#14827: Recognize when inlining would create a join point -------------------------------------+------------------------------------- Reporter: ersetzen | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: JoinPoints Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ersetzen): Posting into this issue feels somewhat like necromancy but I found a similar issue and the cause is actually quite simple: {{{ main :: IO () main = print foo {-# INLINE[1] foo #-} foo :: Int foo = bar 3 {-# INLINE[~1] bar #-} bar :: Int -> Int bar i = i + 1 }}} We inline bar into foo but not into the unfolding of foo: {{{ -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0} foo [InlPrag=INLINE[1] (sat-args=0)] :: Int [LclId, Unf=Unf{Src=InlineStable, TopLvl=True, Value=False, ConLike=False, WorkFree=False, Expandable=False, Guidance=ALWAYS_IF(arity=0,unsat_ok=False,boring_ok=False) Tmpl= bar (GHC.Types.I# 3#)}] foo = GHC.Types.I# 4# }}} Laterwe inline the unfolding of foo which still references bar - but the phase for inling bar is over. {{{ main _s26f= ... case bar (GHC.Types.I# 3#) of { GHC.Types.I# ww3_a26u -> ... }}} So bar never gets inlined at the use side even though everything along the way had inline pragmas. There isn't really a single decision that was wrong but the result is very unintuitive and can break fusion. A compiler warning for this type of phase collision might be worthwhile if it is easily doable? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14827#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler