llvm dev - Apr 2018 - Malloc null checks, why sometimes are moved and sometimes not?

Hello,


Here is simple test code:
https://godbolt.org/g/mjAUpu

LLVM generally assumes that malloc never fails.

But I dont understand difference between these two example functions - and
why null check was not removed in f1, since in f2 it was removed.

Thanks
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On 4/13/2018 6:39 PM, Dávid Bolvanský via llvm-dev
wrote:
> 
> Here is simple test code:
> https://godbolt.org/g/mjAUpu
> 
> LLVM generally assumes that malloc never fails.
> 
> But I dont understand difference between these two example functions - 
> and why null check was not removed in f1, since in f2 it was removed.
That's because the return value from malloc is discarded in f2. In that 
case it simply doesn't matter if the malloc happened or not, and can be 
assumed to have succeeded.

In the first case, the pointer is returned from the function, which 
means that someone may actually want to access the allocated storage.

-Krzysztof

2018-04-14 7:51 GMT+08:00 Krzysztof Parzyszek via llvm-dev
<llvm-dev at lists.llvm.org>:
> On 4/13/2018 6:39 PM, Dávid Bolvanský via llvm-dev wrote:
>>
>>
>> Here is simple test code:
>> https://godbolt.org/g/mjAUpu
>>
>> LLVM generally assumes that malloc never fails.
>>
>> But I dont understand difference between these two example functions -
and
>> why null check was not removed in f1, since in f2 it was removed.
>
>
> That's because the return value from malloc is discarded in f2. In that
case
> it simply doesn't matter if the malloc happened or not, and can be
assumed
> to have succeeded.
Wouldn't such assumption be too aggressive?


-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj