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

[Hal Finkel]

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "clang-dev Developers" <cfe-dev at cs.uiuc.edu>
> Sent: Tuesday, December 11, 2012 5:17:43 AM
> Subject: Re: [cfe-dev] no-alias generated as result of restrict function	arguments
> 
> 
> 
> 
> On Sat, Dec 8, 2012 at 5:39 PM, Chandler Carruth <
> chandlerc at google.com > wrote:
> 
> 
> 
> 
> 
> 
> 
> 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...
> 
> As Joerg pointed out in IRC, 'y' is an lvalue, but we do not use it
> to access the object it designates, we only use it to compute the
> address, and thus restrict should have no bearing on it. That would
> in turn make Clang's optimization incorrect and also seems likely to
> make 'noalias' essentially useless for modeling restrict. =/

Does it really make it useless, or can we just not use it for computing icmp results on pointer values?

 -Hal

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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory