[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 +75,34 @@ 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;
+  // this will be properly calculated when writing it.
+  Header.ParameterOffset = 0;
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bogner wrote:

I think this wording might be a little bit too brief. How about "The parameter offset doesn't matter here - we recalculate it during serialization"

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