8.2.1 changes to iface/FlagChecker.hs

George Colpitts george.colpitts at gmail.com
Thu Aug 3 19:37:23 UTC 2017

I reported this in https://ghc.haskell.org/trac/ghc/ticket/13604

On Thu, Aug 3, 2017 at 4:32 PM Evan Laforge <qdunkan at gmail.com> wrote:

> My program uses the GHC API to provide a REPL, and upgrading to 8.2.1
> broke that... well, just made it really slow.  Where it used to
> quickly load the compiled files, it now wants to recompile everything
> as bytecode which makes 1s startup time go to 1m.  The reason was it
> now thinks flags have changed.  I tracked it down to changes in
> FlagChecker.hs, documented with
> https://ghc.haskell.org/trac/ghc/ticket/11798 and
> https://ghc.haskell.org/trac/ghc/ticket/10923, which add -fhpc and -O
> to the tracked flags.  -fhpc means loading tests in ghci can never
> work, because ghci rejects the -fhpc flag, and likewise -O means ghci
> can never load optimized files for the same reason.
> It seems like the easiest solution might be to just have ghci not
> reject -fhpc and -O.  Of course we could also revert those changes,
> but they're justified in the trac tickets.
> Of course this means I'm loading optimized code together with bytecode
> compiled code and maybe that's a bad thing (or for tests, -fhpc with
> non-hpc).  But I've been doing this for something like 7 years and
> it's been fine.
> Unrelatedly, I was hoping compiles would get faster, but they seem a
> bit slower, from around 3m20s to around 4m :(  I can track down
> per-file timing changes if it would be interesting to someone.
> Also thanks to whoever put trac tickets in the comments, it makes it
> really easy to find out why a change was made.
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