C `restrict` keyword in D

[Johan Engelen via Digitalmars-d]

On Monday, 4 September 2017 at 09:47:12 UTC, Moritz Maxeiner 
wrote:
> On Monday, 4 September 2017 at 09:15:30 UTC, ag0aep6g wrote:
>> On 09/04/2017 06:10 AM, Moritz Maxeiner wrote:
>>> Indeed, but it also means that - other than null 
>>> dereferencing - pointer issues can by made into reference 
>>> issues my dereferencing a pointer and passing that into a 
>>> function that takes that parameter by reference.
>>
>> Why "other than null dereferencing"? You can dereference a 
>> null pointer and pass it in a ref parameter. That doesn't 
>> crash at the call site, but only when the callee accesses the 
>> parameter:
>>
>> [...]
>
> Because I was ignorant and apparently wrong, thanks for the 
> correction.
> Still, though, this is surprising to me, because this means 
> taking the address of a parameter passed by reference (which is 
> in your case is typed as an existing int) can be null. Is this 
> documented somewhere (couldn't find it in the spec and it seems 
> like a bug to me)?

LDC treats passing `null` to a reference parameter as UB.
It doesn't matter when the program crashes after passing null to 
ref, exactly because it is UB.
Because the caller has to do the dereferencing (semantically) you 
only have to do the null-check in the caller, and not in callee. 
This removes a ton of manual null-ptr checks from the code, and 
enables more optimizations too.
For class parameters, they are pointers not references, as in: it 
is _not_ UB to pass-in `null`. Very unfortunate, because it 
necessitates null-ptr checks everywhere in the code, and hurts 
performance due to missed optimization opportunities.

(The spec requires crashing on null dereferencing, but this spec 
bit is ignored by DMD and LDC, I assume in GDC too. Crashing on 
`null` dereferencing requires a null-check on every dereferencing 
through an unchecked pointer, because 0 might be a valid memory 
access, and also because ptr->someDataField is not going to 
lookup address 0, but 0+offsetof(someDataField) instead, e.g. 
potentially addressing a valid low address at 1000000, say.)

- Johan