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Re: [patch] Enable lightweight checks with _GLIBCXX_ASSERTIONS.


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


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