[clang] [HLSL] Add HLSLAttributedResourceType (PR #106181)

[Justin Bogner via cfe-commits]

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@@ -6155,6 +6155,63 @@ class BTFTagAttributedType : public Type, public llvm::FoldingSetNode {
   }
 };
 
+class HLSLAttributedResourceType : public Type, public llvm::FoldingSetNode {
+public:
+  struct Attributes {
+    // This is where data gathered from HLSL resource attributes are stored,
+    // such as resource class, is_rov, dimension, is_array, is_feedback
+    // or is_multisample. The values will be accessed individually via fields
+    // on the embedded struct. The Data alias is used for the AST type
+    //  serialization.
+    union {
+      struct {
+        uint8_t ResourceClass; // maps to llvm::dxil::ResourceClass
+        uint8_t IsROV : 1;
+        // FIXME: add additional resource properties here
+      };
+      unsigned Data;
+    };
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bogner wrote:

Unfortunately, type punning through a union is undefined behaviour in C++ ([ISO/IEC 14882:2017 [class.union]](https://timsong-cpp.github.io/cppwp/n4659/class.union)), but [better explained on cppreference](https://en.cppreference.com/w/cpp/language/union#Member_lifetime)). So while most compilers do happen to do what you're hoping for here, we can't actually do this this way.

I think FoldingSetNode does fine with a series of `AddBoolean`, so we can presumably keep these as bitfields to save space in memory but add each individually in `Profile` without it causing too much trouble.

https://github.com/llvm/llvm-project/pull/106181