[LLVMdev] Strange pointer aliasing behaviour

[David Vandevoorde]

On Jun 17, 2010, at 1:34 PM, Eugene Toder wrote:

>> Do you have a reference to the standard that makes it undefined?
> 
> I'm second this question. I tried to find anything banning calculating
> address of one field from address of another in the standard some time
> ago, but could not find it.


In the currect C++0x FCD, 5.7/5:

	"When an expression that has integral type is added to or subtracted
	 from a pointer, the result has the type of the pointer operand.
	 If the pointer operand points to an element of an array object, and
	 the array is large enough, the result points to an element offset
	 from the original element such that the difference of the subscripts
	 of the resulting and original array elements equals the integral
	 expression. In other words, if the expression P points to the i-th
	 element of an array object, the expressions (P)+N (equivalently,
	 N+(P)) and (P)-N (where N has the value n) point to, respectively,
	 the i + n-th and i − n-th elements of the array object, provided 
	 they exist. Moreover, if the expression P points to the last element
	 of an array object, the expression (P)+1 points one past the last
	 element of the array object, and if the expression Q points one past
	 the last element of an array object, the expression (Q)-1 points to
	 the last element of the array object. If both the pointer operand
	 and the result point to elements of the same array object, or one
	 past the last element of the array object, the evaluation shall not
	 produce an overflow; otherwise, the behavior is undefined."

(Note in particular the last phrase, and recall that subscripting is defined in terms of pointer arithmetic.)


	Daveed

> 
> On Thu, Jun 17, 2010 at 6:19 PM, Jeffrey Yasskin <jyasskin at google.com> wrote:
>> On Thu, Jun 17, 2010 at 12:22 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Wed, Jun 16, 2010 at 11:14 PM, Pierre C <lists at peufeu.com> wrote:
>>>>> There are essentially two ways to "solve" this issue: one is
>>>>> type-based alias analysis, i.e. assuming "double" and "int" don't
>>>>> alias; LLVM doesn't implement this at the moment.  The other is to
>>>>> attempt to analyze the loop and prove that %indvar.i is never
>>>>> negative; LLVM doesn't implement anything like this at the moment
>>>>> either.
>>>>> 
>>>>> -Eli
>>>> 
>>>> Actually I think it's much simpler than that...
>>>> 
>>>> http://llvm.org/releases/1.3/docs/AliasAnalysis.html#basic-aa
>>>> 
>>>> it says says "Different fields of a structure do not alias."
>>>> 
>>>> This is the case here : we have two different fields of a struct however it
>>>> mistakenly thinks they alias.
>>> 
>>> Consider a case like the following:
>>> struct X { int a; int b[10]; };
>>> int f(struct X* a) { a->b[-1] = 1; return a->a; }
>>> 
>>> This is technically illegal code, but various programs depend on
>>> constructs like this working.
>> 
>> I don't know if it's illegal, but this is how libstdc++'s string
>> implementation finds its header data. std::string stores a pointer
>> directly to the character data (making subscripting slightly faster),
>> and then subtracts the size of the header when it needs to do any
>> bookkeeping.
>> 
>> Do you have a reference to the standard that makes it undefined?
>> 
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> 
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