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

Dan Gohman dan433584 at gmail.com
Thu Dec 13 09:52:14 PST 2012

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.


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