Add a -trace-register-updates option

The idea is for this to play a similar debugging role to
-trace-registers, but it's less verbose (more comparable to
-trace-insns and -trace-loads) because it only prints messages when a
register is written to, and skips temporaries and condition code flags.

execution/exec_options.ml

@@ -182,6 +182,7 @@ let opt_check_read_operands = ref false
 let opt_check_write_operands = ref false
 let opt_fix_write_operands = ref false
 let opt_trace_registers = ref false
+let opt_trace_register_updates = ref false
 let opt_trace_segments = ref false
 let opt_trace_taint = ref false
 let opt_trace_unexpected = ref false

execution/exec_options.mli

@@ -160,6 +160,7 @@ val opt_check_read_operands : bool ref
 val opt_check_write_operands : bool ref
 val opt_fix_write_operands : bool ref
 val opt_trace_registers : bool ref
+val opt_trace_register_updates : bool ref
 val opt_trace_segments : bool ref
 val opt_trace_taint : bool ref
 val opt_trace_unexpected : bool ref

execution/exec_set_options.ml

@@ -493,6 +493,8 @@ let cmdline_opts =
      " Print symbolic memory regions");
     ("-trace-registers", Arg.Set(opt_trace_registers),
      " Print register contents");
+    ("-trace-register-updates", Arg.Set(opt_trace_register_updates),
+     " Print when registers are assigned to");
     ("-trace-setup", Arg.Set(opt_trace_setup),
      " Print progress of program loading");
     ("-trace-stmts", Arg.Set(opt_trace_stmts),

execution/fragment_machine.ml

@@ -2350,8 +2350,20 @@ struct
 			 jump (self#eval_label_exp l2)
 		 | V.Move(V.Temp((n,s,t) as v), e) ->
 		     let rhs = self#eval_int_exp_simplify e in
+		     let trace_eval () =
+		       Printf.printf "    %s <- %s\n" s (D.to_string_32 rhs)
+		     in
 		       if !opt_trace_eval then
-			 Printf.printf "    %s <- %s\n" s (D.to_string_32 rhs);
+			 trace_eval ()
+		       else if !opt_trace_register_updates then
+			 if String.sub s 0 1 = "T" then
+			   () (* skip updates to temps *)
+			 else if String.length s = 4 &&
+			   String.sub s 0 2 = "R_" &&
+			   String.sub s 3 1 = "F" then
+			     () (* skip updates to flags *)
+			 else
+			   trace_eval ();
 		       self#set_int_var v rhs;
 		       loop rest
 		 | V.Move(V.Mem(memv, idx_e, ty), rhs_e) ->