debug extension overhead

[Jonathan Wakely]

2009/6/3 Silvius Rus:
> Coming back to performance guarantees.  We are considering turning on
> _GLIBCXX_DEBUG with say "-O0 -g" builds, but need to assure users that
> the overhead is acceptable.  I am trying to avoid users writing their
> own vector wrapper that adds simple bounds checks with -DDEBUG.

What's wrong with std::vector::at() ?  :-)

> Is there a list of asymptotic overhead bounds for the checks done with
> _GLIBCXX_DEBUG?

There are no performance guarantees for debug mode.  The overhead is
whatever is necessary to achieve the checking.

>> Usability in time sensitive applications is not and has never been a
>> design goal for debug extensions.
>>
>> -benjamin
>
> That's fine.  However, I would like to be able to debug an application
> that makes decisions based on the amount of time it takes to perform
> some operations.  If the operation ratios change too much, the dynamic
> program paths change, so I'm not debugging the original behavior.
> Turning on just a subset of guaranteed constant overhead checks might
> just do, since the overhead will likely distribute fairly uniformly,
> so operation ratios won't change too much.

One option would be to use debug mode in unit tests for individual
components, to verify correct stdlib usage separate from
time-sensitive behaviour as part of a larger system, but not to try
and enable it routinely for entire applications.

Jonathan