Having said that, if we really access a file during compilation, then I think it is *not* a transitive dependency by definition! Any file which is accessed during a build rule is a direct dependency.

Well, ok, then we need a new definition! Suppose

Module M imports A
A.hi mentions (in a type or unfolding) B.T
Hence, to find out about B.T we need to read B.hi

So you would say that B is a direct dependency of M. Fine. I’ll call B.hi a “deep dependency” of M, but A.hi is a “shallow dependency” of M.

But it’s very hard to predict deep dependencies. Whether M deep-depends on B.hi depends on

the contents of A.hi
the optimisation level at which M is compiled
perhaps even the contents of some apparently unrelated X.hi, directly imported by M, which influences the extent to which GHC’s optimiser dives into A’s exprots

So no ghc -M is going to do the job. To get the real answer you must compile M!

Happily,

we already list the deep dependencies in M.hi
and, as I mentioned earlier, if a deep dependency changes (such as B.hi) then GHC guarantees that a shallow dependency (such as A.hi) will change.

Does that help?

Simon

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Andrey Mokhov
Sent: 27 March 2019 15:06
To: David Eichmann <davide@well-typed.com>; Neil Mitchell <ndmitchell@gmail.com>
Cc: GHC developers <ghc-devs@haskell.org>
Subject: RE: Hadrian Transitive Dependencies

Hi David,

We had a discussion about this with Neil some time ago, and I think we had the following list of progressively more complex invariants for different types of build systems:

Non-cloud build systems: *all direct inputs must be declared*. If you miss a direct input dependency then a build may complete successfully but with an incorrect result.

Cloud build systems: *all direct inputs and direct outputs must be declared*. If you miss a direct output then a build may fail because the cloud will not be able to restore the corresponding output.

Cloud build systems with shallow (deferred) materialisation of build artefacts: *all transitive inputs and direct outputs must be declared*. Let’s say you’d like to download the resulting GHC binary directly, without materialising any intermediate artefacts. Then you’ll need to know GHC’s ultimate transitive inputs.

I think for now we are really keen to make Hadrian a cloud build system, but whether shallow builds are valuable enough is not clear. Maybe not. Therefore, I’d say we don’t need to track transitive inputs right now. Furthermore, if we were to track all transitive inputs, we would lose the desirable early cutoff property, which prevents rebuilding after adding a comment in a file on which a lot of other files transitively depend on.

> GHC is reading *.hi files that are not reported as dependencies by

> `ghc -M -include-pkg-deps`. This is because they are not direct, but transitive

> dependencies!

So, here I’m confused. If we read a file A when compiling a file B, then it’s by definition a direct dependency. Perhaps we just read too much? Maybe the solution is to switch to fine-grained `ghc -M` mode, to analyse import dependencies for a single module instead of doing it transitively, which I believe was discussed in a ticket some time ago? I can’t find this ticket, but I think Alp was looking into it at some point. Alp: do you remember it?

Thank you for all your work on Hadrian!

Cheers,

Andrey

From: David Eichmann [mailto:davide@well-typed.com]
Sent: 27 March 2019 12:54
To: Neil Mitchell <ndmitchell@gmail.com>; Andrey Mokhov <andrey.mokhov@newcastle.ac.uk>; GHC developers <ghc-devs@haskell.org>
Subject: Hadrian Transitive Dependencies

Hello Shake/Hadrian contributors and the like,

Recently I've been putting Hadrian's fsatrace linting feature to good use, tracking down missing dependencies in Hadrian. Ultimately, we want to use shake's cloud build / shared cache feature and ensure it works across CI builds. Unfortunately the feature isn't working smoothly with Hadrian: see #16295. This is very desirable to improve CI build times. It is my understanding that in order to get caching to work:

1. All accessed files must declared with `need` AND
2. All created files must be declared with `produces` (or be the target of the build rule)

Is my understanding correct? Or is there a weaker condition (perhaps only 2 is necessary)?

