[patch] Enable lightweight checks with _GLIBCXX_ASSERTIONS.

[Martin Sebor]

On 09/10/2015 10:38 AM, Florian Weimer wrote:
> On 09/10/2015 06:30 PM, Martin Sebor wrote:
>> 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.
>
> I think there are only random blog articles discussing aspects of it,
> most of them slightly incorrect or outdated.

_FORTIFY_SOURCE is a GLIBC feature test macro. It's documented
in <features.h> and mentioned in some of its online manuals.
For example:

http://man7.org/linux/man-pages/man7/feature_test_macros.7.html

or here:

http://manpages.ubuntu.com/manpages/hardy/man7/feature_test_macros.7.html

Martin