Unsupported types for log_msg calls

[jafingerhut]

I had a fairly decent sized v1model P4 program that was causing simple_switch to give an error message very soon after starting up, about some unexpected contents inside of the BMv2 JSON file, and it took a while to whittle it down and discover it was because I was trying to do log_msg on a value of type bool. Casting it to a bit<1> type caused things to work. It would be nice to at least document what types are supported, and preferably give a compile time error message for unsupported types.

From my experiments using the attached P4 program, it appears that these types cause either cryptic errors when starting simple_switch, or a run-time error and simple_switch crash when the log_msg statement is executed:

• bool - cryptic error at simple_switch start time
• header - crash when attempting to execute log_msg call, but at least fairly easy to localize in the code
• struct - same as header, even if all fields of the struct are of type bit<W>

I did not try header_union, header stack, or anything more 'exotic' than header or struct, but would expect they have the same behavior as header/struct.

One enhancement possibility - Change p4c bmv2 back end to give an error if an unsupported type is used as argument to log_msg calls.

Whatever list of supported/unsupported types we decide upon, I am happy to document, either in the v1model.p4 #include file, or here, whichever seems most appropriate: https://github.com/p4lang/behavioral-model/blob/master/docs/simple_switch.md

[jafingerhut]

Actually attaching example program I mentioned in comment above.
log-msg-supported-types.p4.txt

[jafingerhut]

Confirmed that this PR fixed this issue. Closing this issue.