Allow users to ignore optimization changes

* Add a new flag, `-fignore-optim-changes`, allowing them to avoid
  recompilation if the only changes are to the `-O` level or to
  flags controlling optimizations.

* When `-fignore-optim-changes` is *off*, recompile when optimization
  flags (e.g., `-fno-full-laziness`) change. Previously, we ignored
  these unconditionally when deciding whether to recompile a module.

Reviewers: austin, bgamari, simonmar

Reviewed By: simonmar

Subscribers: duog, carter, simonmar, rwbarton, thomie

GHC Trac Issues: #13604

Differential Revision: https://phabricator.haskell.org/D4123

1 files changed, 9 insertions, 3 deletions

diff --git a/ghc/Main.hs b/ghc/Main.hs
index 74b017a489..b720dea3db 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -179,10 +179,16 @@ main' postLoadMode dflags0 args flagWarnings = do
       -- can be overriden from the command-line
       -- XXX: this should really be in the interactive DynFlags, but
       -- we don't set that until later in interactiveUI
-      dflags2  | DoInteractive <- postLoadMode = imp_qual_enabled
-               | DoEval _      <- postLoadMode = imp_qual_enabled
+      -- We also set -fignore-optim-changes and -fignore-hpc-changes,
+      -- which are program-level options. Again, this doesn't really
+      -- feel like the right place to handle this, but we don't have
+      -- a great story for the moment.
+      dflags2  | DoInteractive <- postLoadMode = def_ghci_flags
+               | DoEval _      <- postLoadMode = def_ghci_flags
                | otherwise                     = dflags1
-        where imp_qual_enabled = dflags1 `gopt_set` Opt_ImplicitImportQualified
+        where def_ghci_flags = dflags1 `gopt_set` Opt_ImplicitImportQualified
+                                       `gopt_set` Opt_IgnoreOptimChanges
+                                       `gopt_set` Opt_IgnoreHpcChanges
 
         -- The rest of the arguments are "dynamic"
         -- Leftover ones are presumably files