D - Unsafe and doomed

[deadalnix]

On Monday, 6 January 2014 at 00:43:22 UTC, Thiez wrote:
> 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.

Your example is a bug in the optimizer. Mine isn't.