diff --git a/mlir/lib/Dialect/StandardOps/IR/Ops.cpp b/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
--- a/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
+++ b/mlir/lib/Dialect/StandardOps/IR/Ops.cpp
@@ -1441,12 +1441,18 @@
 
 OpFoldResult DimOp::fold(ArrayRef<Attribute> operands) {
   auto index = operands[1].dyn_cast_or_null<IntegerAttr>();
+  auto argTy = memrefOrTensor().getType();
 
-  // All forms of folding require a known index.
-  if (!index)
-    return {};
+  // All forms of folding require a known index or a known rank of 1 (in which
+  // case we can assume the index is 0 if the code is correct).
+  if (!index) {
+    auto shapedTy = argTy.dyn_cast<ShapedType>();
+    if (!shapedTy || !shapedTy.hasRank() || shapedTy.getRank() != 1)
+      return {};
+
+    index = IntegerAttr::get(operands[1].getType(), 0);
+  }
 
-  auto argTy = memrefOrTensor().getType();
   // Fold if the shape extent along the given index is known.
   if (auto shapedTy = argTy.dyn_cast<ShapedType>()) {
     // Folding for unranked types (UnrankedMemRefType, UnrankedTensorType) is
@@ -1502,6 +1508,10 @@
     return *(alloc.getDynamicSizes().begin() +
              memrefType.getDynamicDimIndex(unsignedIndex));
 
+  if (auto alloca = dyn_cast_or_null<AllocaOp>(definingOp))
+    return *(alloca.getDynamicSizes().begin() +
+             memrefType.getDynamicDimIndex(unsignedIndex));
+
   if (auto view = dyn_cast_or_null<ViewOp>(definingOp))
     return *(view.getDynamicSizes().begin() +
              memrefType.getDynamicDimIndex(unsignedIndex));
diff --git a/mlir/test/Dialect/Standard/canonicalize.mlir b/mlir/test/Dialect/Standard/canonicalize.mlir
--- a/mlir/test/Dialect/Standard/canonicalize.mlir
+++ b/mlir/test/Dialect/Standard/canonicalize.mlir
@@ -134,6 +134,22 @@
 
 // -----
 
+// Test case: Folding of dim((alloca(rank(%v)), %idx) -> rank(%v)
+// CHECK-LABEL: func @dim_of_1d_alloca_with_dynamic_size(
+//  CHECK-SAME:     %[[MEM:[0-9a-z]+]]: memref<*xf32>
+//  CHECK-NEXT:   %[[RANK:.*]] = rank %[[MEM]] : memref<*xf32>
+//  CHECK-NEXT:   return %[[RANK]] : index
+func @dim_of_1d_alloca_with_dynamic_size(%arg0: memref<*xf32>)
+    -> index {
+  %0 = rank %arg0 : memref<*xf32>
+  %1 = alloca(%0) : memref<?xindex>
+  %c0 = constant 0 : index
+  %2 = dim %1, %c0 : memref<?xindex>
+  return %2 : index
+}
+
+// -----
+
 // Test case: Folding of dim(memref_reshape %v %shp, %idx) -> load %shp[%idx]
 // CHECK-LABEL: func @dim_of_memref_reshape(
 //  CHECK-SAME:     %[[MEM:[0-9a-z]+]]: memref<*xf32>,