[llvm] [IR] Add `dead_on_return` attribute (PR #143271)

[Yingwei Zheng via llvm-commits]

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@@ -1741,6 +1741,20 @@ Currently, only the following parameter attributes are defined:
 
     This attribute cannot be applied to return values.
 
+``dead_on_return``
+    This attribute indicates that the memory pointed to by the argument is dead
+    upon normal function return.
+
+    It is similar to ``byval`` in the regard that it is generally used to pass
+    structs and arrays by value, and the memory is caller-invisible when the
+    function returns. However, unlike ``byval``, it is intended for ABIs where the
+    *callee* allocates the hidden copy, rather than the caller. Stores that would
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dtcxzyw wrote:

IIUC both `byval` and `dead_on_return` indicate the caller allocates the hidden copy: https://godbolt.org/z/79b4cvWfo

The only difference is that we allocate the copy explicitly on AArch64.


https://github.com/llvm/llvm-project/pull/143271