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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/43892] PowerPC suboptimal "add with carry" optimization
Date: Thu, 03 Jun 2021 04:33:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D43892

--- Comment #33 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #32)
> So it is more about the back-end of PowerPC at this point.

For the testcase

=3D=3D=3D
typedef unsigned int u32;
typedef unsigned long long u64;

u32 f(u32 a, u32 b)
{
        u32 s =3D a + b;
        if (a + b < b)
                s++;
        return s;
}

u32 g(u32 *p, u32 n)
{
        u32 s =3D 0;
        while (n--)
                s =3D f(s, *p++);
        return s;
}

u32 g4(u32 *p)
{
        u32 s =3D 0;
        s =3D f(s, *p++);
        s =3D f(s, *p++);
        s =3D f(s, *p++);
        s =3D f(s, *p++);
        return s;
}

u32 h4(u32 *p)
{
        u64 s =3D 0;
        s +=3D *p++;imple
        s +=3D *p++;
        s +=3D *p++;
        s +=3D *p++;
        s =3D (s >> 32) + (u32)s;
        s =3D (s >> 32) + (u32)s;
        return s;
}
=3D=3D=3D

... GCC does not do anything with ADD_OVERFLOW.  But all *do* compile
to reasonable code (albeit not optimal).  So no, you cannot say Gimple
is super here and it is all the backend's fault :-)=