#13213: Lifting thunks out of thunks to reduce their size. -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This is a spin off of this thread: ticket:10980#comment:12 The idea is to add a transformation to STG (or Core) that replaces {{{ let t1 = let t2 = e1 in e2 in e3 }}} with {{{ let t2 = e1 t1 = e2 in e3 }}} Can we give an easy criterion when this transformation is beneficial? The problem is that this will increase allocation in the case when `t1` is not called, as it is more space efficient to pack the union of the free variables of `e1` and `e2` into one closure, instead of having one for the free variables of `e1` and one for those of `e2`. We could say “Do this transformation if the free variables of `e2` and `e1`” are disjoint. Then we’d allocate two extra words (one for the info table of `t2`, and one pointer to that in the closure for `t1`), but have some nice gains if `t1` is indeed called. But this rule would not fire in the [ticket:10980#comment:12 original example], because the `Num a` dictionary is also a free variable, which would now have to be copied into both closures! I guess one could try and see whether real-world programs benefit from this transformation, and how many shared free variables between `e1` and `e2` are heuristically ok. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13213 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler