flambda-backend: Re-enable -flambda2-join-points (#374)

Author: mshinwell

driver/main_args.ml

@@ -934,15 +934,15 @@ module Flambda2 = Clflags.Flambda2
 
 let mk_flambda2_join_points f =
   "-flambda2-join-points", Arg.Unit f,
-  Printf.sprintf "Propagate information from incoming edges at a join\n\
+  Printf.sprintf "Propagate information from all incoming edges to a join\n\
       \     point%s (Flambda 2 only)"
     (format_default Flambda2.Default.join_points)
 ;;
 
 let mk_no_flambda2_join_points f =
   "-no-flambda2-join-points", Arg.Unit f,
-  Printf.sprintf " Propagate information only from the fork point to\n\
-      \     a join point%s (Flambda 2 only)"
+  Printf.sprintf " Propagate information to a join point only if there are\n\
+      \     zero or one incoming edge(s)%s (Flambda 2 only)"
     (format_not_default Flambda2.Default.join_points)
 ;;
 

utils/clflags.ml

@@ -462,7 +462,7 @@ end
 module Flambda2 = struct
   module Default = struct
     let classic_mode = false
-    let join_points = true
+    let join_points = false
     let unbox_along_intra_function_control_flow = true
     let backend_cse_at_toplevel = false
     let cse_depth = 2
@@ -657,7 +657,7 @@ module Flambda2 = struct
 
   let o2_flags () =
     cse_depth := 2;
-    join_points := true;
+    join_points := false;
     unbox_along_intra_function_control_flow := true;
     Expert.fallback_inlining_heuristic := false;
     backend_cse_at_toplevel := false