Debugging Compilation Bugs

These tips are directed towards compilation bugs. Before trying these, make sure your program produces the correct values with the Calyx Interpreter

Disabling Optimizations

The first step is disabling optimization passes and running the bare bones compilation pipeline.

To disable the passes, add the flag -p no-opt to compiler invocation:

  1. For the compiler: futil <filename> -p no-opt.
  2. For fud: fud ... -s futil.flags " -p no-opt".

If the output is still incorrect then one of the core compilation passes is incorrect. Our best bet at this point is to reduce the test file such that the output from the interpreter and the Calyx compiler still disagree and report the bug. We can use the waveform debugging to figure out which part of the compilation pipeline generates the incorrect result.

If the execution generates the right result, then one of the optimizations passes is incorrect. To identify which optimization pass is wrong, add back individual passes and see when the execution fails. The calyx/src/default_passes.rs file defines the compilation pipeline. Start by incrementally adding passes to this flag invocation:

-p validate -p remove-comb-groups -p <PASS 1> ... -p <PASS N> -p compile -p lower

Reducing Test Files

It is often possible to reduce the size of the example program that is generating incorrect results. In order to perform a reduction, we need to run the program twice, once with a "golden workflow" that we trust to generate the right result and once with the buggy workflow.

For example, if we've identified the problem to be in one of the Calyx passes, the "golden workflow" is running the program without the pass while the buggy workflow is running the program with the pass enabled. This case is so common that we've written a script that can run programs with different set of flags to the Calyx compiler and show the difference in the outputs after simulation.

The script is invoked as:

tools/flag-compare.sh <calyx program> <data>

By default, the script will try to run the programs by simulating them through Verilator by providing fud with the target --to dat. If you'd like to use the Calyx Interpreter instead, run the following command:

tools/flag-compare.sh <calyx program> <data> interpreter-out

Reducing Calyx Programs

The best way to reduce Calyx program deleting group enables from the control program and seeing if the generated program still generates the wrong output. While doing this, make sure that you're not deleting an update to a loop variable which might cause infinite loops.

By default, the compiler will complain if the program contains a group that is not used in the control program which can get in the way of minimizing programs. To get around this, run the dead-group-removal pass before the validation passes:

futil -p dead-group-removal -p validate ...

Reducing Dahlia Programs

If you're working with Dahlia programs, it is also possible to reduce the program with the script since it simply uses fud to run the program with the simulator. As with Calyx reduction, try deleting parts of the program and seeing if the flag configurations for the Calyx program still generate different outputs.

Waveform Debugging

Waveform debugging is the final way of debugging Calyx programs. A waveform captures the value of every port at every clock cycle and can be viewed using a wave viewer program like GTKWave or WaveTrace to look at the wave form. Because of this level of granularity, it generates a lot of information. To make the information a little more digestible, we can use information generated by Calyx during compilation.

For waveform debugging, we recommend disabling the optimization passes and static timing compilation (unless you're debugging these passes). In this debugging strategy, we'll do the following:

  1. Dump out the control FSM for the program we're debugging.
  2. Find the FSM states that enable the particular groups that might be misbehaving.
  3. Open the waveform viewer and find clock cycles where the FSM takes the corresponding values and identify other signals that we care about.

Consider the control section from examples/futil/dot-product.futil:

    seq {
      let0;
      while le0.out with cond0 {
        seq {
          par {
            upd0;
            upd1;
          }
          let1;
          let2;
          upd2;
          upd3;
        }
      }
    }

Suppose that we want to make sure that let0 is correctly performing its computation. We can generate the control FSM for the program using:

  futil <filename> -p top-down-cc

This generates a Calyx program with several new groups. We want to look for groups with the prefix tdcc which look something like this:

group tdcc {
  let0[go] = !let0[done] & fsm.out == 4'd0 ? 1'd1;
  cs_wh.in = fsm.out == 4'd1 ? le0.out;
  cs_wh.write_en = fsm.out == 4'd1 ? 1'd1;
  cond0[go] = fsm.out == 4'd1 ? 1'd1;
  par[go] = !par[done] & cs_wh.out & fsm.out == 4'd2 ? 1'd1;
  let1[go] = !let1[done] & cs_wh.out & fsm.out == 4'd3 ? 1'd1;
  let2[go] = !let2[done] & cs_wh.out & fsm.out == 4'd4 ? 1'd1;
  upd2[go] = !upd2[done] & cs_wh.out & fsm.out == 4'd5 ? 1'd1;
  upd3[go] = !upd3[done] & cs_wh.out & fsm.out == 4'd6 ? 1'd1;
  ...
}

The assignments to let0[go] indicate what conditions make the let0 group execute. In this program, we have:

let0[go] = !let0[done] & fsm.out == 4'd0 ? 1'd1;

Which states that let0 will be active when the state of the fsm register is 0 along with some other conditions. The remainder of the group defines how the state in the fsm variable changes:

  ...
  fsm.in = fsm.out == 4'd0 & let0[done] ? 4'd1;
  fsm.write_en = fsm.out == 4'd0 & let0[done] ? 1'd1;
  fsm.in = fsm.out == 4'd1 & cond0[done] ? 4'd2;
  fsm.write_en = fsm.out == 4'd1 & cond0[done] ? 1'd1;
  ...

For example, we can see that when the value of the FSM is 0 and let0[done] becomes high, the FSM will take the value 1.

Once we have this information, we can open the VCD file and look at points when the fsm register has the value 1 and check to see if the assignments in let0 activated in the way we expected.