Disallow null references in safe code?

[Andrei Alexandrescu]

On 2/2/14, 1:23 PM, deadalnix wrote:
> On Sunday, 2 February 2014 at 10:58:51 UTC, Dicebot wrote:
>> On Sunday, 2 February 2014 at 03:45:06 UTC, Andrei Alexandrescu wrote:
>>> On 2/1/14, 7:35 PM, deadalnix wrote:
>>>> http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html
>>>>
>>>
>>> Whoa, thanks. So the compiler figures null pointer dereference in C
>>> is undefined behavior, which means the entire program could do
>>> whatever if that does happen.
>>>
>>> Andrei
>>
>> As far as I have understood previous posts, it is even worse than that
>> - LLVM optimiser assumes that C semantics whatever high-level language
>> is.
>>
>> deadalnix is that true?
>
> It depends. For instance you can specify semantic of wrap around, so
> both undefined and defined overflow exists.
>
> In the precise case we are talking about about, that really do not make
> any sense to propose any other semantic as it would prevent the
> optimizer to optimize away most load.

A front-end pass could replace the dead dereference with a guard that 
asserts the reference is not null. More generally this is a matter that 
can be fixed but currently is not receiving attention by backend writers.

Andrei