[llvm-dev] restrict func param losing noalias when inlined

[Johannes Doerfert via llvm-dev]

On 9/28/20 8:39 PM, Ryan Taylor wrote:
> Johannes,
>
> Thanks, I have been following along some of the thread(s) and the phab
> reviews. The scope of this work is more encompassing than our current needs
> and I've looked at trying to carve a piece out.
>
> It's not clear to me what purpose the llvm.noalias intrinsic serves right
> now. Also, if a mem instruction has !noalias metadata, then it should not
> be aliased, but I must be missing some complexity. For example, if you look
> at the alias of two memory locations and they are both tagged with noalias
> MD, that should be safe to NoAlias providing they have no specific !scope
> tag? I've looked at the ScopedNoAliasAA but during the inline no !scope MD
> is created therefore it's not marked as NoAlias from that analysis (it
> returns MayAlias if no scope MD or no noalias MD).

`!noalias` metadata needs a scope to be useful, otherwise it is not:

https://llvm.org/docs/LangRef.html#noalias-and-alias-scope-metadata

Though I don't know what the implementation status in 6.0 was.

~ Johannes


> Thanks,
>
> Ryan
>
> On Mon, Sep 28, 2020, 8:53 PM Johannes Doerfert <johannesdoerfert at gmail.com>
> wrote:
>
>> Hi Ryan,
>>
>> the alias metadata was (and is) broken for various reasons.
>> he replacement is currently under review, I can point you to it,
>> however, to be honest, I'm unclear how we are supposed to help
>> you with LLVM 6 right now.
>>
>> Cheers,
>>     Johannes
>>
>>
>> On 9/28/20 7:00 PM, Ryan Taylor via llvm-dev wrote:
>>> Given some code:
>>>
>>> void func (float * restrict a, float *b) {
>>>     for (int i =0; i < 100; ++i) {
>>>       a[i] = b[i] + 1;
>>>     }
>>> }
>>>
>>> float * aa;
>>> float * bb;
>>> int main() {
>>>      func(aa, bb);
>>>      return 0;
>>> }
>>>
>>> produces IR that has the llvm.noalias intrinsic along with the !noalias
>>> metadata:for both the load and store, however, AA returns MayAlias, I
>> would
>>> expect a NoAlias?
>>>
>>> This is also an older version of llvm: 6 (yes, I know, sigh).
>>>
>>> Thanks,
>>>
>>> Ryan
>>>
>>>
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