[llvm] [NVPTX] Add Intrinsics for discard.* (PR #128404)

[Artem Belevich via llvm-commits]

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@@ -671,6 +671,45 @@ level on which the priority is to be applied. The only supported value for the s
 For more information, refer to the PTX ISA
 `<https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-applypriority>`_.
 
+``llvm.nvvm.discard.*``'
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+.. code-block:: llvm
+
+  declare void  @llvm.nvvm.discard.global.L2(ptr addrspace(1) %global_ptr, i64 immarg)
+  declare void  @llvm.nvvm.discard.L2(ptr %ptr, i64 immarg)
+
+Overview:
+"""""""""
+
+The *effects* of the ``@llvm.nvvm.discard.L2*`` intrinsics are those of a non-atomic 
+non-volatile ``llvm.memset`` that writes ``undef`` to the destination 
+address range ``[%ptr, %ptr + immarg)``. 
+Subsequent reads from the address range may read ``undef`` until the memory is overwritten 
+with a different value.
+These operations *hint* the implementation that data in the L2 cache can be destructively 
+discarded without writing it back to memory. 
+The operand ``immarg`` is an integer constant that specifies the length in bytes of the 
+address range ``[%ptr, %ptr + immarg)`` to write ``undef`` into. 
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Artem-B wrote:

Per @gonzalobg comment the pointer must be aligned by 128. That's something we should mention in the doc.
Other than that, LGTM.

https://github.com/llvm/llvm-project/pull/128404