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, 11 insertions, 3 deletions

diff --git a/compiler/main/Annotations.hs b/compiler/main/Annotations.hs
index a81ae80614..052b0615e7 100644
--- a/compiler/main/Annotations.hs
+++ b/compiler/main/Annotations.hs
@@ -12,7 +12,8 @@ module Annotations (
 
         -- * AnnEnv for collecting and querying Annotations
         AnnEnv,
-        mkAnnEnv, extendAnnEnvList, plusAnnEnv, emptyAnnEnv, findAnns,
+        mkAnnEnv, extendAnnEnvList, plusAnnEnv, emptyAnnEnv,
+        findAnns, findAnnsByTypeRep,
         deserializeAnns
     ) where
 
@@ -20,7 +21,7 @@ import Binary
 import Module           ( Module )
 import Name
 import Outputable
-import Serialized
+import GHC.Serialized
 import UniqFM
 import Unique
 
@@ -115,10 +116,17 @@ findAnns deserialize (MkAnnEnv ann_env)
   = (mapMaybe (fromSerialized deserialize))
     . (lookupWithDefaultUFM ann_env [])
 
+-- | Find the annotations attached to the given target as 'Typeable'
+--   values of your choice. If no deserializer is specified,
+--   only transient annotations will be returned.
+findAnnsByTypeRep :: AnnEnv -> CoreAnnTarget -> TypeRep -> [[Word8]]
+findAnnsByTypeRep (MkAnnEnv ann_env) target tyrep
+  = [ ws | Serialized tyrep' ws <- lookupWithDefaultUFM ann_env [] target
+    , tyrep' == tyrep ]
+
 -- | Deserialize all annotations of a given type. This happens lazily, that is
 --   no deserialization will take place until the [a] is actually demanded and
 --   the [a] can also be empty (the UniqFM is not filtered).
 deserializeAnns :: Typeable a => ([Word8] -> a) -> AnnEnv -> UniqFM [a]
 deserializeAnns deserialize (MkAnnEnv ann_env)
   = mapUFM (mapMaybe (fromSerialized deserialize)) ann_env
-