[GHC] #16069: Proposal to resolve some confusion about stages in Hadrian

[GHC]

#16069: Proposal to resolve some confusion about stages in Hadrian
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The current situation with hadrian is that

 1. We build libraries required to build the stage2 compiler with stage1
 and place them in `_build/stage1`.
 2. The `stage2` compiler is built with `stage1` and then placed in
 `_build/stage1`.
 3. Executables built using the `stage2` compiler such as haddock then also
 use the libraries built by the `stage1` compiler even though they are
 being built with the `stage2` compiler.

 However, this is problematic because when you want to build a `stage3`
 compiler, we want to actually build the libraries again using the `stage2`
 compiler and place them in `_build/stage2`. If we follow the existing
 logic for executables built using the `stage2` compiler then in order to
 build a `stage3` compiler we would just reuse all the libraries from
 `_build/stage1` which seems incorrect.

 The proposal is simply thus:

 1. When building an executable at stageN, we use libraries also built at
 stageN. Concretely, if we want to build `haddock`, a stage2 executable,
 then we also have to build all the libraries it needs with the `stage2`
 compiler and install them into a package database in `_build/stage2`.
 2. The definition of "build" can be configured. By default, in order to
 "build" a stage2 library we copy it from the stage1 database but it is
 configurable to rebuild it with the stage2 compiler as we would want to
 for stage3.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16069>
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