llvm dev - Apr 2018 - Missed strlen optimizations

Hello,

Code: https://godbolt.org/g/EG4Wi6

unsigned fff3(void) {
    char buf[10] = "";
    return strlen(buf);
}

Since we are memset-ing before strlen call, we could replace strlen with
just 0.

Has LLVM any API to get "last instruction before strlen" which
modifies
strlen argument "buf"? So we can check "yes, it is memset there,
replace
strlen with zero" ?


Thanks
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Is:


2018-04-20 18:07 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:
> Hello,
>
> Code: https://godbolt.org/g/EG4Wi6
>
> unsigned fff3(void) {
>     char buf[10] = "";
>     return strlen(buf);
> }
>
> Since we are memset-ing before strlen call, we could replace strlen with
> just 0.
>
> Has LLVM any API to get "last instruction before strlen" which
modifies
> strlen argument "buf"? So we can check "yes, it is memset
there, replace
> strlen with zero" ?
>
>
> Thanks
>
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Use *last = nullptr;
  for (Use &U : Src->uses())
      last = &U;
  last->getUser()->dump();


Or any better solution?

2018-04-20 19:19 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:
> Is:
>
>
> 2018-04-20 18:07 GMT+02:00 Dávid Bolvanský <david.bolvansky at
gmail.com>:
>
>> Hello,
>>
>> Code: https://godbolt.org/g/EG4Wi6
>>
>> unsigned fff3(void) {
>>     char buf[10] = "";
>>     return strlen(buf);
>> }
>>
>> Since we are memset-ing before strlen call, we could replace strlen
with
>> just 0.
>>
>> Has LLVM any API to get "last instruction before strlen"
which modifies
>> strlen argument "buf"? So we can check "yes, it is
memset there, replace
>> strlen with zero" ?
>>
>>
>> Thanks
>>
>
>
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