[LLVMdev] BasicAliasAnalysis: Null pointers do not alias with anything

[Chris Lattner]

On Nov 4, 2009, at 7:19 AM, Daniel Berlin wrote:

> On Wed, Nov 4, 2009 at 4:51 AM, Hans Wennborg <hans at hanshq.net> wrote:
>
>>
>>
>> The reason is that it is unsure whether the null pointer which is  
>> passed in
>> the call to @foo may alias with %t. Obviously, a null pointer  
>> doesn't alias
>> with anything, because it's not legal to read or write through it  
>> (right?).
>
> I don't remember whether LLVM's language spec says anything different,
> but whether null may alias anything is generally platform dependent.
>
> On some platforms, null may actually point to things and be
> dereferenced legally.
>
> (This is often used to speculatively executive conditionals involving
> pointer-derefs)

Our current policy is to disallow dereferences of the null pointer,  
unless they are in the non-default address space.

-Chris