If I'm correct, this amounts to fixing all fsatrace lint errors. See here for a breakdown of lint errors / missing dependencies. A large portion of these are Haskell interface files (i.e. *.hi files). Before building a Haskell object file, dependencies are discovered via `ghc` using the `-M -include-pkg-deps` options. Unfortunately, shake's fsatrace linting complains about other *.hi files being accessed! For example when building `stage1/libraries/mtl/build/Control/Monad/RWS/Class.o` we get the following dependencies from ghc:

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : libraries/mtl/Control/Monad/RWS/Class.hs

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Prelude.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Data/Monoid.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/RWS/Strict.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/RWS/Lazy.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Identity.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Maybe.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Except.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Error.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Class.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/Writer/Class.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/State/Class.hi

_build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o : _build/stage1/libraries/mtl/build/Control/Monad/Reader/Class.hi

And shake complains of the following missing deps:

_build/stage0/bin/ghc -Wall -hisuf hi -osuf o -hcsuf hc -static -hide-all-packages -no-user-package-db '-package-db _build/stage1/lib/package.conf.d' '-this-unit-id mtl-2.2.2' '-package-id base-4.13.0.0' '-package-id transformers-0.5.5.0' -i -i_build/stage1/libraries/mtl/build -i_build/stage1/libraries/mtl/build/autogen -ilibraries/mtl/. -Iincludes -I_build/generated -I_build/stage1/libraries/mtl/build -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/include -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/integer-gmp-1.0.2.0/include -I/home/david/ghc/_build/stage1/lib/x86_64-linux-ghc-8.9.20190325/rts-1.0/include -I_build/generated -optc-I_build/generated -optP-include -optP_build/stage1/libraries/mtl/build/autogen/cabal_macros.h -outputdir _build/stage1/libraries/mtl/build -Wnoncanonical-monad-instances -optc-Werror=unused-but-set-variable -optc-Wno-error=inline -c libraries/mtl/Control/Monad/RWS/Class.hs -o _build/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o -O2 -H32m -Wall -fno-warn-unused-imports -fno-warn-warnings-deprecations -Wcompat -Wnoncanonical-monad-instances -Wnoncanonical-monadfail-instances -XHaskell2010 -XSafe -ghcversion-file=/home/david/MEGA/File_Dump/Well-Typed/GHC/_nosync_git/ghc/_build/generated/ghcversion.h -Wno-deprecated-flags

Lint checking error - _build/HEAD_default/stage1/libraries/mtl/build/Control/Monad/RWS/Class.o - 22 values were used but not depended upon:

  Used:  _build/HEAD_default/stage0/lib/settings

  Used:  _build/HEAD_default/stage0/lib/platformConstants

  Used:  _build/HEAD_default/stage0/lib/llvm-targets

  Used:  _build/HEAD_default/stage0/lib/llvm-passes

  Used:  _build/HEAD_default/stage0/lib/package.conf.d/package.cache

  Used:  _build/HEAD_default/stage1/lib/package.conf.d/package.cache

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Float.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Base.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/ghc-prim-0.5.3/GHC/Types.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Maybe.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Writer/Lazy.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Writer/Strict.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/State/Lazy.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/State/Strict.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Reader.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/List.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/transformers-0.5.5.0/Control/Monad/Trans/Cont.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/ghc-prim-0.5.3/GHC/Tuple.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/IO/Exception.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/integer-gmp-1.0.2.0/GHC/Integer/Type.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/Data/Either.hi

  Used:  _build/HEAD_default/stage1/lib/x86_64-linux-ghc-8.9.20190325/base-4.13.0.0/GHC/Natural.hi

GHC is reading *.hi files that are not reported as dependencies by `ghc -M -include-pkg-deps`. This is because they are not direct, but transitive dependencies! How do we fix these lint errors (again with the goal of using shakes shared cache feature)? Some ideas:

* Wildly over approximate dependencies. This may be easier to implement but cause unneeded recompilation (when a false dependency changes). Either:
* `need` all dependent packages' interface files recursively as well as transitive dependencies reported by `ghc -M -include-pkg-deps` within the current package. OR
* OR `need` all transitive dependencies reported by `ghc -M -include-pkg-deps`. This will likely result in fewer dependencies but requires a bit more work in recovering dependent packages' dependency graphs.
* Perhaps transitive dependencies are not important for shared caching to work. Change shakes linting feature to allow (untracked?) transitive dependencies to be accessed.

Feed back would be greatly appreciated.

David Eichmann

--

David Eichmann, Haskell Consultant

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