[MLIR][LLVM] Fold extract of constant

Author: wsmoses

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -1932,6 +1932,19 @@ OpFoldResult LLVM::ExtractValueOp::fold(FoldAdaptor adaptor) {
     getContainerMutable().set(extractValueOp.getContainer());
     return getResult();
   }
+
+  {
+    DenseElementsAttr constval;
+    matchPattern(getContainer().getOperand(), m_Constant(&constval));
+    if (constval && constval.getElementType() == getType()) {
+      if (constval.isSplat()) {
+        return constval.getSplatValue();
+      } else if (getPosition().size() == 1) {
+        return constval.getValues()[getPosition()[0]];
+      }
+    }
+  }
+
   auto insertValueOp = getContainer().getDefiningOp<InsertValueOp>();
   OpFoldResult result = {};
   while (insertValueOp) {

mlir/test/Dialect/LLVMIR/canonicalize.mlir

@@ -94,6 +94,26 @@ llvm.func @fold_extract_extractvalue(%arr: !llvm.struct<(i64, array<1 x ptr<1>>)
   llvm.return %b : !llvm.ptr<1>
 }
 
+// -----
+// CHECK-LABEL: fold_extract_const
+// CHECK-NOT: extractvalue
+// CHECK: llvm.mlir.constant(-5.0)
+llvm.func @fold_extract_const() -> f64 {
+  %a = llvm.mlir.constant(dense<[-8.900000e+01, 5.000000e-01]> : tensor<2xf64>) : !llvm.array<2 x f64>
+  %b = llvm.extractvalue %a[1] : !llvm.array<2 x f64>
+  llvm.return %b : f64
+}
+
+// -----
+// CHECK-LABEL: fold_extract_splat
+// CHECK-NOT: extractvalue
+// CHECK: llvm.mlir.constant(-8.9)
+llvm.func @fold_extract_const() -> f64 {
+  %a = llvm.mlir.constant(dense<[-8.900000e+01> : tensor<2xf64>) : !llvm.array<2 x f64>
+  %b = llvm.extractvalue %a[1] : !llvm.array<2 x f64>
+  llvm.return %b : f64
+}
+
 // -----
 
 // CHECK-LABEL: fold_bitcast