updates the Sub.compute_liveness function to handle SSA form (#1445)

Author: ivg

lib/bap/bap.mli

@@ -8529,7 +8529,20 @@ module Std : sig
         whole subroutine is the set of variables that are live at the
         [Graphs.Tid.start] node.
 
+        When the subroutine is in the SSA form then the phi-nodes have
+        the following semantics. For a phi-node at block [b0],
+        [x0 := phi([b1,x1; b2,x2;...;bN,xN])], the defined variable
+        [x0] is considered to be at the entry of [b0], i.e., [x0] is
+        live-in of [b0], and the variable [xi] is live-out of basic
+        block [bi], for [i > 0].
+
+        Informally, a phi-node defines the values on the corresponding
+        edges of the predecessors.
+
+
         @since 2.1
+        @since 2.5.0 supports SSA
+        @before 2.5.0 the subroutine must not be in the SSA form
     *)
     val compute_liveness : t -> (tid, Var.Set.t) Solution.t
 

lib/bap_sema/bap_sema_free_vars.ml

@@ -30,13 +30,26 @@ let blk_defs blk =
   Seq.fold ~init:Var.Set.empty  ~f:(fun defs def ->
       Set.add defs (Ir_def.lhs def))
 
+let update blk defs uses trans = Map.update trans blk ~f:(function
+    | None -> {defs; uses}
+    | Some had -> {
+        defs = Set.union had.defs defs;
+        uses = Set.union had.uses uses
+      })
+
 let block_transitions sub =
   Term.enum blk_t sub |>
   Seq.fold ~init:Tid.Map.empty ~f:(fun fs blk ->
-      Map.add_exn fs (Term.tid blk) {
-        defs = blk_defs blk;
-        uses = Ir_blk.free_vars blk;
-      })
+      let init = update
+          (Term.tid blk)
+          (blk_defs blk)
+          (Ir_blk.free_vars blk) fs in
+      Term.enum phi_t blk |>
+      Seq.fold ~init ~f:(fun init phi ->
+          let defs = Var.Set.singleton @@ Ir_phi.lhs phi in
+          Ir_phi.values phi |>
+          Seq.fold ~init ~f:(fun fs (src,exp) ->
+              update src defs (Exp.free_vars exp) fs)))
 
 let compute_liveness sub =
   let g = G.create sub in