http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58686
--- Comment #2 from Cong Hou <congh at google dot com> ---
I think this issue is more like a missed optimization.
If the iteration number can be calculated as a constant value at compile time,
then the function assert_loop_rolls_lt() won't be called due to an early exit
(specifically in the function number_of_iterations_lt() at the call to
number_of_iterations_lt_to_ne()). That is why I could not craft a testcase
showing miscompile.
A better test case is shown below:
#define N 4
void foo(int* a, unsigned int i)
{
int j = 0;
do
{
a[j++] = 0;
i -= 4;
}
while (i >= N);
}
Compile it with -O3 and the produced result is using __builtin_memset() as the
niter can be calculated. But if the value of N is replaced by others like 3 or
5, GCC won't optimize this loop into __builtin_memset() any more.
