[llvm] [NFC] Refactoring MCDXBC to support out of order storage of root parameters (PR #137284)

[via llvm-commits]

================
@@ -9,18 +9,81 @@
 #include "llvm/BinaryFormat/DXContainer.h"
 #include <cstdint>
 #include <limits>
+#include <variant>
 
 namespace llvm {
 
 class raw_ostream;
 namespace mcdxbc {
 
-struct RootParameter {
+struct RootParameterInfo {
   dxbc::RootParameterHeader Header;
-  union {
-    dxbc::RootConstants Constants;
-    dxbc::RTS0::v2::RootDescriptor Descriptor;
-  };
+  size_t Location;
+
+  RootParameterInfo() = default;
+
+  RootParameterInfo(dxbc::RootParameterHeader H, size_t L)
+      : Header(H), Location(L) {}
+};
+
+using RootDescriptor = std::variant<dxbc::RTS0::v1::RootDescriptor,
+                                    dxbc::RTS0::v2::RootDescriptor>;
+using ParametersView = std::variant<const dxbc::RootConstants *,
+                                    const dxbc::RTS0::v1::RootDescriptor *,
+                                    const dxbc::RTS0::v2::RootDescriptor *>;
+struct RootParametersContainer {
+  SmallVector<RootParameterInfo> ParametersInfo;
+
+  SmallVector<dxbc::RootConstants> Constants;
+  SmallVector<RootDescriptor> Descriptors;
+
+  void addInfo(dxbc::RootParameterHeader H, size_t L) {
+    ParametersInfo.push_back(RootParameterInfo(H, L));
+  }
+
+  void addParameter(dxbc::RootParameterHeader H, dxbc::RootConstants C) {
+    addInfo(H, Constants.size());
+    Constants.push_back(C);
+  }
+
+  void addParameter(dxbc::RootParameterHeader H,
+                    dxbc::RTS0::v1::RootDescriptor D) {
+    addInfo(H, Descriptors.size());
+    Descriptors.push_back(D);
+  }
+
+  void addParameter(dxbc::RootParameterHeader H,
+                    dxbc::RTS0::v2::RootDescriptor D) {
+    addInfo(H, Descriptors.size());
+    Descriptors.push_back(D);
+  }
+
+  std::optional<ParametersView> getParameter(const RootParameterInfo *H) const {
+    switch (H->Header.ParameterType) {
----------------
joaosaffran wrote:

Optional is necessary here, because yaml2obj needs to be able to write invalid root signatures representations, since we used it as a testing tool. 

https://github.com/llvm/llvm-project/pull/137284