[llvm-commits] [llvm] r59940 - in /llvm/trunk: docs/LangRef.html lib/Analysis/BasicAliasAnalysis.cpp

[Török Edwin]

On 2008-11-24 09:55, Nick Lewycky wrote:
> Török Edwin wrote:
>   
>> On 2008-11-24 09:43, Nick Lewycky wrote:
>>     
>>> Török Edwin wrote:
>>>   
>>>       
>>>> On 2008-11-24 07:00, Nick Lewycky wrote:
>>>>     
>>>>         
>>>>> Author: nicholas
>>>>> Date: Sun Nov 23 23:00:44 2008
>>>>> New Revision: 59940
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=59940&view=rev
>>>>> Log:
>>>>> Seriously strengthen the guarantee offered by noalias on a function's return
>>>>> value. It must now be as if the pointer were allocated and has not escaped to
>>>>> the caller. Thanks to Dan Gohman for pointing out the error in the original
>>>>> and helping devise this definition.
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>> malloc(0) is allowed to return a unique pointer, instead of NULL.
>>>> Does this code handle that?
>>>>     
>>>>         
>>> Sure, a noalias function can return NULL or a unique pointer. Returning 
>>> NULL is sufficient to not alias.
>>>
>>>   
>>>       
>> Agreed about NULL.
>> However if I call the function twice, and it returns the same pointer
>> (which is unique, but same for both calls), doesn't that mean
>> that the return value aliases a caller visible pointer?
>>     
>
> Oh I see what you mean.
>
> I hadn't thought of that, but since you can't actually store a byte 
> there, it should be treated just like NULL case.
>
> Feel free to patch the LangRef, or else I'll get to it later tomorrow.
>
> Similarly, there's the case of:
>    p1 = malloc(1)
>    free(p1)
>    p2 = malloc(1)
> p1 could equal p2, but they don't alias since the object pointed to has 
> been destroyed.
>
> "Aliasing" in general is not defined in the LangRef. Perhaps we'll need 
> to do that in order to elucidate the intended behaviour in all of these 
> corner cases.
>
>   

I added an explanation to LangRef.html in r59955, feel free to improve it:
-    caller.</dd>
+    caller. Note that this applies only to pointers that can be used to
actually
+    load/store a value: NULL, unique pointers from malloc(0), and freed
pointers
+    are considered to not alias anything.</dd>

Best regards,
--Edwin