[LLVMdev] Strange pointer aliasing behaviour

[Eugene Toder]

John, David,

That's not the whole story, though. For example, 6.7.2.1/13 of C99
allows a conversion between a pointer to struct and it's first member
and also specifies that members of the struct are laid out in the
order of declaration. To derive pointer to one field from another it's
sufficient to 1) cast pointer to a struct to a pointer to array of
chars 2) perform arithmetics as per 6.5.6 3) cast the pointer to
pointer to a field. I have not found paragraphs explicitly allowing or
denying 1 and 3, however there are many hints -- offsetof macro, which
would have no use otherwise; 6.2.6/4, which allows to copy any object
into array of chars; aliasing rules of 6.5, which allow accessing any
object through char*.

On the original subject -- in this example, isn't it guaranteed that
_length and _data[i] do not alias? i is non-negative (if it's unsigned
-- by definition, if it's signed -- because signed overflow is
undefined behaviour), so &_data[i] >= &_data > &_length.

Eugene

On Thu, Jun 17, 2010 at 6:58 PM, John McCall <rjmccall at apple.com> wrote:
>
> On Jun 17, 2010, at 10:42 AM, Andrew Haley wrote:
>
> On 06/17/2010 06:19 PM, Jeffrey Yasskin 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.
>
> Character types are special: they can alias everything.  if this weren't
> the case you couldn't write malloc().
>
> Do you have a reference to the standard that makes it undefined?
>
> Several places, but 6.3.2.3 of C99 is the most important: it lists all
> the legal pointer conversions.
>
> The better reference is 6.5.6p7-8, about pointer addition:
>   For the purposes of these operators, a pointer to an object that is not an
> element of an array behaves the same as a pointer to the first element of an
> array of length one with the type of the object as its element type.
>   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.
> John.
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