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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
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