Just to check, when you say that “I found it was an instance of…” do you mean “I compiled with –fno-state-hack as the only change, and it got faster again”? Otherwise how would you know this was the cause?
Simon From: ghc-devs [mailto:[email protected]] On Behalf Of David Spies Sent: 07 December 2014 19:44 To: [email protected] Subject: Re: -O/-O2 causes program to run too slow Ok, so I found that it was an instance of this: https://ghc.haskell.org/trac/ghc/ticket/1168 and I read through this whole thread: https://www.haskell.org/pipermail/glasgow-haskell-users/2008-February/014259.html I don't understand the state-hack optimization. It's clearly not safe and I'm not convinced that it actually is an optimization. In what circumstances does the state-hack identify a single-entry function that can't be identified as single-entry by some other (safe) method? On Sun, Dec 7, 2014 at 10:52 AM, David Spies <[email protected]<mailto:[email protected]>> wrote: I have a program I wrote to submit for the Car Game problem on Kattis: https://open.kattis.com/problems/cargame but it runs over the 5-second time-limit I downloaded the test data and found that on GHC 7.8.3, if I switch from -O2 to -O0, it runs three times faster (almost certainly fast enough for Kattis to accept). Can someone tell me what's going on? Is this a bug?
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