(Apologies for the size of this patch, I couldn't make a smaller one
that was validate-clean and also made sense independently)
(Some of this code is derived from GHCJS.)
This commit adds support for running interpreted code (for GHCi and
TemplateHaskell) in a separate process. The functionality is
experimental, so for now it is off by default and enabled by the flag
Reaosns we want this:
- compiling Template Haskell code with -prof does not require building the code without -prof first
- when GHC itself is profiled, it can interpret unprofiled code, and the same applies to dynamic linking. We would no longer need to force -dynamic-too with TemplateHaskell, and we can load ordinary objects into a dynamically-linked GHCi (and vice versa).
- An unprofiled GHCi can load and run profiled code, which means it can use the stack-trace functionality provided by profiling without taking the performance hit on the compiler that profiling would entail.
Amongst other things; see
https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi for more details.
Notes on the implementation are in Note [Remote GHCi] in the new
module compiler/ghci/GHCi.hs. It probably needs more documenting,
feel free to suggest things I could elaborate on.
Things that are not currently implemented for -fexternal-interpreter:
- The GHCi debugger
- :set prog, :set args in GHCi
- recover in Template Haskell
- Redirecting stdin/stdout for the external process
These are all doable, I just wanted to get to a working validate-clean
I also haven't done any benchmarking yet. I expect there to be slight hit
to link times for byte code and some penalty due to having to
serialize/deserialize TH syntax, but I don't expect it to be a serious
problem. There's also lots of low-hanging fruit in the byte code
generator/linker that we could exploit to speed things up.