August 07
On Sunday, 5 August 2018 at 14:06:25 UTC, kinke wrote:
> On Sunday, 5 August 2018 at 03:01:46 UTC, Allen Garvey wrote:
>> The thing that's counter-intuitive to me is that if you write the pointer address as 0 right in the code it gets interpreted as null, but if you pass in the same value as 0, everything works as expected.
>
> If you provide the start address as null literal directly in the D code, the optimizer infers that you're going to read from address 0x3 in the first iteration. If you provide the start address as argument from outside code not available during optimization, LLVM cannot make any assumptions in this regard.

To add: dereferencing `null` is UB with LDC, and the optimizer makes use of that. (I didn't look at the code, but reading address 3 is OK. Probably you are indexing off of the null ptr, which is UB)

There is the `null-pointer-is-valid` attribute that you can put on functions to make `null` dereference valid defined behavior. In D, that'd be :
```
import ldc.attributes;

void invalid() {
    int* i = null;
    *i = 1;
}

@llvmAttr("null-pointer-is-valid", "true")
void valid() {
    int* i = null;
    *i = 1;
}
```

Probably needs trunk LLVM to work. (I think the attribute is new)

-Johan



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