[llvm] Check for side effects when lowering target intrinsics, update NVVM ldu/ldg intrinsics with IntrWillReturn and test for DCE (PR #98968)

[Kevin McAfee via llvm-commits]

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@@ -5227,7 +5227,7 @@ void SelectionDAGBuilder::visitTargetIntrinsic(const CallInst &I,
   // definition.
   const Function *F = I.getCalledFunction();
   bool HasChain = !F->doesNotAccessMemory();
-  bool OnlyLoad = HasChain && F->onlyReadsMemory();
+  bool OnlyLoad = HasChain && F->onlyReadsMemory() && !I.mayHaveSideEffects();
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kalxr wrote:

Thanks for the comment Matt. This particular change is the main motivation for this patch, so I'd like to better understand potential issues with it. Could you elaborate on the behavior change and the failures it would cause? I haven't noticed any failures of that nature reflected in tests, but maybe there isn't coverage for it.

https://github.com/llvm/llvm-project/pull/98968