Don't track free variables in STG syntax by default

Summary:
Currently, `CoreToStg` annotates `StgRhsClosure`s with their set of non-global
free variables.  This free variable information is only needed in the final
code generation step (i.e. `StgCmm.codeGen`), which leads to transformations
such as `StgCse` and `StgUnarise` having to maintain this information.

This is tiresome and unnecessary, so this patch introduces a trees-to-grow-like
approach that only introduces the free variable set into the syntax tree in the
code gen pass, along with a free variable analysis on STG terms to generate
that information.

Fixes #15754.

Reviewers: simonpj, osa1, bgamari, simonmar

Reviewed By: osa1

Subscribers: rwbarton, carter

GHC Trac Issues: #15754

Differential Revision: https://phabricator.haskell.org/D5324

2 files changed, 8 insertions, 4 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index d78b5984e1..2b1992274b 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -616,7 +616,7 @@ data GeneralFlag
    -- Except for uniques, as some simplifier phases introduce new
    -- variables that have otherwise identical names.
    | Opt_SuppressUniques
-   | Opt_SuppressStgFreeVars
+   | Opt_SuppressStgExts
    | Opt_SuppressTicks     -- Replaces Opt_PprShowTicks
    | Opt_SuppressTimestamps -- ^ Suppress timestamps in dumps
 
@@ -3166,7 +3166,7 @@ dynamic_flags_deps = [
                   setGeneralFlag Opt_SuppressTypeApplications
                   setGeneralFlag Opt_SuppressIdInfo
                   setGeneralFlag Opt_SuppressTicks
-                  setGeneralFlag Opt_SuppressStgFreeVars
+                  setGeneralFlag Opt_SuppressStgExts
                   setGeneralFlag Opt_SuppressTypeSignatures
                   setGeneralFlag Opt_SuppressTimestamps)
 
@@ -3976,7 +3976,9 @@ dFlagsDeps = [
   depFlagSpec' "ppr-ticks"              Opt_PprShowTicks
      (\turn_on -> useInstead "-d" "suppress-ticks" (not turn_on)),
   flagSpec "suppress-ticks"             Opt_SuppressTicks,
-  flagSpec "suppress-stg-free-vars"     Opt_SuppressStgFreeVars,
+  depFlagSpec' "suppress-stg-free-vars" Opt_SuppressStgExts
+     (useInstead "-d" "suppress-stg-exts"),
+  flagSpec "suppress-stg-exts"          Opt_SuppressStgExts,
   flagSpec "suppress-coercions"         Opt_SuppressCoercions,
   flagSpec "suppress-idinfo"            Opt_SuppressIdInfo,
   flagSpec "suppress-unfoldings"        Opt_SuppressUnfoldings,
diff --git a/compiler/main/HscMain.hs b/compiler/main/HscMain.hs
index 9dd750736a..837e903631 100644
--- a/compiler/main/HscMain.hs
+++ b/compiler/main/HscMain.hs
@@ -124,6 +124,7 @@ import CorePrep
 import CoreToStg        ( coreToStg )
 import qualified StgCmm ( codeGen )
 import StgSyn
+import StgFVs           ( annTopBindingsFreeVars )
 import CostCentre
 import ProfInit
 import TyCon
@@ -1426,10 +1427,11 @@ doCodeGen hsc_env this_mod data_tycons
               cost_centre_info stg_binds hpc_info = do
     let dflags = hsc_dflags hsc_env
 
+    let stg_binds_w_fvs = annTopBindingsFreeVars stg_binds
     let cmm_stream :: Stream IO CmmGroup ()
         cmm_stream = {-# SCC "StgCmm" #-}
             StgCmm.codeGen dflags this_mod data_tycons
-                           cost_centre_info stg_binds hpc_info
+                           cost_centre_info stg_binds_w_fvs hpc_info
 
         -- codegen consumes a stream of CmmGroup, and produces a new
         -- stream of CmmGroup (not necessarily synchronised: one