[Bug tree-optimization/100801] Aggressive loop optimizations cause incorrect warning

[matthijsvanduin at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100801

Matthijs van Duin <matthijsvanduin at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |matthijsvanduin at gmail dot 
com

--- Comment #3 from Matthijs van Duin <matthijsvanduin at gmail dot com> ---
Simpler testcase:

/* compiler flags required to trigger warning: -O1 -ftree-vrp  */

static void foo(int *x, long n)
{
        long i = 0;
        for (; i + 4 <= n; i += 4) {
        }
        for (; i < n; i++) {
                x[i]++;
        }
}

void bar(int *x)
{
        foo(x, 128);
}


A workaround with no runtime overhead is adding the following check between the
loops:

if (__builtin_constant_p(n % 4 == 0) && n % 4 == 0)
        return;