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

1 files changed, 3 insertions, 1 deletions

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