Running ghc-debug on ghc

[Matthew Pickering]

Hi,

I now have some simple instructions for running ghc-debug on GHC.

1. Cherry-pick 4be70967f1f1ab70cbe31aad8ae69aea87c6f4c4

commit 4be70967f1f1ab70cbe31aad8ae69aea87c6f4c4 (HEAD -> wip/ghc-with-debug)
Author: Matthew Pickering <matthewtpickering at gmail.com>
Date:   Fri Jan 8 11:26:17 2021 +0000

    Add support for ghc-debug to ghc executable

2. Build GHC
* Add the following to _build/hadrian.settings

```
stage1.*.ghc.hs.opts += -finfo-table-map -fdistinct-constructor-tables
```

* Build GHC as normal

```
./hadrian/build -j8
```

* The result is a ghc-debug enabled compiler

# Building a debugger

* Use the compiler you just built to build ghc-debug

```
cd ghc-debug
cabal update
cabal new-build debugger -w ../_build/stage1/bin/ghc
```

# Running the debugger

Modify `test/Test.hs` to implement the debugging thing you want to do. Perhaps
start with `p30`, which is a program to generate a profile.


* Start the process you want to debug
```
GHC_DEBUG_SOCKET=/tmp/ghc-debug build-cabal
```

* Start the debugger
```
cabal new-run debugger -w ...
```

* Open a ticket about the memory issue you find.

There is the start of some more documentation here -
http://ghc.gitlab.haskell.org/ghc-debug/docs.html

These instructions are also in the instructions.md file in the commit.

Cheers,

Matt