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This PR implements HLSL's initialization list behvaior as specified in the draft language specifcation under [*Decl.Init.Agg*](https://microsoft.github.io/hlsl-specs/specs/hlsl.html#Decl.Init.Agg). This behavior is a bit unusual for C/C++ because intermediate braces in initializer lists are ignored and a whole array of additional conversions occur unintuitively to how initializaiton works in C. The implementaiton in this PR generates a valid C/C++ initialization list AST for the HLSL initializer so that there are no changes required to Clang's CodeGen to support this. This design will also allow us to use Clang's rewrite to convert HLSL initializers to valid C/C++ initializers that are equivalent. It does have the downside that it will generate often redundant accesses during codegen. The IR optimizer is extremely good at eliminating those so this will have no impact on the final executable performance. There is some opportunity for optimizing the initializer list generation that we could consider in subsequent commits. One notable opportunity would be to identify aggregate objects that occur in the same place in both initializers and do not require converison, those aggregates could be initialized as aggregates rather than fully scalarized. Closes #56067 --------- Co-authored-by: Finn Plummer <50529406+inbelic@users.noreply.github.com> Co-authored-by: Helena Kotas <hekotas@microsoft.com> Co-authored-by: Justin Bogner <mail@justinbogner.com>
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