[patch] Enable lightweight checks with _GLIBCXX_ASSERTIONS.

[Martin Sebor]

On 09/09/2015 09:58 AM, Florian Weimer wrote:
> On 09/08/2015 05:45 PM, Jonathan Wakely wrote:
>
>>> I doubt we can achieve the complexity goals in all cases.  I expect that
>>>
>>>   for (int i = 0; i < 10000; ++i) {
>>>     vector[i];
>>>   }
>>>
>>> is optimized away in default mode, but with _GLIBCXX_ASSERTIONS, it is
>>> not.
>>>
>>> The last time I looked at this, GCC was unable to move bounds checks out
>>> of loops.
>>
>> Maybe we don't want to make _FORTIFY_SOURCE imply _GLIBCXX_ASSERTIONS
>> then, so they can be enabled independently. We don't have to make that
>> decision right away.
>
> I think we should try with _FORTIFY_SOURCE first.  The above case looks
> rather artificial.  If there is a visible performance impact, maybe we
> can get the compiler to eliminate the vector bounds checks in many cases.

There is quite a bit of documentation of _FORTIFY_SOURCE that explains
its effect on user code. People who have read the documentation and
used the macro to achieve the effect might find the secondary effects
on libstdc++ surprising and unwelcome.

Martin