[cfe-dev] no-alias generated as result of restrict function arguments

[Chandler Carruth]

On Sat, Dec 8, 2012 at 5:09 PM, Joerg Sonnenberger
<joerg at britannica.bec.de>wrote:

> Hi all,
> consider the following input:
>
> int y;
>
> void foo(int * restrict x)
> {
>         if (x == &y)
>                 (*x)++;
>         (*x)++;
> }
>
> The clang codegen gives the argument of foo the noalias attribute and
> GVN will optimise the condition to false.
>
> When discussing this on IRC, Chandler suggested that Clang shouldn't be
> creating the noalias attribute here, i.e. do alias interference. I'm not
> sure he is correct, but this is subtle enough to warrant some discussion
> before filling a PR.
>

Sorry, I was wrong. On further consideration, I think Clang is correct here.

1) (*x) is an lvalue based on the pointer 'x' and is modified in the
function, so the restrict qualifier actually governs the set of addresses
viable...
2) The expression 'y' is an lvalue expression AFAICT in the C99 standard,
is not based on the pointer 'x', and thus cannot access the same object
which x points to.

Thus, clang is correct to optimize the condition to false I think...
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