Dealing with EXIT code of target

[PhilippvK]

The exit code of the simulated program is either the return value of the main() function or the argument to the exit(-1) function. For unit testing and benchmarks, we should make sure to not ignore if the program reports a non-zero exit code.

Previous approach (without Semihosting):

• Custom _exit hook implemented in syscalls.c:

  etiss/examples/SW/riscv/cmake/pulpino_tumeda/syscalls.c

  Lines 113 to 121 in 40c0b92

  	void _exit_(int exit_status)
  	{
  	#ifdef DEBUG_SYSTEM
  	printf_fromisr("exit called with code: %i\n", exit_status);
  	#endif
  	asm("ebreak");
  	while (1)
  	;
  	}

• Based on printf -> ROM size overhead -> Only enabled in DEBUG mode

Current approach (with Semihosting based on https://github.com/wysiwyng/etiss/tree/coredsl_exceptions)

• No need to implement custom syscalls.c anymore.
• SYS_EXIT currently just tells ETISS to exit, ignoring the argument. See: https://github.com/wysiwyng/etiss/blob/a09aea80a433d6b35a2df82176a4f6e39a8fa3e5/src/jitlibs/semihost/semihost.cpp#L405

Proposed Improvements:

• Print exit code in handler for SYS_EXIT using etiss logger
• Log level Warn/Error for non zero (FAIL) values
• Log level Verbose/Info elsewhere (OK)
• Optionally pass through exit code to bare_etiss_simulator to fails as well (if configured by the user)

[wysiwyng]

bare_etiss_processor currently also uses exit codes to signal when ETISS encountered some internal error. Distinguishing these from the application's exit code would not be possible, this would at least have to be noted somewhere. How do other simulators handle this?

[PhilippvK]

RISC-V OVPSim has a command line flag to choose whether a non-zero _exit() call will lead to a failed simulation:
--override riscvOVPsim/cpu/pk/reportExitErrors=F (Boolean) (default=F) (default) Report non-zero exit() return codes as simulator errors

At least to my knowledge, when using Spike & riscv-pk the exit code of spike should always be the exit code of the program (wihich of course, has disadvantages, as you just mentioned.)

If this flag is enabled, it will also print the exit code to stdout, which at least allows extracting the actual value by parsing the output. I prefer this approach over handing the printing of the Exit code in the target SW (using custom syscalls) which is not even feasible when using semihosting.