LSpecPrag])
......@@ -354,18 +361,27 @@ dictArity dicts = count isId dicts
lookupArity :: IdEnv Arity -> Id -> Arity
lookupArity ar_env id = lookupVarEnv ar_env id `orElse` 0
addInline :: Id -> Arity -> CoreExpr -> Id
addInline id arity rhs
| isInlinePragma (idInlinePragma id)
-- Add an Unfolding for an INLINE (but not for NOINLINE)
= id `setIdUnfolding` mkInlineRule InlSat rhs arity
| otherwise
= id
\end{code}
Nested arities
~~~~~~~~~~~~~~
Note [Eta-expanding INLINE things]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
foo :: Eq a => a -> a
{-# INLINE foo #-}
foo x = ...
If (foo d) ever gets floated out as a common sub-expression (which can
happen as a result of method sharing), there's a danger that we never
get to do the inlining, which is a Terribly Bad thing given that the
user said "inline"!
To avoid this we pre-emptively eta-expand the definition, so that foo
has arity 2 (one for the Eq and one for x); and that in turn should
mean that (foo d) is a PAP and we don't share it.
Note [Nested arities]
~~~~~~~~~~~~~~~~~~~~~
For reasons that are not entirely clear, method bindings come out looking like
this:
......
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