D - Unsafe and doomed

[Thiez]

On Monday, 6 January 2014 at 00:20:59 UTC, deadalnix wrote:
> void foo(int* ptr) {
>     *ptr;
>     if (ptr is null) {
>         // do stuff
>     }
>
>     // do stuff.
> }
>
> The code look stupid, but this is quite common after a first 
> pass of optimization/inlining, do end up with something like 
> that when a null check if forgotten.
>
> The problem here is that the if can be removed, as you can't 
> reach that point if the pointer is null, but *ptr can also be 
> removed later as it is a dead load.
>
> The resulting code won't crash and do random shit instead.

If you read 
http://people.csail.mit.edu/akcheung/papers/apsys12.pdf there is 
a nice instance where a compiler moved a division above the check 
that was designed to prevent division by zero, because it assumed 
a function would return (when in fact it wouldn't). I imagine a 
similar scenario could happen with a null pointer, e.g.:

if (ptr is null) {
   perform_function_that_never_returns();
}
auto x = *ptr;

If the compiler assumes that 
'perform_function_that_never_returns()' returns, it will 
recognize the whole if-statement and its body as dead code. 
Optimizers can be a little too smart for their own good at times.