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advanced_bugpoint.rst: Paragraphing
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Krystine Sherwin
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@ -6,27 +6,40 @@ in solving or investigating Yosys bugs. This also applies if you are using a
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fuzzing tool; fuzzers have a tendency to find many variations of the same bug,
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so identifying the root cause is important for avoiding issue spam.
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- if the failing command was part of a larger script, such as one of the
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:doc:`/using_yosys/synthesis/synth`, you could try to follow the design
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through the script
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If you're familiar with C/C++, you might try to have a look at the source code
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of the command that's failing. Even if you can't fix the problem yourself, it
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can be very helpful for anyone else investigating if you're able to identify
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where the issue is arising.
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+ sometimes when a command is raising an error, you're seeing a symptom rather
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than the underlying issue
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+ an earlier command may be putting the design in an invalid state which isn't
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picked up until the error is raised
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+ check out :ref:`yosys_internals/extending_yosys/advanced_bugpoint:why context matters`
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- if you're familiar with C/C++ you might try to have a look at the source
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code of the command that's failing
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Finding the failing command
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---------------------------
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Using the ``-L`` flag can help here, allowing you to specify a file to log to,
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such as ``yosys -L out.log -s script.ys``. Most commands will print a header
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message when they begin; something like ``2.48. Executing HIERARCHY pass
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(managing design hierarchy).`` The last header message will usually be the
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failing command. There are some commands which don't print a header message, so
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you may want to add ``echo on`` to the start of your script. The `echo` command
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echoes each command executed, along with any arguments given to it. For the
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`hierarchy` example above this might be ``yosys> hierarchy -check``.
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.. note::
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It may also be helpful to use the `log` command to add messages which you can
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then search for either in the terminal or the logfile. This can be quite
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useful if your script contains script-passes, like the
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:doc:`/using_yosys/synthesis/synth`, which call many sub-commands and you're
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not sure exactly which script-pass is calling the failing command.
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+ even if you can't fix the problem yourself, it can be very helpful for
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anyone else investigating if you're able to identify where exactly the
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issue is
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Minimizing scripts
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------------------
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.. TODO:: disclaimer this is intended as advanced usage, and generally not necessary for bug reports
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.. warning::
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This section is intended as **advanced usage**, and generally not necessary
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for normal bug reports.
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If you're using a command line prompt, such as ``yosys -p 'synth_xilinx' -o
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design.json design.v``, consider converting it to a script. It's generally much
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@ -40,28 +53,10 @@ command line, as well as being better for sharing with others.
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synth_xilinx
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write_json design.json
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Next up you want to remove everything *after* the error occurs. Using the
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``-L`` flag can help here, allowing you to specify a file to log to, such as
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``yosys -L out.log -s script.ys``. Most commands will print a header message
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when they begin; something like ``2.48. Executing HIERARCHY pass (managing
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design hierarchy).`` The last header message will usually be the failing
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command. There are some commands which don't print a header message, so you may
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want to add ``echo on`` to the start of your script. The `echo` command echoes
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each command executed, along with any arguments given to it. For the
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`hierarchy` example above this might be ``yosys> hierarchy -check``.
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.. note::
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It may also be helpful to use the `log` command to add messages which you can
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then search for either in the terminal or the logfile. This can be quite
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useful if your script contains script-passes, like the
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:doc:`/using_yosys/synthesis/synth`, which call many sub-commands and you're
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not sure exactly which script-pass is calling the failing command.
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If your final command calls sub-commands, replace it with its contents and
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repeat the previous step. In the case of the
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:doc:`/using_yosys/synthesis/synth`, as well as certain other script-passes, you
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can use the ``-run`` option to simplify this. For example we can replace
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Next up you want to remove everything *after* the error occurs. If your final
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command calls sub-commands, replace it with its contents first. In the case of
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the :doc:`/using_yosys/synthesis/synth`, as well as certain other script-passes,
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you can use the ``-run`` option to simplify this. For example we can replace
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``synth -top <top> -lut`` with the :ref:`replace_synth`. The options ``-top
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<top> -lut`` can be provided to each `synth` step, or to just the step(s) where
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it is relevant, as done here.
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@ -177,32 +172,85 @@ recovering context, checkout
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Why context matters
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-------------------
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- if you did minimized your script, and removed commands prior to the failure
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to get a smaller design, try to work backwards and find which commands may
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have contributed to the design failing
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- especially important when the bug is happening inside of a ``synth_*`` script
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- example (#4590)
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Sometimes when a command is raising an error, you're seeing a symptom rather
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than the underlying issue. It's possible that an earlier command may be putting
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the design in an invalid state, which isn't picked up until the error is raised.
