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
---
 testsuite/tests/rts/LinkerUnload.hs | 4 ++--
 testsuite/tests/rts/T2615.hs        | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

(limited to 'testsuite/tests/rts')

diff --git a/testsuite/tests/rts/LinkerUnload.hs b/testsuite/tests/rts/LinkerUnload.hs
index 7e9d1dd38a..9d6b243256 100644
--- a/testsuite/tests/rts/LinkerUnload.hs
+++ b/testsuite/tests/rts/LinkerUnload.hs
@@ -16,5 +16,5 @@ loadPackages = do
     let dflags' = dflags { hscTarget = HscNothing
                          , ghcLink  = LinkInMemory }
     pkgs <- setSessionDynFlags dflags'
-    dflags <- getSessionDynFlags
-    liftIO $ Linker.linkPackages dflags pkgs
+    hsc_env <- getSession
+    liftIO $ Linker.linkPackages hsc_env pkgs
diff --git a/testsuite/tests/rts/T2615.hs b/testsuite/tests/rts/T2615.hs
index 53c2d13ceb..6a81185e9e 100644
--- a/testsuite/tests/rts/T2615.hs
+++ b/testsuite/tests/rts/T2615.hs
@@ -1,4 +1,4 @@
-import ObjLink
+import GHCi.ObjLink
 
 library_name = "libfoo_script_T2615.so" -- this is really a linker script
 
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