[clang] [Clang][CodeGen] Emit alignment information for indirect arguments (PR #201999)

[Sayan Sivakumaran via cfe-commits]

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@@ -3061,13 +3061,9 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
       //
       // This is different from indirect *not* byval, where the object
       // exists already, and the align attribute is purely
-      // informative.
+      // informative (but is still useful for optimization passes).
----------------
sivakusayan wrote:

Yes, if I'm reading this correctly we know this from this [chunk of code](https://github.com/llvm/llvm-project/blob/fe2f7e40994574dd5d7efa4da8be5d9237905416/clang/lib/CodeGen/CGCall.cpp#L5641) inside of `EmitCall()`. In particular, since we aren't byval we create the [aligned temporary here](https://github.com/llvm/llvm-project/blob/fe2f7e40994574dd5d7efa4da8be5d9237905416/clang/lib/CodeGen/CGCall.cpp#L5713). Godbolt [shows an example](https://godbolt.org/z/o3GYMn69Y) of this if we use the linked ticket's code sample, looking at the top of the unoptimized `qux`.

Let me add a better comment here, I'm not sure if there is a useful `assert` that I can add but I'll check.

https://github.com/llvm/llvm-project/pull/201999