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

[Artem Belevich via llvm-commits]

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@@ -5043,6 +5043,15 @@ def int_nvvm_prefetch_global_L2_evict_last: Intrinsic<[], [llvm_global_ptr_ty],
 def int_nvvm_prefetchu_L1 : Intrinsic<[], [llvm_ptr_ty],
   [IntrArgMemOnly, ReadOnly<ArgIndex<0>>, NoCapture<ArgIndex<0>>]>;
 
+// Intrinsics for discard
+def int_nvvm_discard_global_L2 : DefaultAttrsIntrinsic<[], 
+    [llvm_global_ptr_ty, llvm_i64_ty], [IntrArgMemOnly, ReadOnly<ArgIndex<0>>, 
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Artem-B wrote:

Discarding in-cache data that was not written back yet is equivalent to modifying memory. I.e. the read from the given address will return the data that's different than what was previously written. 

Another concern is that we will invalidate `[a .. a + (size - 1)]` I'm not quite sure whether LLVM tracks the amount of data accessed via a pointer. If it does, then we may need to apply `hasSideEffects` to let LLVM know that we're messing with more than the pointee-type-sized chunk of memory.

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