Add 'questa' to list of supported simulators (VUNIT_SIMULATOR)

[tasgomes]

Currently, if VUNIT_SIMULATOR is set to 'questa' the following error occurs:

RuntimeError: Simulator from VUNIT_SIMULATOR environment variable 'questa' is not supported. Supported simulators are dict_keys(['modelsim', 'rivierapro', 'activehdl', 'ghdl', 'incisive'])

However, both Questa and Modelsim call vsim to run the simulations. In fact, if I set VUNIT_SIMULATOR='modelsim' and VUNIT_MODELSIM_PATH=<path to questa install directory>, I can execute VUnit testbenches with Questa without any problem.

Would it be possible to add the name 'questa' to the current list of supported simulators?

or is there any limitation?

[umarcor]

Would the enhancement include supporting VUNIT_QUESTASIM_PATH as an alias of VUNIT_MODELSIM_PATH?

Context:

vunit/vunit/sim_if/factory.py

Lines 102 to 127 in fbb46c6

	def select_simulator(self):
	"""
	Select simulator class, either from VUNIT_SIMULATOR environment variable
	or the first available
	"""
	available_simulators = self._detect_available_simulators()
	name_mapping = {simulator_class.name: simulator_class for simulator_class in self.supported_simulators()}
	if not available_simulators:
	return None
	environ_name = "VUNIT_SIMULATOR"
	if environ_name in os.environ:
	simulator_name = os.environ[environ_name]
	if simulator_name not in name_mapping:
	raise RuntimeError(
	(
	"Simulator from " + environ_name + " environment variable %r is not supported. "
	"Supported simulators are %r"
	)
	% (simulator_name, name_mapping.keys())
	)
	simulator_class = name_mapping[simulator_name]
	else:
	simulator_class = available_simulators[0]
	return simulator_class

I can execute VUnit testbenches with Questa without any problem

QuestaSim is explicitly supported. However, the procedure is as you guessed.

[tasgomes]

I guess so in order to keep it consistent with others.

[tasgomes]

@umarcor There are also some other considerations, example:

lib.set_sim_option("modelsim.vsim_flags", (...))

[std-max]

I wonder if Modelsim will be discontinued in a near future and be replaced with Questasim? see this reddit thread

If that's the case, maybe it would be better to change all modelsim reference in the code to questasim, and to make modelsim an alias of questasim?

Do you know if modelsim.ini is also named like this with Questasim? or if it's named questasim.ini?

[tasgomes]

@std-max as far as my experience goes. with Questa-Intel FPGA Edition, the file is still called modelsim.ini.

If I compare the vunit_out folders between Questa and Modelsim:

• Questa: .\vunit_out\modelsim\modelsim.ini
• Modelsim: .\vunit_out\questa\modelsim.ini

[LarsAsplund]

From a maintenance point of view the simplest solution is to just call it modelsim everywhere in the code and very clearly show, in documentation, that it includes Questa as well. Is there something we can do to make this clearer?

[std-max]

Agree with you @LarsAsplund
This is the simplest way I think. I will try to file a PR that adds a bit of documentation for that point.

[tasgomes]

Currently all my (minor) issues can be solved with workarounds. But I am wondering if we are delaying something that might need to be done in the future.

At the moment, I have a Makefile that allows me to change the simulator (modelsim, questa and ghdl) using a switch. This switch is also used for Vivado to compile the Xilinx libraries where Vivado distinguishes between Modelsim (-simulator modelsim) and Questa (-simualtor questa). Because I compile the libraries to specific folders and use different vunit_out folders depending on the simulator used, I can run the same regression test in any of the simulators at the same time.

At the moment, one of the workarounds in run.py is to overwrite the name of the simulator when questa is select as follow:

if args.simulator == 'questa':
    environ["VUNIT_SIMULATOR"] = 'modelsim'
else:
    environ["VUNIT_SIMULATOR"] = args.simulator

One main difference between Questa and Modelsim, is that Questa has optimizations turned on by default. If you try to disable them, you even get a (suppressible) compilation error that your simulation will run very slowly, etc.. The point is because optimizations are on, if you debug your DUT in GUI mode many internal signals are optimized out and not visible so you need to add some flags to vsim in order to preserve visibility for debugging.

Also, comparing the help menu of vsim and vcom shows that there many new flags in Questa that do not exist in Modelsim so at some point it will make sense to have modelsim.vsim_flags and questa.vsim_flags like we have for ghdl.elab_flags so that new Questa flags are not applied when using Modelsim simulator. Again this can still be solved with the workaround shown above.

[LarsAsplund]

@tasgomes Can you share some of the flags that are different? Just to get a feel for how "useful" they are.

Some weeks ago a new colleague tried to install free ModelSim from Intel to use that with VUnit. He found out that the free simulator in Quartus is now a variant of Questa which he didn't manage to install. Not sure if it wasn't free after all. Anyway, it was solved by using an older version of Quartus to get access to the free ModelSim.

If there actually is a free Questa from Intel it means that it will be easier for us to verify Questa and also support it as a simulator different from ModelSim. Anyone having experience with this new Questa version?

[tasgomes]

@LarsAsplund Although there are now new flags in Questa, currently there is none that I used that only exists in one of them. But since I started using Questa, I had to add this one to preserve signal visibility when doing debug activities in GUI mode:

lib.set_sim_option("modelsim.vsim_flags", ["-voptargs=+acc"])

I use Questa Intel FPGA Edition. I believe when you are installing it, you can select the free "Starter Edition" option.

[LarsAsplund]

@tasgomes If there are no strong reasons in support of differentiating between the two I suggest we postpone it until we find such reasons. Reduces maintenance. Only make the Questa and ModelSim relation very clear in the docs.

In general one cannot not avoid that users have to take special steps to handle their different licenses. For example, you can have several vendor versions of ModelSim installed and VUNIT_SIMULATOR alone cannot be used to differentiate between the two.

Turning off optimization is also something that you have to do with other simulators, for example Riviera-PRO.

Looks like we did something wrong when installing Intel Questa then. Good to know. I should give it another try.