Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

[Paul Eggert]

Robert Dewar <address@hidden> writes:

> We have not seen ONE imaginary example, let
> alone a real example, where the optimziation of loop invariants
> (by far the most important optimization in the class we are
> discussing) would break existing code.

But didn't this thread get started by a real program that was broken
by an optimization of loop invariants?  Certainly I got a real bug
report of a real problem, which you can see here:

http://lists.gnu.org/archive/html/bug-gnulib/2006-12/msg00084.html

Here is a bit more discussion:

http://gcc.gnu.org/ml/gcc/2006-12/msg00607.html

If this doesn't count as "optimization of loop invariants"
then what would count?

This particular example was just a test program run by "configure", so
the penalty for getting it wrong wasn't that severe -- the application
compiled its own version of mktime rather than using the system
mktime.  But I daresay I can find an example of real-world production
code that does something similar.