Tidy: fix handling of ranges with non-zero lower bounds in undriven ranges check (MikePopoloski#1527)

Author: jameshanlon

tools/tidy/src/synthesis/UndrivenRange.cpp

@@ -42,7 +42,8 @@ struct UndrivenRangeVisitor : public TidyVisitor,
     void handle(const VariableSymbol& symbol) {
 
         // If the variable has a fixed range, then determine if any ranges are
-        // undriven.
+        // undriven. Note that driver bit ranges are zero indexed, so we need to
+        // offset them by the lower bound of the variable's range.
         if (symbol.getType().hasFixedRange()) {
             auto range = symbol.getType().getFixedRange();
 
@@ -62,11 +63,11 @@ struct UndrivenRangeVisitor : public TidyVisitor,
             }
 
             for (auto [driver, bounds] : drivers) {
-                if (bounds.first > current) {
-                    undriven.push_back({current, (int)bounds.first - 1});
+                if (bounds.first + start > current) {
+                    undriven.push_back({current, (int)bounds.first + start - 1});
                 }
 
-                current = std::max(current, (int)bounds.second + 1);
+                current = std::max(current, (int)bounds.second + start + 1);
             }
 
             if (current <= end) {

tools/tidy/tests/UndrivenRangeTest.cpp

@@ -85,3 +85,35 @@ endmodule
 
     CHECK(result);
 }
+
+TEST_CASE("Undriven range: ensure variables with non-zero lower bounds are handled correctly") {
+    std::string output;
+    auto result = runCheckTest("UndrivenRange", R"(
+typedef logic [28:3] data_t;
+module offset_bus(input data_t i_input,
+                  output data_t o_output);
+  assign o_output = i_input;
+endmodule
+    )",
+                               {}, &output);
+    CHECK(result);
+}
+
+TEST_CASE("Undriven range: ensure variables with non-zero lower bounds report the correct undriven "
+          "bits") {
+    std::string output;
+    auto result = runCheckTest("UndrivenRange", R"(
+typedef logic [28:3] data_t;
+module offset_bus(input data_t i_input,
+                  output data_t o_output);
+  assign o_output[28:4] = i_input;
+endmodule
+    )",
+                               {}, &output);
+    CHECK_FALSE(result);
+    CHECK("\n" + output == R"(
+source:4:33: warning: [SYNTHESIS-20] variable o_output has undriven bits: 3
+                  output data_t o_output);
+                                ^
+)");
+}