Pick some low-hanging const fruit

Makes more of the code const-correct; there are still plenty of difficult-to-fix const problems left.
No behavior change expected.

tgt-stub/expression.c

@@ -231,7 +231,7 @@ static void show_property_expression(ivl_expr_t net, unsigned ind)
 {
       ivl_signal_t sig = ivl_expr_signal(net);
       const char* pnam = ivl_expr_name(net);
-      char*signed_flag = ivl_expr_signed(net)? "signed" : "unsigned";
+      const char*signed_flag = ivl_expr_signed(net)? "signed" : "unsigned";
 
       if (ivl_expr_value(net) == IVL_VT_REAL) {
 	    fprintf(out, "%*s<property base=%s, prop=%s, real>\n", ind, "",

tgt-vlog95/expr.c

@@ -401,7 +401,7 @@ static unsigned calc_can_skip_unsigned(ivl_expr_t oper1, ivl_expr_t oper2)
 static void emit_expr_binary(ivl_scope_t scope, ivl_expr_t expr, unsigned wid,
                              unsigned is_full_prec)
 {
-      char *oper = "<invalid>";
+      const char *oper = "<invalid>";
       ivl_expr_t oper1 = ivl_expr_oper1(expr);
       ivl_expr_t oper2 = ivl_expr_oper2(expr);
       unsigned can_skip_unsigned = calc_can_skip_unsigned(oper1, oper2);
@@ -976,7 +976,7 @@ static void emit_expr_ternary(ivl_scope_t scope, ivl_expr_t expr, unsigned wid,
 static void emit_expr_unary(ivl_scope_t scope, ivl_expr_t expr, unsigned wid,
                             unsigned is_full_prec)
 {
-      char *oper = "invalid";
+      const char *oper = "invalid";
       ivl_expr_t oper1 = ivl_expr_oper1(expr);
       switch (ivl_expr_opcode(expr)) {
 	case '-': oper = "-"; break;

tgt-vlog95/scope.c

@@ -24,7 +24,7 @@
 
 const char *func_rtn_name = 0;
 
-static char*get_time_const(int time_value)
+static const char*get_time_const(int time_value)
 {
       switch (time_value) {
 	case   2: return "100s";

tgt-vlog95/stmt.c

@@ -408,7 +408,8 @@ static void emit_assign_and_opt_opcode(ivl_scope_t scope, ivl_statement_t stmt,
                                        unsigned allow_opcode)
 {
       unsigned wid;
-      char opcode, *opcode_str;
+      char opcode;
+      const char *opcode_str;
 
       assert (ivl_statement_type(stmt) == IVL_ST_ASSIGN);
 // HERE: Do we need to calculate the width? The compiler should have already
@@ -941,7 +942,7 @@ static void emit_stmt_block_named(ivl_scope_t scope, ivl_statement_t stmt)
 
 static void emit_stmt_case(ivl_scope_t scope, ivl_statement_t stmt)
 {
-      char *case_type;
+      const char *case_type;
       unsigned idx, default_case, count = ivl_stmt_case_count(stmt);
       switch (ivl_statement_type(stmt)) {
 	case IVL_ST_CASE:

tgt-vvp/draw_class.c

@@ -30,7 +30,7 @@ static void show_prop_type_vector(ivl_type_t ptype)
       unsigned packed_dimensions = ivl_type_packed_dimensions(ptype);
       assert(packed_dimensions < 2);
 
-      char*signed_flag = ivl_type_signed(ptype)? "s" : "";
+      const char*signed_flag = ivl_type_signed(ptype)? "s" : "";
       char code = data_type==IVL_VT_BOOL? 'b' : 'L';
 
       if (packed_dimensions == 0) {

tgt-vvp/draw_net_input.c

@@ -230,7 +230,7 @@ static void str_repeat(char*buf, const char*str, unsigned rpt)
  * If the drive strength is strong we can draw a C4<> constant as the
  * pull value, otherwise we need to draw a C8<> constant.
  */
-static char* draw_net_pull(ivl_net_logic_t lptr, ivl_drive_t drive, char*level)
+static char* draw_net_pull(ivl_net_logic_t lptr, ivl_drive_t drive, const char*level)
 {
       char*result;
       char tmp[32];

tgt-vvp/vvp_scope.c

@@ -1804,7 +1804,7 @@ static int sfunc_has_modpath_output(ivl_lpm_t lptr)
 static void draw_sfunc_output_def(ivl_lpm_t net, char type)
 {
       ivl_nexus_t nex = ivl_lpm_q(net);
-      char *suf = (type == 'd') ? "/d" : "";
+      const char *suf = (type == 'd') ? "/d" : "";
 
       switch (data_type_of_nexus(nex)) {
 	case IVL_VT_REAL:

vpi/sys_countdrivers.c

@@ -102,7 +102,7 @@ static PLI_INT32 sys_countdrivers_compiletf(ICARUS_VPI_CONST PLI_BYTE8 *name)
 
         /* The optional arguments must be variables. */
       for (arg_num = 2; arg_num < 7; arg_num += 1) {
-	    char *arg_name = NULL;
+	    const char *arg_name = NULL;
 	    switch (arg_num) {
 	        case 2: arg_name = "second"; break;
 	        case 3: arg_name = "third";  break;

vpi/sys_queue.c

@@ -587,7 +587,7 @@ static unsigned check_numeric_args(vpiHandle argv, unsigned count,
 	/* Check that the first count arguments are numeric. Currently
 	 * only three are needed/supported. */
       for (idx = 0; idx < count; idx += 1) {
-	    char *loc = NULL;
+	    const char *loc = NULL;
 	    vpiHandle arg = vpi_scan(argv);
 
 	      /* Get the name for this argument. */