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

[Daniel Berlin]

On Thu, Dec 13, 2012 at 12:52 PM, Dan Gohman <dan433584 at gmail.com> wrote:
> On Wed, Dec 12, 2012 at 2:45 PM, Joerg Sonnenberger
> <joerg at britannica.bec.de> wrote:
>> On Wed, Dec 12, 2012 at 01:59:55PM -0800, Dan Gohman wrote:
>>> On Wed, Dec 12, 2012 at 1:26 PM, Joerg Sonnenberger
>>> > The original issue is that clang maps restrict on function arguments to
>>> > NoAlias and that makes compares against the address of global variables
>>> > false. Minimal test case:
>>> >
>>> > @y = external global i32
>>> >
>>> > define zeroext i1 @t(i32* noalias %x) nounwind uwtable readnone {
>>> > entry:
>>> >   %cmp = icmp eq i32* %x, @y
>>> >   ret i1 %cmp
>>> > }
>>>
>>> The bug here isn't in clang's use of noalias or in BasicAliasAnalysis'
>>> implementation of noalias; it's in the code that's optimizing the
>>> icmp.
>>
>> I am just saying that the comments in BasicAliasAnalysis makes me wonder
>> if it has the same kind of problem.
>
> And I am answering with an explanation of why the BasicAliasAnalysis
> code in question does not have this same kind of problem. A NoAlias
> response from AliasAnalysis isn't intended to imply inequality of
> pointers, so there's no inherent problem with saying NoAlias for
> pointers which can be equal. For the same reason, this is why using
> AliasAnalysis information in instcombine/simplify to simplify pointer
> comparisons is a bug.

+1
This is precisely one of the differences between pointer analysis and
alias analysis.