[Simh] VAX emulation issues on Raspberry Pi

[Timothe Litt]

On 31-Jul-18 12:41, Paul Koning wrote:
>
>
> One thing that happens with newer compilers is that they take more advantage of opportunities offered by the letter of the standard.  If you do something that is "undefined", the compiler can do with that whatever it wants to.  If you have code that appears to allow something undefined under certain conditions, the compiler is free to assume that the undefined thing cannot happen and therefore that scenario doesn't occur.
>
> For example: 
> 	extern int foo[100];
>
> 	if (i < 0)
> 		a += foo[i];
>
> The compiler is allowed to delete that code because subscripting an array with a negative number is undefined.  And modern compilers often will.
>
> There is an excellent paper, I think from MIT, about many undefined things in C that are not all that well understood by many programmers.  Unfortunately I've misplaced the reference.  It mentions that Linux turns off a whole pile of gcc optimizations by specific magic flags to avoid breaking things due to undefined things it does in the existing code base.
>
> For integer arithmetic, what you mentioned is a key point.  Unsigned is defined to have wraparound semantics.  With signed integers, overflow is "undefined".  So, for example, if you want to emulate the PDP11 arithmetic operations, you have to use unsigned short (uint16_t).  Using signed short (int16_t) is incorrect because of the overflow rules.
>
> More generally, in SIMH you probably want the rule that every integer variable is unsigned.  Or at least, make every variable unsigned unless you know there is a good reason why it needs to be signed, and no undefined behavior is possible for that particular variable.
>
> If you compile (in gcc) with -Wall and get no warnings, that's a pretty good sign.  If you do get warnings, they almost certainly need to be fixed rather than disabled.
>

You probably mean
https://people.csail.mit.edu/nickolai/papers/wang-stack-tocs.pdf

For more reading, see, e.g. http://port70.net/~nsz/c/c99/n1256.html#J.2
<http://port70.net/%7Ensz/c/c99/n1256.html#J.2>,
https://cacm.acm.org/magazines/2016/3/198849-a-differential-approach-to-undefined-behavior-detection/fulltext,
https://blog.regehr.org/archives/1520,
https://runtimeverification.com/blog/undefined-behavior-review-and-rv-match/


The last is a competitive analysis - but quite informative nonetheless.

In additions to -Wall, I also used to compile SimH with -pedantic and
-Werror.  And usually one of the -std flags.

Another big potential gotcha in SimH is the heavy use of casts, and type
aliasing rules (the latter especially problematic in subroutine calls).



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