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

[Matt Arsenault 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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arsenm wrote:

The IR does not have a notion of non-memory side effects. The code previously only considers the attributes of the underlying function definition, which is what all other intrinsic code expects (and changes the opcode between INTRINSIC_WO_CHAIN/INTRINSIC_W_CHAIN/INTRINSIC_VOID). This now will swap the opcode and operand structure depending on whether the callsite has a memory(none) attribute 

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