[LLVMdev] Question about NoWrap flag for SCEVAddRecExpr

[Sanjoy Das]

On Thu, Jun 11, 2015 at 12:02 AM, Adam Nemet <anemet at apple.com> wrote:
>
>> On Jun 10, 2015, at 11:44 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
>>
>>> Base is treated as unsigned so 0xff…ff + 1 would be 0x100…00
>>
>> This is the part I was missing, thanks for pointing out the FAQ.  So
>> the infinitely precise address computed by a GEP is
>>
>> zext(Base) + sext(Idx0) + sext(Idx1) … ?
>
> Yes, that is the way I read it.
>
>>> 0x100…00 which would be out of bounds (sort of).
>>
>> Does this mean, for C++ programs of the form,
>>
>>  for (int *I = array, *E = array + size; I != E; ++I)
>>   ...
>>
>> the memory allocator has to guarantee that array cannot span
>> [0xff..fffff-31,0xff..fffff] (both inclusive) with size == 32?
>
> I think so.  Address 0 cannot be dereferenced, so you can’t have a valid object spanning across address 0.

I the example I meant to give, [0xff..fffff-31,0xff..fffff] == [-32,
-1] does not span address 0 -- address 0 is the address one byte
outside the range assigned to `array`.



>
> Adam
>
>>
>> -- Sanjoy
>