Re: [Haskell-cafe] GHC: Policy on -O flags?

`-O2` is not a very rationally considered flag from what I understand. It only enables `-fspec-constr` and `-fliberate-case`. The later also triggers another simplification pass. `-fspec-constr` is quite limited as it only works in the definition module. I have never trusted it to optimise my code. I have seen `-fliberate-case` achieve something but more often the improvement comes from the extra simplification pass. Therefore I wouldn't try to read the tea leaves too closely. There are probably flags in `-O` which affect compile time more but have less benefit. Matt On Tue, Aug 27, 2019 at 5:13 PM Brandon Allbery wrote:

I think at first you just give it a -f flag, and let experience determine whether it should be part of -O or -O2.

On Tue, Aug 27, 2019 at 12:10 PM Andreas Klebinger wrote:

...

Hello ghc-devs and haskell users.

I'm looking for opinions on when an optimization should be enabled by default.

-O is currently the base line for an optimized build. -O2 adds around 10-20% compile time for a few % (around 2% if I remember correctly) in performance for most things.

The question is now if I implement a new optimization, making code R% faster but slowing down the compiler down by C% at which point should an optimization be:

* Enabled by default (-O) * Enabled only at -O2 * Disabled by default

Cheap always beneficial things make sense for -O Expensive optimizations which add little make sense for -O2

But where exactly is the line here? How much compile time is runtime worth?

If something slows down the compiler by 1%/2%/5% and speeds up code by 0.5%/1%/2% which combinations make sense for -O, -O2?

Can there even be a good policy with the -O/-O2 split?

Personally I generally want code to either: * Typecheck/Run at all (-O0, -fno-code, repl) * Not blow through all my RAM when adding a few Ints while developing: -O ? * Make a reasonable tradeoff between runtime/compiletime: -O ? * Give me all you got: -O2 (-O99999)

The use case for -O0 is rather clear, so is -O2. But what do people consider the use case for -O

What trade offs seem acceptable to you as a user of GHC?

Is it ok for -O to become slower for faster runtimes? How much slower? Should all new improvements which might slow down compilation be pushed to -O2?

Or does an ideal solution add new flags? Tell me what do you think.

Cheers, Andreas Klebinger

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-- brandon s allbery kf8nh allbery.b@gmail.com _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.