[llvm] [NVPTX] Add Custom ABI (abi_preserve) support via function attributes (PR #227499)

[Akshay Deodhar via llvm-commits]

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@@ -104,6 +104,34 @@ When compiled, the PTX kernel functions are callable by host-side code.
   of the number of thread blocks. This attribute is only allowed for kernel
   functions and requires `nvvm.reqntid` and `nvvm.cluster_dim` attributes.
 
+`"nvvm.preserve_n_data"="<n>"`
+
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akshayrdeodhar wrote:

That is a fair argument, other frontends will want to use syntax that matches the PTX spec.

https://github.com/llvm/llvm-project/pull/227499