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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102540] [12 Regression] Dead Code Elimination
 Regression at -O3 since r12-476-gd846f225c25c5885
Date: Thu, 20 Jan 2022 09:44:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102540

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think the GCC 12 IL would require tracking equivalences on parts of
registers,
in this case that _2 is equal to the low part of a.0_1.  That is, one would
need to extend what CCP does with bit propagation to include equivalences.

There's also the if ((long)unsigned-var !=3D 0) -> if (unsigned-var !=3D 0)
transform that could save us here but we have that guarded with
single_use () and the converted value is used after the branching
but not the source so we'd have increased register pressure.

I do not see a good way to fix this issue, slightly altering the testcase to
do the undesired transform on the source level

static long a;
static unsigned b;
void foo(void);
int main() {
    long c, e;
    b =3D a;
    c =3D (unsigned)a;
    e =3D c ? 2 / (c + 1) : 0;
    if (e && !b)
        foo();
    a =3D 0;
}

causes the issue to appear also with GCC 11.=