Re: -O/-O2 causes program to run too slow

Yes, it got faster when compiled with -fno-state-hack. It also got faster when I added a second (useless) reference to cf (changing the definition of doProbs to cf `seq` replicateM_ m doProb) Also, when profiling, the number of entries into findChain decreased significantly after adding -fno-state-hack On Wed, Dec 10, 2014 at 10:49 AM, Simon Peyton Jones wrote:

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:ghc-devs-bounces@haskell.org] *On Behalf Of *David Spies *Sent:* 07 December 2014 19:44 *To:* ghc-devs@haskell.org *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...

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 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?