Insight into the DMD back-end

[Stephan]

On 07.12.2010 06:57, %u wrote:
> Yay for more comments like this one.
>
> Don 2010-12-06 11:53:27 PST
> --
> Bearophile -- That's an interesting [automatic fuzzy testing]link. Currently,
> DMD back-end bugs are
> being found at the rate of about 3 per year. So yes, fuzzy testing of DMC could
> probably flush out some backend bugs a bit faster.
> -------------------
>
> Here's what's happening. First, in this code:
>
>      for (int i = 0; i<  10; i++) {
>          foo(i * 5 - 6);
>      }
> it sees that i and 10 are always>=0, so the signed comparison "i<  10" is
> replaced with an unsigned one. (This happens in the backend in constprop() ).
> Then, while dealing with loop invariants, it rewrites the loop into:
>
> for (int _i2 = -6; _i2<  10*5 - 6; _i2 += 5)
> {
>    foo(_i2);
> }
>
> Fine. Except that it had changed the comparison into an unsigned one!
> Particularly interesting is the case where the call is foo(i*5-50);
> Then, the loop becomes:
> for (int _i2 = -50; _i2<  0; _i2 += 5)
>
> Since an unsigned value is NEVER less than zero, it just drops the loop
> completely!
>
> Nasty.
> --
> http://d.puremagic.com/issues/show_bug.cgi?id=5294

What i think is more disturbing is Walters response:

> I'm not sure how to fix that one yet, but it has been there for 25 years
> now, so I am not sure it is urgent!

I often ran into this strange behaviour when using -O optimization 
without knowing where it came from and it is so disturbing when i think 
of people newly getting interested in D making the experience when 
trying to compare it with C/C++ and then finding out the optimization 
makes strange things. I think out of a image perspective such bugs must 
be high priority, ESPECIALLY if it lies there for 25years already.

Regards,
Stephan