Remote GHCi, -fexternal-interpreter

Summary:
(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
-fexternal-interpreter.

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
patch first.

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.

Test Plan:
* validate
* I've run parts of the test suite with
EXTRA_HC_OPTS=-fexternal-interpreter, notably tests/ghci and tests/th.
There are a few failures due to the things not currently implemented
(see above).

Reviewers: simonpj, goldfire, ezyang, austin, alanz, hvr, niteria, bgamari, gibiansky, luite

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1562

1 files changed, 26 insertions, 0 deletions

diff --git a/iserv/iserv-bin.cabal b/iserv/iserv-bin.cabal
new file mode 100644
index 0000000000..9dac158ebf
--- /dev/null
+++ b/iserv/iserv-bin.cabal
@@ -0,0 +1,26 @@
+Name: iserv-bin
+Version: 0.0
+Copyright: XXX
+License: BSD3
+-- XXX License-File: LICENSE
+Author: XXX
+Maintainer: XXX
+Synopsis: XXX
+Description:
+        XXX
+Category: Development
+build-type: Simple
+cabal-version: >=1.10
+
+Executable iserv
+    Default-Language: Haskell2010
+    Main-Is: Main.hs
+    C-Sources: iservmain.c
+    Build-Depends: array      >= 0.5 && < 0.6,
+                   base       >= 4   && < 5,
+                   unix       >= 2.7 && < 2.8,
+                   binary     >= 0.7 && < 0.9,
+                   bytestring >= 0.10 && < 0.11,
+                   containers >= 0.5 && < 0.6,
+                   deepseq    >= 1.4 && < 1.5,
+                   ghci