• Simon Marlow's avatar
    Remote GHCi, -fexternal-interpreter · 4905b83a
    Simon Marlow authored
    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
    4905b83a
all.T 14.8 KB