[GHC] #16295: Enable cached builds with Hadrian

[GHC]

#16295: Enable cached builds with Hadrian
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        Reporter:  snowleopard       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Build System      |              Version:  8.6.3
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by snowleopard:

Old description:

> As Neil is working on turning Shake into Cloud Shake [1], Shake has got a
> new feature of "locally cached builds", making it possible to reuse build
> artefacts from previous builds on the same machine. This means, in
> particular, that a clean build could be finished very quickly if you've
> built GHC before from the same/similar sources. Hopefully, this could
> also help us speed up GHC's CI some day.
>
> To enable cached builds, it is important to find and fix all instances of
> untracked dependencies in Hadrian, or at least record them using the
> `produces` feature of Shake [2], because otherwise Shake has no idea that
> these files are actually needed and should therefore be stored in the
> cache. Here are a couple of tickets related to untracked dependencies:
>
> https://ghc.haskell.org/trac/ghc/ticket/16271
> https://ghc.haskell.org/trac/ghc/ticket/16272
>
> I am currently working this and hope to produce a patch soon.
>
> [1] See the Build Systems à la Carte paper:
> https://dl.acm.org/ft_gateway.cfm?id=3236774
>
> [2] http://hackage.haskell.org/package/shake/docs/Development-
> Shake.html#v:produces

New description:

 As Neil is working on turning Shake into Cloud Shake [1], Shake has got a
 new feature of "locally cached builds", making it possible to reuse build
 artefacts from previous builds on the same machine. This means, in
 particular, that a clean build could be finished very quickly if you've
 built GHC before from the same/similar sources. Hopefully, this could also
 help us speed up GHC's CI some day.

 To enable cached builds, it is important to find and fix all instances of
 untracked dependencies in Hadrian, or at least record them using the
 `produces` feature of Shake [2], because otherwise Shake has no idea that
 these files are actually needed and should therefore be stored in the
 cache. Here are a couple of tickets related to untracked dependencies:

 https://ghc.haskell.org/trac/ghc/ticket/16271

 https://ghc.haskell.org/trac/ghc/ticket/16272

 I am currently working this and hope to produce a patch soon. I marked
 this is a "bug", not a "feature request", since untracked dependencies are
 bugs, and they prevent us from using Shake's shiny new feature.

 [1] See the Build Systems à la Carte paper:
 https://dl.acm.org/ft_gateway.cfm?id=3236774

 [2] http://hackage.haskell.org/package/shake/docs/Development-
 Shake.html#v:produces

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16295#comment:1>
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