[GHC] #16242: Hadrian is too aggressive in rebuilding

[GHC]

#16242: Hadrian is too aggressive in rebuilding
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                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:                    |
-------------------------------------+-------------------------------------

Comment (by snowleopard):

 > Edit a comment in one file.
 > [...]
 > What happens though is that stage-1 is rebuilt

 Wait, this doesn't sound right. Which file did you edit? Here is what
 should have happened: the corresponding compiler should have been rerun
 generating the same object file, at which point the rebuild should have
 stopped (we call this feature "early cutoff"). Are we hitting GHC non-
 determinism here, i.e. the resulting object file was different after
 editing a comment?

 > Restore the LAX_DEPENDENCIES feature. I don't know the specification of
 that feature, but my experience is that it just did the Right Thing

 Although I agree that having a flag that does the Right Thing would be
 useful, I'd like to point out that Hadrian provides a general mechanism
 for skipping the rebuild of any collection of files. `--freeze1` simply
 uses this mechanism by skipping everything in `stage0//*`. Here is the
 corresponding code fragment:

 https://gitlab.haskell.org/ghc/ghc/blob/10faf44d97095b2f8516b6d449d266f6889dcd70/hadrian/src/Main.hs#L30-31

 You can specify the files that should be ignored from the command line
 using `--skip` flag. For example, the equivalent of the flag `--freeze1`
 is `--skip=stage0//*`.

 > Somewhat separately, it might be nice to package up common idioms into
 smaller command lines.

 I think "build flavours" are the intended mechanism for common idioms,
 although typing `--flavour=...` is indeed inconvenient. We could add a
 shorthand `-f` for this.

 Separate scripts for such idioms sounds like a plausible approach too,
 although there is a risk that having multiple ways to do the same thing
 may be confusing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16242#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler