Merge pull request YosysHQ#4165 from phsauter/shiftadd-offset-fix

peepopt: handle offset too large in `shiftadd`

passes/pmgen/peepopt_shiftadd.pmg

@@ -103,8 +103,20 @@ code
 		new_a.append(old_a);
 	} else {
 		// data >> (...+c) transformed to data[MAX:c] >> (...)
-		new_a.append(old_a.extract_end(offset));
-
+		if(offset < GetSize(old_a)) { // some signal bits left?
+			new_a.append(old_a.extract_end(offset));
+		} else {
+			// warn user in case data is empty (no bits left)
+			std::string location = shift->get_src_attribute();
+			if (location.empty())
+				location = shift->name.str();
+			if(shift->type.in($shiftx))
+				log_warning("at %s: result of indexed part-selection is always constant (selecting from '%s' with index '%s + %d')\n", \
+							location.c_str(), log_signal(old_a), log_signal(var_signal), offset);
+			else
+				log_warning("at %s: result of shift operation is always constant (shifting '%s' by '%s + %d'-bits)\n", \
+							location.c_str(), log_signal(old_a), log_signal(var_signal), offset);
+		}
 	}
 
 	SigSpec new_b = {var_signal, SigSpec(State::S0, log2scale)};