[TableGen] more helpful error message in MapTableEmitter

[nhaehnle]

Summary

This should speak for itself. I found it useful in tracking down a confusing error in .td files.

[ab]

LGTM.

utils/TableGen/CodeGenMapTable.cpp

347

Why not getAsString() ?

utils/TableGen/CodeGenMapTable.cpp

for (unsigned j = 0, endCF = ColFields->size();
       Init *ColFieldJ = ColFields->getElement(j);
       Init *CurInstrInit = CurInstr->getValue(ColFieldJ)->getValue();
       std::string CurInstrVal = CurInstrInit->getAsUnquotedString();
       Init *ColFieldJVallue = CurValueCol->getElement(j);
       MatchFound = (CurInstrVal == ColFieldJVallue->getAsUnquotedString());
     }
 
     if (MatchFound) {
-      if (MatchInstr) // Already had a match
+      if (MatchInstr) {
+        // Already had a match
         // Error if multiple matches are found for a column.
+        std::string KeyValueStr;
+        for (Init *Value : KeyValue) {
+          if (!KeyValueStr.empty())
+            KeyValueStr += ", ";
+          KeyValueStr += Value->getAsUnquotedString();
+        }
+
         PrintFatalError("Multiple matches found for `" + KeyInstr->getName() +
-              "', for the relation `" + InstrMapDesc.getName());
+              "', for the relation `" + InstrMapDesc.getName() + "', row fields [" +
+              KeyValueStr + "], column `" + CurValueCol->getAsString() + "'");
+      }
       MatchInstr = CurInstr;
     }
   }
   return MatchInstr;
 }
 
 //===----------------------------------------------------------------------===//
 // Emit one table per relation. Only instructions with a valid relation of a