[llvm-dev] Malloc null checks, why sometimes are moved and sometimes not?

[Krzysztof Parzyszek via llvm-dev]

On 4/14/2018 3:09 AM, 陳韋任 wrote:
> 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?

Normally, yes, but the standard explicitly allows the compiler to not 
call allocation/deallocation functions if the storage can be provided in 
another way. If the storage that would have been allocated by malloc 
cannot ever be accessed, we can pretend that it has been successfully 
provided in some way.

-Krzysztof