Direct recursion detection possible?

[Cecil Ward]

On Thursday, 25 May 2023 at 13:46:32 UTC, Cecil Ward wrote:
> On Thursday, 25 May 2023 at 10:35:45 UTC, Quirin Schroll wrote:
>> On Wednesday, 24 May 2023 at 23:04:53 UTC, Cecil Ward wrote:
>>> [...]
>>
>> I had this almost happen in my C++ code. It never ran because 
>> I got a compile error. The compiler is MSVC.
>>
>> [...]
>
> My apologies, when I wrote ‘straightforward’ I should meant to 
> say, ‘without body, with no content’. You mentioned without 
> side-effects, which is more general and very useful.

We would have to add something in your quick sketch to mention 
exceptions. So if the basic block is not null, a type of check 
that I would settle for as a first step, in addition to your 
conditions all calls would have to be nothrow if there is an 
exception handler.

I don’t know what the D call is to terminate the current process, 
or even begin the shutdown sequence for the system if we are 
building an embedded system with (other) no o/s. But I should 
think that a routine having an attribute of ‘terminate the 
process’, when the reason isn’t  because we mean ‘panic’, 
core_dump, ud_n, etc because of a bug, but because we simply want 
to terminate the process because we’re all done, with success or 
failure, means that it is not pure.

In GCC there is a noreturn attribute iirc - is that correct? but 
I think that there ought to be subtypes of that - to say whether 
it’s debug code or not, whether it’s a straight ‘end if process’ 
rather than a panic / bug / core_dump etc. (Does GCC or LDC 
generate a ud2 when you wish to panic? Presumably without 
cleanup?)

Merely having an exception handler present above or around your 
non-trivial infinite loop might be a sign that you have something 
equivalent to a break inside the loop. But I don’t think we 
should consider this, as there may actually be no exception 
throwing at all, and that could be a bug - that the way out has 
been omitted.

Anyway, it would be easier to do the "L1: jmp L1" thing first as 
then there are no worries, and we would get some benefit much 
sooner.