[llvm] [NVPTX] Lower LLVM masked vector loads and stores to PTX (PR #159387)

[Princeton Ferro via llvm-commits]

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@@ -3410,10 +3500,56 @@ SDValue NVPTXTargetLowering::LowerVASTART(SDValue Op, SelectionDAG &DAG) const {
                       MachinePointerInfo(SV));
 }
 
+static std::pair<MemSDNode *, uint32_t>
+convertMLOADToLoadWithUsedBytesMask(MemSDNode *N, SelectionDAG &DAG) {
+  SDValue Chain = N->getOperand(0);
+  SDValue BasePtr = N->getOperand(1);
+  SDValue Mask = N->getOperand(3);
+  SDValue Passthru = N->getOperand(4);
+
+  SDLoc DL(N);
+  EVT ResVT = N->getValueType(0);
+  assert(ResVT.isVector() && "Masked vector load must have vector type");
+  // While we only expect poison passthru vectors as an input to the backend,
+  // when the legalization framework splits a poison vector in half, it creates
+  // two undef vectors, so we can technically expect those too.
+  assert((Passthru.getOpcode() == ISD::POISON ||
+          Passthru.getOpcode() == ISD::UNDEF) &&
+         "Passthru operand expected to be poison or undef");
+
+  // Extract the mask and convert it to a uint32_t representing the used bytes
+  // of the entire vector load
+  uint32_t UsedBytesMask = 0;
+  uint32_t ElementSizeInBits = ResVT.getVectorElementType().getSizeInBits();
+  assert(ElementSizeInBits % 8 == 0 && "Unexpected element size");
+  uint32_t ElementSizeInBytes = ElementSizeInBits / 8;
+  uint32_t ElementMask = (1u << ElementSizeInBytes) - 1u;
+
+  for (SDValue Op : reverse(Mask->ops())) {
+    // We technically only want to do this shift for every
+    // iteration *but* the first, but in the first iteration NewMask is 0, so
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Prince781 wrote:

Fix comment?

https://github.com/llvm/llvm-project/pull/159387