[llvm-dev] Propagating noalias annotation

[Hongbin Zheng via llvm-dev]

Do you mean a and b are noalias if:

static int foo(int *a, int *b) {
  return a[0] + b[0];
}

int bar(int *x) {
  return foo(x+1, x);
}

?

To me, because "AA.alias((x+1, MemoryLocation::UnknownSize),
(x, MemoryLocation::UnknownSize)) != NoAlias", so a and b are not noalias.
Maybe my version is too conservative.

Thanks
Hongbin


On Thu, Nov 16, 2017 at 11:55 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 11/17/2017 01:49 AM, Hongbin Zheng wrote:
>
> Could you elaborate "Note that, without further analysis of the uses of
> the potentially-noalias pointers, you can do this only ..."?
> Why the uses, instead of the def, of pointer matter?
>
>
> Both matter.
>
> static int foo(int *a, int *b) {
>   return a[0] + b[1];
> }
>
> int bar(int *x) {
>   return foo(x+1, x);
> }
>
> You can't mark a and b as noalias here, even though NoAlias(x+1, x) ==
> true. This is why I was saying that, unless the pointers come from
> distinct, identified underlying objects, you need to look at the uses of
> the pointers too.
>
>  -Hal
>
> P.S. As discussed in D4609, we probably want to try adding CGSCC AA
> wrappers, to look back through function arguments, instead of using
> attribute propagation.
>
>
>
> Thanks
> Hongbin
>
> On Thu, Nov 16, 2017 at 11:21 PM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi, Alexandre,
>>
>> We don't have anything currently which does this. Note that, without
>> further analysis of the uses of the potentially-noalias pointers, you can
>> do this only for arguments with distinct (and identified) underlying
>> objects (i.e., you need something a bit stronger than just "non-aliasing
>> pointers").
>>
>>  -Hal
>>
>>
>> On 11/16/2017 10:11 AM, via llvm-dev wrote:
>>
>>> Is this what you are looking for?
>>>
>>> https://reviews.llvm.org/D4609
>>>
>>> Best,
>>>
>>> Haicheng Wu
>>>
>>> On 2017-11-14 20:34, Alexandre Isoard via llvm-dev wrote:
>>>
>>>> Hello,
>>>>
>>>> Do we have a pass that propagate the noalias annotation down to the
>>>> callee when:
>>>> - it is static
>>>> - it is always called with non aliasing pointers
>>>> ?
>>>>
>>>> Or maybe that is incorrect?
>>>>
>>>> --
>>>>
>>>> ALEXANDRE ISOARD
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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