Debugging GHC with GHCi

Ben Gamari ben at
Mon Jan 9 04:51:02 UTC 2017

Thomas Jakway <tjakway at> writes:

> I want to be able to load certain GHC modules in interpreted mode in 
> ghci so I can set breakpoints in them.  I have tests in the testsuite 
> that are compiled by inplace/bin/ghc-stage2 with -package ghc.  I can 
> load the tests with ghc-stage2 --interactive -package ghc but since ghc 
> is compiled I can only set breakpoints in the tests themselves.  Loading 
> the relevant files by passing them as absolute paths to :l loads them 
> but ghci doesn't stop at the breakpoints placed in them (I'm guessing 
> because ghci doesn't realize that the module I just loaded is meant to 
> replace the compiled version in -package ghc).
> So if I use
> inplace/bin/ghc-stage2 --interactive -package ghc mytest.hs
> then
> :l abs/path/to/AsmCodeGen.hs
> and set breakpoints, nothing happens.
Many of us would love to be able to load GHC into GHCi. Unfortunately,
we aren't currently in a position where this is possible. The only thing
standing in our way is the inability of GHC's interpreter to run modules
which use unboxed tuples. While there are a few modules within GHC which
use unboxed tuples, none of them are particularly interesting for
debugging purposes, so compiling them with -fobject-code should be fine.
In principle this could be accomplished by,

    {-# OPTIONS_GHC -fobject-code #-}

However, as explained in #10965, GHC sadly does not allow this. I spent
a bit of time tonight trying to see if I could convince GHC to first
manually build object code for the needed modules, and then load
bytecode for the rest. Unfortunately recompilation checking fought me at
every turn.

The current state of my attempt can be found here [1]. It would be great
if someone could pick it up. This will involve,

 * Working out how to convince the GHC to use the object code for
   utils/Encoding.o instead of recompiling

 * Identifying all of the modules which can't be byte-code compiled and
   add them to $obj_modules

 * Chassing down whatever other issues that might pop up along the way

I also wouldn't be surprised if you would need this GHC patch [2].


- Ben

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