From 4905b83a2d448c65ccced385343d4e8124548a3b Mon Sep 17 00:00:00 2001
From: Simon Marlow <marlowsd@gmail.com>
Date: Wed, 18 Nov 2015 16:42:24 +0000
Subject: 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
---
 compiler/utils/Outputable.hs | 4 ++++
 1 file changed, 4 insertions(+)

(limited to 'compiler/utils/Outputable.hs')

diff --git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index cda74755ca..c3bdf5eb79 100644
--- a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ -94,6 +94,7 @@ import Util
 import Platform
 import Pretty           ( Doc, Mode(..) )
 import Panic
+import GHC.Serialized
 
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
@@ -811,6 +812,9 @@ instance Outputable a => Outputable (SCC a) where
    ppr (AcyclicSCC v) = text "NONREC" $$ (nest 3 (ppr v))
    ppr (CyclicSCC vs) = text "REC" $$ (nest 3 (vcat (map ppr vs)))
 
+instance Outputable Serialized where
+    ppr (Serialized the_type bytes) = int (length bytes) <+> ptext (sLit "of type") <+> text (show the_type)
+
 {-
 ************************************************************************
 *                                                                      *
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