#11707: Don't desugar large lists with build -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: 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 bgamari): There is currently a GHC flag which controls whether `build` desugaring is used at all, `-fsimple-list-literals`. To roughly characterize the effect of `build` on allocation, I did a nofib against 3f60ce8751c860a2bd0ddbe87429136a9d97449b run with and without `-fsimple- list-literals`. The major allocation changes were, {{{ -------------------------------------------------------------------------------- Program Size Allocs Runtime Elapsed TotalMem -------------------------------------------------------------------------------- ansi +0.0% +10.8% 0.000 0.004 0.0% bspt +0.1% +2.7% 0.004 0.004 0.0% cacheprof +0.1% +3.3% 0.181 0.181 0.0% eliza -0.2% +0.8% 0.000 0.000 0.0% fluid -0.1% +0.7% 0.004 0.004 0.0% fulsom +0.0% +0.7% 0.125 0.125 -21.4% integrate +0.0% +152.6% 0.081 0.081 +7.4% lift +0.0% +4.5% 0.000 0.000 0.0% nucleic2 -0.3% +2.4% 0.024 0.024 0.0% parser -0.4% +0.6% 0.011 0.012 0.0% reptile -0.0% +0.1% 0.004 0.004 0.0% rewrite +0.1% +96.7% 0.007 0.012 0.0% (those tests with no change in allocations omitted) -------------------------------------------------------------------------------- Min -0.4% -0.2% -1.0% -2.0% -21.4% Max +0.1% +152.6% +0.3% +0.6% +7.4% Geometric Mean -0.0% +1.9% -0.2% -0.2% -0.2% }}} There are a few things to note here: * Allocations, if they change at all, increase without `build` fusion, as we would expect * A `fulsom` had its total memory usage dramatically reduced without `build` fusion; this may be worth further investigation. * Runtime didn't appreciably change in any case * Binary sizes don't change On the other hand, if I compile `T9961` with `-fsimple-list-literals`, I see a dramatic reduction in compiler allocations (from 745044392 to 518841472, about 30%). Unfortunately it's not easy to measure the runtime cost of list construction with this test. I'll try to write up a simple `Criterion` benchmark to do this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11707#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler