Interface Files with Core Definitions

This commit adds three new flags

* -fwrite-if-simplified-core: Writes the whole core program into an interface
  file
* -fbyte-code-and-object-code: Generate both byte code and object code
  when compiling a file
* -fprefer-byte-code: Prefer to use byte-code if it's available when
  running TH splices.

The goal for including the core bindings in an interface file is to be able to restart the compiler pipeline
at the point just after simplification and before code generation. Once compilation is
restarted then code can be created for the byte code backend.
This can significantly speed up
start-times for projects in GHCi. HLS already implements its own version of these extended interface
files for this reason.

Preferring to use byte-code means that we can avoid some potentially
expensive code generation steps (see #21700)

* Producing object code is much slower than producing bytecode, and normally you
  need to compile with `-dynamic-too` to produce code in the static and dynamic way, the
  dynamic way just for Template Haskell execution when using a dynamically linked compiler.

* Linking many large object files, which happens once per splice, can be quite
  expensive compared to linking bytecode.

And you can get GHC to compile the necessary byte code so
`-fprefer-byte-code` has access to it by using
`-fbyte-code-and-object-code`.

Fixes #21067

1 files changed, 5 insertions, 0 deletions

diff --git a/ghc/Main.hs b/ghc/Main.hs
index 45dd5fede1..16075284c0 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -209,6 +209,11 @@ main' postLoadMode units dflags0 args flagWarnings = do
         where def_ghci_flags = dflags1 `gopt_set` Opt_ImplicitImportQualified
                                        `gopt_set` Opt_IgnoreOptimChanges
                                        `gopt_set` Opt_IgnoreHpcChanges
+                                       -- Setting this by default has the nice effect that
+                                       -- -fno-code and --interactive falls back to interpreter rather than
+                                       -- object code but has little other effect unless you are also using
+                                       -- fat interface files.
+                                       `gopt_set` Opt_UseBytecodeRatherThanObjects
 
   logger1 <- getLogger
   let logger2 = setLogFlags logger1 (initLogFlags dflags2)