Optimising away memset() calls?

[Ángel González]

On 11/10/14 12:07, Florian Weimer wrote:
> On 10/10/2014 11:20 AM, Andrew Haley wrote:
>> On 09/10/14 21:52, Ángel González wrote:
>>
>>> The compiler would need to know that memset_s is special (either
>>> intrinsically or thorugh eg. function attributes). Either way, IMHO
>>> an advanced knowledge allowing to optimize it out would be a
>>> violation of K.3.7.4.1.
>>
>> It would be a perverse thing to do and goes against intent, but we
>> again fall into the problem of defining an access.  But this is
>> irrelevant anyway: even if a key is stored in an array X in the source
>> code and the array X is later wiped with memset_s(), there is
>> absolutely nothing to force the compiler to use X during the
>> computation: it may well store the key somewhere else altogether.
>
> It's not even that hard to come up with an example where calling 
> memset_s causes the creation of another copy which is the only one 
> which is being cleared:
>
Actually it's use_key() what produces the copy by getting struct key by 
value. It's just that in the first case gcc is smart enough to notice 
key isn't used later and gives its own version to the function. If you 
compile with -O0 you get the copy in both without_clear() and with_clear()