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

[Justin Bogner via llvm-commits]

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@@ -75,31 +75,32 @@ static bool parseRootConstants(LLVMContext *Ctx, mcdxbc::RootSignatureDesc &RSD,
   if (RootConstantNode->getNumOperands() != 5)
     return reportError(Ctx, "Invalid format for RootConstants Element");
 
-  mcdxbc::RootParameter NewParameter;
-  NewParameter.Header.ParameterType =
+  dxbc::RootParameterHeader Header;
+  Header.ParameterType =
       llvm::to_underlying(dxbc::RootParameterType::Constants32Bit);
 
   if (std::optional<uint32_t> Val = extractMdIntValue(RootConstantNode, 1))
-    NewParameter.Header.ShaderVisibility = *Val;
+    Header.ShaderVisibility = *Val;
   else
     return reportError(Ctx, "Invalid value for ShaderVisibility");
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bogner wrote:

It looks like a bug when the structure is created here but then only partially initialized. We should probably have a comment here, or even write a zero to it with a comment that it will be overwritten later.

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