[llvm] [TableGen] Add `!instances` operator to get defined records (PR #129680)

[Rahul Joshi via llvm-commits]

================
@@ -1455,6 +1455,49 @@ const Init *TGParser::ParseOperation(Record *CurRec, const RecTy *ItemType) {
     return (ExistsOpInit::get(Type, Expr))->Fold(CurRec);
   }
 
+  case tgtok::XInstances: {
+    // Value ::= !instances '<' Type '>' '(' Regex? ')'
+    Lex.Lex(); // eat the operation.
+
+    const RecTy *Type = ParseOperatorType();
+    if (!Type)
+      return nullptr;
+
+    if (!consume(tgtok::l_paren)) {
+      TokError("expected '(' after type of !instances");
+      return nullptr;
+    }
+
+    // The Regex can be optional.
+    const Init *Regex;
+    if (Lex.getCode() != tgtok::r_paren) {
+      SMLoc RegexLoc = Lex.getLoc();
+      Regex = ParseValue(CurRec);
+
+      const auto *RegexType = dyn_cast<TypedInit>(Regex);
+      if (!RegexType) {
+        Error(RegexLoc, "expected string type argument in !instances operator");
+        return nullptr;
+      }
+
+      const auto *SType = dyn_cast<StringRecTy>(RegexType->getType());
+      if (!SType) {
+        Error(RegexLoc, "expected string type argument in !instances operator");
+        return nullptr;
+      }
+    } else {
+      // Use wildcard when Regex is not specified.
+      Regex = StringInit::get(Records, ".*");
----------------
jurahul wrote:

wondering if this case can he handled inside the Fold method instead and we use a null StringInit in the constructor. The only reason is that get `InstancesOpInit::getAsString` in this case will print something different than what was in the input (which maybe ok?).

https://github.com/llvm/llvm-project/pull/129680