Thanks, Andreas! I believe, the wiki modestly tries to suggest this here: https://ghc.haskell.org/trac/ghc/wiki/Building/Using#Workingonanoptimizedsta.... Maybe, it should be more insistent, e.g., by adding these numbers or moving the advice to a more visible place. Not sure. -- Best, Artem On Sun, 5 Aug 2018 at 01:58 Andreas Klebinger wrote:

I've wondered for a good while if using O2 on stage1 might be worth it.

So I did some measurements and it should be worth it for most cases.

For a single "quick" flavour build they are more or less on equal footing. If you rebuild stage2 multiple times reusing stage1 it will be faster. If you build stage2 with optimizations/profiling it will be faster.

Below are the timings using "time make -j9" for a quick build. I forgot to write down the seconds as I didn't expect them to be so close. But it is what it is.

Timings stage1 options O1 vs O2, quick build after make clean:

stage1 opt | time (wall) | time (user) -O1 | 13m | 53m -O2 | 13m | 51m

I've also run the numbers for a optimized stage2 compiler a while ago, where stage1 with O2 was faster. But I no longer have these numbers around.

So it seems safe to say one should use O2 if either: * stage2 is built with optimizations * you freeze stage1 and reuse it while working on stage2

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