GHCi recompilation avoidance UI

[David Feuer]

I started digging back into this today, particularly considering Simon PJ's view
that it's a bit odd for optimization flags to imply -fobject-code (specifically
because we could potentially support optimization for the bytecode
interpreter some day). I'm left even more lost about exactly what we want.
I believe it's fairly clear that, as Simon M wrote,

> [W]e'll want at least -fignore-optim-changes to be the default, so that GHCi
> does the expected thing when you have compiled object files.

Based on Simon PJ's comment, I believe we want to *continue* to discard
optimization flags when -fobject-code is not enabled. As for my suggestion in (2),
I spent the last couple hours attempting to figure out what would be necessary
to allow :load *M to load a module  interpreted even when using -fobject-code,
but found myself utterly lost in the module loading logic. I see that the IIModule
constructor is deeply involved in this, but I haven't been able to figure out
where/how that interacts with -fobject-code to determine whether the module
will actually be loaded interpreted or compiled. Can someone give me a clue?

On Thursday, November 2, 2017 10:21:07 AM EST Simon Marlow wrote:
> On 31 October 2017 at 15:42, David Feuer <david at well-typed.com> wrote:
> 
> > Changes in GHC 8.2.1 lead to a lot of recompilation, because GHCi now
> > refuses to load optimized
> > code unless -fobject-code (and optimization flags) are enabled. I propose
> > the following slight
> > modification to https://ghc.haskell.org/trac/ghc/ticket/13604#comment:48
> >
> > 1. Optimization flags (except -O0) imply -fobject-code. This ensures that
> > GHC respects optimization flags regardless of --interactive.
> >
> > 2. Even when -fobject-code is on, :load *M will load M as bytecode. This
> > provides the "escape hatch" from -fobject-code that you need to use
> > debugging features, etc.
> >
> 
> Yes, I think this is probably what we want. I'm not sure how smooth it will
> be to implement though.
> 
> 
> > 3. New -fignore-optim-changes and -fignore-hpc-changes (​​Phab:D4123)
> > flags should enable users to put together object code and bytecode with
> > diverse optimization levels/options and HPC options while still updating
> > automatically based on source changes and whether profiling is enabled.
> >
> 
> As I mentioned on the diff, I think we'll want at least
> -fignore-optim-changes to be the default, so that GHCi does the expected
> thing when you have compiled object files.
> 
> Cheers
> Simon