[llvm-dev] One more No-alias case on Alias analysis

jingu@codeplay.com via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 12 02:53:40 PDT 2018

Thanks Eli for kind comment. I have created the review on Phabricator. 
https://reviews.llvm.org/D48066 Please review it.

Kind regards,

JinGu Kang

On 11/06/18 20:33, Friedman, Eli wrote:
> On 6/11/2018 10:06 AM, jingu at codeplay.com via llvm-dev wrote:
>> Hello All,
>> I have met one may-alias case from llvm's alias analysis. The code 
>> snippet is as following:
>> char buf[4];
>> void test (int idx) {
>> char *a = &buf[3 - idx];
>> char *b = &buf[idx];
>> *a = 1;
>> *b = 2;
>> }
>> I can see below output from alias set tracker for above code snippet.
>> Alias sets for function 'test':
>> Alias Set Tracker: 1 alias sets for 2 pointer values.
>>   AliasSet[0x53d8070, 2] may alias, Mod       Pointers: (i8* 
>> %arrayidx, 1), (i8* %arrayidx2, 1)
>> As you can see on above code snippet, the 'a' and 'b' are not 
>> aliased. I think if we have following offset form, we can say 
>> No-alias between them.
>> offset1 = odd_number - index
>> offset2 = index
>> I have implemented simple code for it and the output is as following:
>> Alias sets for function 'test':
>> Alias Set Tracker: 2 alias sets for 2 pointer values.
>>   AliasSet[0x541a070, 1] must alias, Mod       Pointers: (i8* 
>> %arrayidx, 1)
>>   AliasSet[0x541cc00, 1] must alias, Mod       Pointers: (i8* 
>> %arrayidx2, 1)
>> How do you think about this? Is it legal for current alias analysis 
>> or not? I have attached the diff file as reference. If I missed 
>> something, please let me know. 
> The concept works. I'm not sure your patch handles all the edge cases 
> correctly, at first glance.  (If you want a full review, please post 
> on Phabricator.)
> -Eli

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