I'm curious about this. I don't think I ever cared *that* much about the no-op time.

I cared a little a few weeks back and found ways to make hadrians no-op runtime ~25% shorter in relatively little time.
Little enough time to have a suspicion others also don't care all that much ;-) And there is still low hanging fruit left to pluck.

I would be more curious about experiences people have with packaging software using buck2. As that has been a pain point
that come up repeatedly over the years. That being said not sure if packagers would more happy to see hadrian go, or annoyed
to have to adjust how to package ghc yet again.

On 19/10/2026 16:35, Sebastian Graf via ghc-devs wrote:
Great!! FWIW, I would be more than happy to adjust to some inconveniences if no-ops become this cheap again.

Am Mo., 19. Okt. 2026 um 16:18 Uhr schrieb Simon Marlow <marlowsd@gmail.com>:
On Mon, 19 Oct 2026 at 15:12, Sebastian Graf <sgraf1337@gmail.com> wrote:
Hi Simon,

Thanks for doing this. If my suspicion is correct and the time it takes to do no-op rebuilds / single test runs is just half as slow as with hadrian, then this will improve the manual development experience on GHC a lot. Not that I'm building GHC much these days, much less so manually...

No-op rebuild:
  • Hadrian: 7.36s
  • Buck2: 0.02s
caveats apply, Hadrian is also checking the .cabal files for changes whereas with Buck2 you currently have to re-run ./generate-buck2.hs if you modify a .cabal file.

Cheers
Simon



Cheers,
Sebastian

Am Mo., 19. Okt. 2026 um 16:07 Uhr schrieb Simon Marlow via ghc-devs <ghc-devs@haskell.org>:
As I mentioned to a few people recently, I wanted to explore replacing Hadrian with Buck2. I've got a prototype to the point where it can build the stage2 compiler and all the libraries on Linux:

> buck2 run :stage2-ghc -- --interactive
GHCi, version 10.1.20261017: https://www.haskell.org/ghc/  :? for help
Using default language edition: GHC2024
ghci> 1+2
3
ghci>

This includes dynamic/static, all the RTS ways and profiling is allegedly working although I haven't tested it very much.

It's a LOT smaller than Hadrian - hard to say exactly how much because some of the code is in the shared projects cabal-buck2 and haskell-buck2, but there's about 1400 lines of Buck2 stuff in the ghc repo and nothing in the library submodules.

I've written up the design along with some notes on how to set it up and try it over here: https://github.com/simonmar/ghc/blob/buck2/README.buck2.md

Keen to hear what people think about this. Are there key metrics that we should measure to understand whether it's better than Hadrian? What are the workflow aspects that people find slow currently?

Cheers
Simon
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