#9388: Narrow the scope of the notorious "state hack" -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by nomeata): I gave it a shot. The obvious place to make the demand analyzer believe that something of type `IO` is going to be called at most once is by adjusting `body_dmd` in `dmdAnalRhs`, similarly to `[Product demands for function body]`. The branch `wip/T9388` contains patches that remove the old state hack and introduce this one. On first glance, it seems to work. At least the bug in #10102 does not occur any more. Overall, it has a negative effect on performance (at least on bytes allocated): nofib’s `fibheaps` regresses by `16.7%`, `banner` by `12.4%` and `hpg` by `7.1%`, rewrite by `5.8%`. Others improve: `k-nucleotide` by `5%`. Geometric mean is a regression by `0.4%`. A quick diff of fibheap’s core shows large changes, which I did not investigate further. The branch validates, with the exception of a number of compiler performance benchmarks (T9203 haddock.Cabal haddock.base T5030 T5631 T9872c T9872a T5642 T9020 T3064 T9872d T9872b T1969), where allocation increases by up to 30%. So real world programs run better with the old state hack. In a way that is expected: Eta expansion is usually a good thing, even if cannot see that a the lambda is indeed one-shot. And even if it is not really one-shot, the benefits of eta-expansion might, in some cases, outweigh the cost of lost sharing. Only in a few (uncommon?) cases, e.g. replicate with a large number, it really hurts. I’m surprised that no test case starts to fail, and no known-to-fail test case suddenly passes; it seems that we have little coverage on the desired and/or unwanted effects of the state hack. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9388#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler