clock net aliases: added comment

Signed-off-by: Alessandro Comodi <[email protected]>

Author: acomodi

vpr/src/timing/read_sdc.cpp

@@ -914,6 +914,17 @@ class SdcParseCallback : public sdcparse::Callback {
             for (tatum::DomainId domain : tc_.clock_domains()) {
                 const auto& clock_name = tc_.clock_domain_name(domain);
 
+                // Clock net aliases are built  when reading the input circuit file.
+                // These aliases represent only real clock net names.
+                //
+                // If the SDC contains virtual clocks, the name of these does not
+                // appear in the net aliases data structure, therefore there is no
+                // need to iterate through the vector and a direct regex match can
+                // be applied.
+                //
+                // Furthermore, a virtual clock name would cause an error as there
+                // is no net associated with that when getting the net aliases from
+                // the netlist.
                 if (tc_.is_virtual_clock(domain)) {
                     if (std::regex_match(clock_name, clock_regex)) {
                         found = true;