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Tutorial: XLS[cc] pipelined loops.

This tutorial is aimed at walking you through the implementation and synthesis into Verilog of a C++ function containing a pipelined loop. Pipelined loops are an automatic way of generating stateful logic that can often be more intuitive and software-like than using explicit state (eg via static).

C++ Source

Create a file named with the following contents.

template<typename T>
using InputChannel = __xls_channel<T, __xls_channel_dir_In>;
template<typename T>
using OutputChannel = __xls_channel<T, __xls_channel_dir_Out>;

class TestBlock {
    InputChannel<int> in;
    OutputChannel<int> out;

    #pragma hls_top
    void Run() {
    int sum = 0;
        #pragma hls_pipeline_unroll yes
        for(int i=0;i<4;++i) {
            sum +=;

Generate optimized XLS IR.

Use a combination of xlscc and opt_main to generate optimized XLS IR.

$ ./bazel-bin/xls/contrib/xlscc/xlscc \
  --block_from_class TestBlock --block_pb block.pb \
$ ./bazel-bin/xls/tools/opt_main --inline_procs >

The --inline_procs option is necessary to make pipelined loops synthesizable.

Examine the optimized IR should look like this, containing only one proc:

package my_package

file_number 1 "/usr/local/google/home/seanhaskell/tmp/"

chan in(bits[32], id=0, kind=streaming, ops=receive_only, flow_control=ready_valid, metadata="""""")
chan out(bits[32], id=1, kind=streaming, ops=send_only, flow_control=ready_valid, metadata="""""")

top proc TestBlock_proc(tkn: token, __for_1_proc_state__4: bits[1], __for_1_proc_state__5: bits[32], __for_1_proc_state__6: bits[32], __for_1_proc_activation__1: bits[1], __for_1_ctx_out_receive_holds_activation__1: bits[1], TestBlock_proc_activation__1: bits[1], after_all_186_holds_activation_0__1: bits[1], after_all_186_holds_activation_1__1: bits[1], after_all_84_holds_activation_0__1: bits[1], after_all_84_holds_activation_1__1: bits[1], after_all_84_holds_activation_2__1: bits[1], __for_1_ctx_in_receive_holds_activation__1: bits[1], init={1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0}) {
  next (after_all.354, __for_1_proc___first_tick_next_state, __for_1_proc_sum_next_state, __for_1_proc_i_next_state, __for_1_ctx_out_receive_activation_out, __for_1_ctx_out_receive_holds_activation_next__1, after_all_84_is_activated, after_all_186_holds_activation_0_next__1, after_all_186_holds_activation_1_next__1, after_all_84_holds_activation_0_next__1, after_all_84_holds_activation_1_next__1, after_all_84_holds_activation_2_next__1, __for_1_ctx_in_receive_holds_activation_next__1)

Perform code-generation into a pipelined Verilog block.

In this case, we will generate a single-stage pipeline without input and output flops. This will result in a module with a 32-bit increment adder along with 32-bit of state.

$ ./bazel-bin/xls/tools/codegen_main \
  --generator=pipeline \
  --delay_model="sky130" \
  --output_verilog_path=xls_counter.v \
  --module_name=xls_counter \
  --top=TestBlock_proc \
  --reset=rst \
  --reset_active_low=false \
  --reset_asynchronous=false \
  --reset_data_path=true \
  --pipeline_stages=1  \
  --flop_inputs=false \

Additional XLS[cc] examples.

For developers, it is possible to check if a specific feature is supported by checking for unit tests.