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This is particularly true for the pre-packaged
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:doc:`/using_yosys/synthesis/synth`, which rely on a combination of generic and
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architecture specific passes. As new features are added to Yosys and more
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designs are supported, the types of cells output by a pass can grow and change;
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and sometimes this leads to a mismatch in what a pass is intended to handle.
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If you minimized your script, and removed commands prior to the failure to get a
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smaller reproducer, try to work backwards and find which commands may have
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contributed to the design failing. From the minimized design you should have
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some understanding of the cell or cells which are producing the error; but where
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did those cells come from? The name and/or type of the cell can often point you
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in the right direction:
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.. code-block::
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# internal cell types start with a $
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# lowercase for word-level, uppercase for bit-level
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$and
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$_AND_
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# cell types with $__ are typically intermediate cells used in techmapping
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$__MUL16X16
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# cell types without a $ are either user-defined or architecture specific
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my_module
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SB_MAC16
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# object names might give you the name of the pass that created them
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$procdff$1204
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$memory\rom$rdmux[0][0][0]$a$1550
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# or even the line number in the Yosys source
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$auto$muxcover.cc:557:implement_best_cover$2152
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$auto$alumacc.cc:495:replace_alu$1209
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Try running the unminimized script and search the log for the names of the
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objects in your minimized design. In the case of cells you can also search for
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the type of the cell. Remember that calling `stat` will list all the types of
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cells currently used in the design, and `select -list =*` will list the names of
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of all the current objects. You can add these commands to your script, or use
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an interactive terminal to run each command individually. Adding them to the
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script can be more repeatable, but if it takes a long time to run to the point
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you're interested in then an interactive shell session can give you more
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flexibility once you reach that point. You can also add a call to the `shell`
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command at any point in a script to start an interactive session at a given
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point; allowing you to script any preparation steps, then come back once it's
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done.
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A worked example
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~~~~~~~~~~~~~~~~
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+ say you did all the minimization and found that the error occurs when a call
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to ``techmap -map +/xilinx/cells_map.v`` with ``MIN_MUX_INPUTS`` defined
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parses a `$_MUX16_` with all inputs set to ``1'x``
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+ step through the original script, calling `stat` after each step to find
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when the `$_MUX16_` is added
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+ find that the `$_MUX16_` is introduced by a call to `muxcover`, but all the
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inputs are defined, so calling `techmap` now works as expected
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Say you did all the minimization and found that an error in `synth_xilinx`
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occurs when a call to ``techmap -map +/xilinx/cells_map.v`` with
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``MIN_MUX_INPUTS`` defined parses a `$_MUX16_` with all inputs set to ``1'x``.
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You could fix the bug in ``+/xilinx/cells_map.v``, but that might only solve
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this one case while leaving other problems that haven't been found yet. So you
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step through the original script, calling `stat` after each step to find when
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the `$_MUX16_` is added.
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* and from running `bugpoint` with the failing techmap you know that the
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cell with index ``2297`` will fail, so you can now call ``select
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top/*$2297`` to limit to just that cell, and optionally call ``design
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-save pre_bug`` or ``write_rtlil -selected pre_bug.il`` to save this state
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You find that the `$_MUX16_` is introduced by a call to `muxcover`, but all the
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inputs are defined, so calling `techmap` now works as expected. From running
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`bugpoint` with the failing techmap you know that the cell with index ``2297``
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will fail, so you call ``select top/*$2297`` to limit to just that cell. This
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can then be saved with ``design -save pre_bug`` or ``write_rtlil -selected
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pre_bug.il``, so that you don't have to re-run all the earlier steps to get back
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here.
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+ next you step through the remaining commands and call `dump` after each to
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find when the inputs are disconnected
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+ find that ``opt -full`` has optimized away portions of the circuit, leading
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to `opt_expr` setting the undriven mux inputs to ``x``, but failing to
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remove the now unnecessary `$_MUX16_`
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Next you step through the remaining commands and call `dump` after each to find
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when the inputs are disconnected. You find that ``opt -full`` has optimized
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away portions of the circuit, leading to `opt_expr` setting the undriven mux
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inputs to ``x``, but failing to remove the now unnecessary `$_MUX16_`. Now
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you've identified a problem in `opt_expr` that affects all of the wide muxes,
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and could happen in any synthesis flow, not just `synth_xilinx`.
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- in this example, you might've stopped with the minimal reproducer, fixed the
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bug in ``+/xilinx/cells_map.v``, and carried on
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- but by following the failure back you've managed to identify a problem with
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`opt_expr` that could be causing other problems either now or in the future
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.. seealso::
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This example is taken from `YosysHQ/yosys#4590
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<https://github.com/YosysHQ/yosys/issues/4590>`_ and can be reproduced with a
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version of Yosys between 0.45 and 0.51.
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