[mlir][Vector] Lower vector.to_elements to LLVM (#145766)

Only elements with at least one use are lowered to `llvm.extractelement`
op.

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -1985,6 +1985,38 @@ struct VectorFromElementsLowering
   }
 };
 
+/// Conversion pattern for a `vector.to_elements`.
+struct VectorToElementsLowering
+    : public ConvertOpToLLVMPattern<vector::ToElementsOp> {
+  using ConvertOpToLLVMPattern::ConvertOpToLLVMPattern;
+
+  LogicalResult
+  matchAndRewrite(vector::ToElementsOp toElementsOp, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    Location loc = toElementsOp.getLoc();
+    auto idxType = typeConverter->convertType(rewriter.getIndexType());
+    Value source = adaptor.getSource();
+
+    SmallVector<Value> results(toElementsOp->getNumResults());
+    for (auto [idx, element] : llvm::enumerate(toElementsOp.getElements())) {
+      // Create an extractelement operation only for results that are not dead.
+      if (element.use_empty())
+        continue;
+
+      auto constIdx = rewriter.create<LLVM::ConstantOp>(
+          loc, idxType, rewriter.getIntegerAttr(idxType, idx));
+      auto llvmType = typeConverter->convertType(element.getType());
+
+      Value result = rewriter.create<LLVM::ExtractElementOp>(loc, llvmType,
+                                                             source, constIdx);
+      results[idx] = result;
+    }
+
+    rewriter.replaceOp(toElementsOp, results);
+    return success();
+  }
+};
+
 /// Conversion pattern for vector.step.
 struct VectorScalableStepOpLowering
     : public ConvertOpToLLVMPattern<vector::StepOp> {
@@ -2035,7 +2067,8 @@ void mlir::populateVectorToLLVMConversionPatterns(
                VectorScalableInsertOpLowering, VectorScalableExtractOpLowering,
                MaskedReductionOpConversion, VectorInterleaveOpLowering,
                VectorDeinterleaveOpLowering, VectorFromElementsLowering,
-               VectorScalableStepOpLowering>(converter);
+               VectorToElementsLowering, VectorScalableStepOpLowering>(
+      converter);
 }
 
 void mlir::populateVectorToLLVMMatrixConversionPatterns(

mlir/test/Conversion/VectorToLLVM/vector-to-llvm-interface.mlir

@@ -2421,6 +2421,44 @@ func.func @from_elements_0d(%arg0: f32) -> vector<f32> {
 
 // -----
 
+//===----------------------------------------------------------------------===//
+// vector.to_elements
+//===----------------------------------------------------------------------===//
+
+// CHECK-LABEL: func.func @to_elements_no_dead_elements
+ // CHECK-SAME:     %[[A:.*]]: vector<4xf32>)
+ //      CHECK:   %[[C0:.*]] = llvm.mlir.constant(0 : i64) : i64
+ //      CHECK:   %[[ELEM0:.*]] = llvm.extractelement %[[A]][%[[C0]] : i64] : vector<4xf32>
+ //      CHECK:   %[[C1:.*]] = llvm.mlir.constant(1 : i64) : i64
+ //      CHECK:   %[[ELEM1:.*]] = llvm.extractelement %[[A]][%[[C1]] : i64] : vector<4xf32>
+ //      CHECK:   %[[C2:.*]] = llvm.mlir.constant(2 : i64) : i64
+ //      CHECK:   %[[ELEM2:.*]] = llvm.extractelement %[[A]][%[[C2]] : i64] : vector<4xf32>
+ //      CHECK:   %[[C3:.*]] = llvm.mlir.constant(3 : i64) : i64
+ //      CHECK:   %[[ELEM3:.*]] = llvm.extractelement %[[A]][%[[C3]] : i64] : vector<4xf32>
+ //      CHECK:   return %[[ELEM0]], %[[ELEM1]], %[[ELEM2]], %[[ELEM3]] : f32, f32, f32, f32
+func.func @to_elements_no_dead_elements(%a: vector<4xf32>) -> (f32, f32, f32, f32) {
+  %0:4 = vector.to_elements %a : vector<4xf32>
+  return %0#0, %0#1, %0#2, %0#3 : f32, f32, f32, f32
+}
+
+// -----
+
+// CHECK-LABEL: func.func @to_elements_dead_elements
+ // CHECK-SAME:     %[[A:.*]]: vector<4xf32>)
+ //  CHECK-NOT:   llvm.mlir.constant(0 : i64) : i64
+ //      CHECK:   %[[C1:.*]] = llvm.mlir.constant(1 : i64) : i64
+ //      CHECK:   %[[ELEM1:.*]] = llvm.extractelement %[[A]][%[[C1]] : i64] : vector<4xf32>
+ //  CHECK-NOT:   llvm.mlir.constant(2 : i64) : i64
+ //      CHECK:   %[[C3:.*]] = llvm.mlir.constant(3 : i64) : i64
+ //      CHECK:   %[[ELEM3:.*]] = llvm.extractelement %[[A]][%[[C3]] : i64] : vector<4xf32>
+ //      CHECK:   return %[[ELEM1]], %[[ELEM3]] : f32, f32
+func.func @to_elements_dead_elements(%a: vector<4xf32>) -> (f32, f32) {
+  %0:4 = vector.to_elements %a : vector<4xf32>
+  return %0#1, %0#3 : f32, f32
+}
+
+// -----
+
 //===----------------------------------------------------------------------===//
 // vector.step
 //===----------------------------------------------------------------------===//