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
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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