Index: flang/lib/Optimizer/CodeGen/CodeGen.cpp
===================================================================
--- flang/lib/Optimizer/CodeGen/CodeGen.cpp
+++ flang/lib/Optimizer/CodeGen/CodeGen.cpp
@@ -827,6 +827,35 @@
   }
 };
 
+/// Lower fir.len_param_index
+struct LenParamIndexOpConversion
+    : public FIROpConversion<fir::LenParamIndexOp> {
+  using FIROpConversion::FIROpConversion;
+
+  // FIXME: this should be specialized by the runtime target
+  mlir::LogicalResult
+  matchAndRewrite(fir::LenParamIndexOp lenp, OpAdaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    mlir::Type ity = lowerTy().indexType();
+    mlir::Type onty = lenp.getOnType();
+    // size of portable descriptor
+    const unsigned boxsize = 24; // FIXME
+    unsigned offset = boxsize;
+    // add the size of the rows of triples
+    if (auto arr = onty.dyn_cast<fir::SequenceType>())
+      offset += 3 * arr.getDimension();
+
+    // advance over some addendum fields
+    const unsigned addendumOffset{sizeof(void *) + sizeof(uint64_t)};
+    offset += addendumOffset;
+    // add the offset into the LENs
+    offset += 0; // FIXME
+    auto attr = rewriter.getI64IntegerAttr(offset);
+    rewriter.replaceOpWithNewOp<mlir::LLVM::ConstantOp>(lenp, ity, attr);
+    return success();
+  }
+};
+
 /// Lower `fir.gentypedesc` to a global constant.
 struct GenTypeDescOpConversion : public FIROpConversion<fir::GenTypeDescOp> {
   using FIROpConversion::FIROpConversion;
@@ -2179,14 +2208,15 @@
         EmboxProcOpConversion, ExtractValueOpConversion, FieldIndexOpConversion,
         FirEndOpConversion, HasValueOpConversion, GenTypeDescOpConversion,
         GlobalLenOpConversion, GlobalOpConversion, InsertOnRangeOpConversion,
-        InsertValueOpConversion, IsPresentOpConversion, LoadOpConversion,
-        NegcOpConversion, NoReassocOpConversion, MulcOpConversion,
-        SelectCaseOpConversion, SelectOpConversion, SelectRankOpConversion,
-        SelectTypeOpConversion, ShapeOpConversion, ShapeShiftOpConversion,
-        ShiftOpConversion, SliceOpConversion, StoreOpConversion,
-        StringLitOpConversion, SubcOpConversion, UnboxCharOpConversion,
-        UnboxProcOpConversion, UndefOpConversion, UnreachableOpConversion,
-        ZeroOpConversion>(typeConverter);
+        InsertValueOpConversion, IsPresentOpConversion,
+        LenParamIndexOpConversion, LoadOpConversion, NegcOpConversion,
+        NoReassocOpConversion, MulcOpConversion, SelectCaseOpConversion,
+        SelectOpConversion, SelectRankOpConversion, SelectTypeOpConversion,
+        ShapeOpConversion, ShapeShiftOpConversion, ShiftOpConversion,
+        SliceOpConversion, StoreOpConversion, StringLitOpConversion,
+        SubcOpConversion, UnboxCharOpConversion, UnboxProcOpConversion,
+        UndefOpConversion, UnreachableOpConversion, ZeroOpConversion>(
+        typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
Index: flang/test/Fir/convert-to-llvm-target.fir
===================================================================
--- flang/test/Fir/convert-to-llvm-target.fir
+++ flang/test/Fir/convert-to-llvm-target.fir
@@ -147,3 +147,34 @@
 // INT32:  %[[len:.*]] = llvm.extractvalue %[[box_char]][1 : i32] : !llvm.struct<(ptr<i32>, i32)>
 // INT32:  %{{.*}} = llvm.sext %0 : i32 to i64
 // INT32-NEXT:  llvm.return
+
+// -----
+
+// Test fir.len_param_index
+
+func @len_param_index() {
+  %0 = fir.len_param_index i, !fir.type<nolen{i:i32, f:f32, l:i64}>
+  %1 = fir.len_param_index f, !fir.type<nolen>
+  %3 = fir.len_param_index l, !fir.type<nolen>
+
+  %4 = fir.len_param_index l1, !fir.type<twolens(l1:i32, l2:i32){i:i32, f:f32, l:i64}>
+  %5 = fir.len_param_index l2, !fir.type<twolens>
+  %6 = fir.len_param_index l, !fir.type<twolens>
+
+  return
+
+// INT64-LABEL: llvm.func @len_param_index
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT64: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32-LABEL: llvm.func @len_param_index
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+// INT32: {{%.*}} = llvm.mlir.constant(40 : i64) : i64
+}