[TableGen] Add an option to SubtargetFeature to disable setting the field to the maximum value

Author: wangpc-pp

llvm/docs/WritingAnLLVMBackend.rst

@@ -1772,7 +1772,8 @@ is assumed to be a bool and only one SubtargetFeature should refer to it.
 Otherwise, it is assumed to be an integer. The integer value may be the name
 of an enum constant. If multiple features use the same integer field, the
 field will be set to the maximum value of all enabled features that share
-the field.
+the field if `SetMaxValue` are set to true (by default), or we will set the
+field to the last specified value.
 
 .. code-block:: text
 
@@ -1783,6 +1784,7 @@ the field.
     string Value = v;
     string Desc = d;
     list<SubtargetFeature> Implies = i;
+    bit SetMaxValue = true;
   }
 
 In the ``Sparc.td`` file, the ``SubtargetFeature`` is used to define the

llvm/include/llvm/Target/Target.td

@@ -494,12 +494,17 @@ class SubtargetFeature<string n, string f, string v, string d,
   //
   // A value of "true" or "false" implies the field is a bool. Otherwise,
   // it is assumed to be an integer. the integer value may be the name of an
-  // enum constant. If multiple features use the same integer field, the
-  // field will be set to the maximum value of all enabled features that
-  // share the field.
-  //
+  // enum constant.
   string Value = v;
 
+  // SetMaxValue - Should we set the maximum value to the integer field.
+  //
+  // If multiple features use the same integer field, the field will be set to
+  // the maximum value of all enabled features that share the field if `SetMaxValue`
+  // are set to true (by default), or we will set the field to the last specified
+  // value.
+  bit SetMaxValue = true;
+
   // Desc - Feature description.  Used by command line (-mattr=) to display help
   // information.
   //

llvm/utils/TableGen/SubtargetEmitter.cpp

@@ -1902,13 +1902,14 @@ void SubtargetEmitter::parseFeaturesFunction(raw_ostream &OS) {
     StringRef Instance = R->getName();
     StringRef Value = R->getValueAsString("Value");
     StringRef FieldName = R->getValueAsString("FieldName");
+    bool SetMaxValue = R->getValueAsBit("SetMaxValue");
 
-    if (Value == "true" || Value == "false")
+    if (Value == "true" || Value == "false" || !SetMaxValue)
       OS << "  if (Bits[" << Target << "::" << Instance << "]) " << FieldName
          << " = " << Value << ";\n";
     else
       OS << "  if (Bits[" << Target << "::" << Instance << "] && " << FieldName
-         << " < " << Value << ") " << FieldName << " = " << Value << ";\n";
+         << " != " << Value << ") " << FieldName << " = " << Value << ";\n";
   }
 
   OS << "}\n";