[llvm] [LLVM][Tablegen] Add Default arguments support for Intrinsics in TableGen (PR #198557)

[Rahul Joshi via llvm-commits]

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@@ -130,9 +130,23 @@ class Returned<ArgIndex idx> : IntrinsicProperty {
   int ArgNo = idx.Value;
 }
 
-// ImmArg - The specified argument must be an immediate.
-class ImmArg<ArgIndex idx> : IntrinsicProperty {
+// DefaultValue - A value carrier paired with ImmArg to declare a default for
+// a missing trailing argument. AutoUpgrade fills the default when an old
+// .bc / .ll file is loaded.
+class DefaultValue<int val> {
+  int Value = val;
+}
+
+// Sentinel — used as the default for ImmArg's optional DefaultValue parameter.
+def NoDefault : DefaultValue<0> {
+  let Value = ?;
+}
+
+// ImmArg - The specified argument must be an immediate. The optional
+// second template parameter declares a default value for AutoUpgrade.
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jurahul wrote:

nit: instead of "declares a default value" can this be "specifies a default value?

https://github.com/llvm/llvm-project/pull/198557