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From: "matthijsvanduin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/100801] Aggressive loop optimizations cause
 incorrect warning
Date: Sat, 31 Jul 2021 00:41:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100801

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matthijsvanduin at gmail d=
ot 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 =3D 0;
        for (; i + 4 <=3D n; i +=3D 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 =3D=3D 0) && n % 4 =3D=3D 0)
        return;=