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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/106892] Wrong code at -O3 on x86_64-linux-gnu
Date: Fri, 09 Sep 2022 08:15:14 +0000 [thread overview]
Message-ID: <bug-106892-4-vGCKb3EpkB@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-106892-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106892
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |NEW
CC| |rguenth at gcc dot gnu.org
Keywords| |wrong-code
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Li Shaohua from comment #3)
> Yes, I reduced it too much. Here is the new one with return value in g()
> function.
>
> a, b, c, d, e;
> f[8];
> g() {
> while (a)
> a >>= 4;
> return 0;
> }
> h(i) {
> if (i >= '0')
> return i - '0';
> }
> j(i) {
> b = 2;
> for (; g() <= 7; b++)
> if (i) {
> for (; e <= 7; e++) {
> c = 1;
> for (; c <= 7; c++) {
> d = h(b + 48);
> f[-d + 4] ^= 3;
> }
> }
> return;
> }
> }
> main() {
> j(1);
> printf("%d\n", f[2]);
> }
When I apply the same "fix" to h() (add a return 0 or __builtin_unreachable ())
the code works again.
Note I think the missing return stmt isn't reached at runtime.
Disabling either unswitching or loop splitting makes the issue go away,
enabling both ontop of -O2 doesn't trigger it.
We early inline h into j where h looks like
int h (int i)
{
int _3;
<bb 2> :
if (i_1(D) > 47)
goto <bb 3>; [INV]
else
goto <bb 4>; [INV]
<bb 3> :
_3 = i_1(D) + -48;
// predicted unlikely by early return (on trees) predictor.
return _3;
<bb 4> :
return;
}
which results in
b.2_1 = b;
_2 = b.2_1 + 48;
_3 = h (_2);
d = _3;
becoming
b.2_1 = b;
_2 = b.2_1 + 48;
if (_2 > 47)
goto <bb 6>; [34.00%]
else
goto <bb 7>; [66.00%]
<bb 6> :
_30 = _2 + -48;
_43 = _30;
<bb 7> :
# _39 = PHI <_43(6), _37(5)>
_3 = _39;
d = _3;
note that the return; case results in _37 to be used which is completely
random.
That said, our handling of an unreachable missing return in the IL is
likely counter productive.
Cleaned up testcase:
int a, b, c, d, e;
int f[8];
int g() {
while (a)
a >>= 4;
return 0;
}
int h(int i) {
if (i >= '0')
return i - '0';
//__builtin_unreachable ();
}
void j(int i) {
b = 2;
for (; g() <= 7; b++)
if (i) {
for (; e <= 7; e++) {
c = 1;
for (; c <= 7; c++) {
d = h(b + 48);
f[-d + 4] ^= 3;
}
}
return;
}
}
int main() {
j(1);
if (f[2] != 0)
__builtin_abort ();
}
next prev parent reply other threads:[~2022-09-09 8:15 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-08 21:37 [Bug c/106892] New: " shaohua.li at inf dot ethz.ch
2022-09-08 22:02 ` [Bug c/106892] " jakub at gcc dot gnu.org
2022-09-09 7:40 ` rguenth at gcc dot gnu.org
2022-09-09 7:53 ` shaohua.li at inf dot ethz.ch
2022-09-09 8:15 ` rguenth at gcc dot gnu.org [this message]
2022-09-09 8:20 ` marxin at gcc dot gnu.org
2022-09-09 8:20 ` jakub at gcc dot gnu.org
2022-09-09 8:31 ` [Bug c/106892] [11/12/13 Regression] Wrong code at -O3 on x86_64-linux-gnu since r11-963-g80d6f89e78fc3b77 rguenth at gcc dot gnu.org
2022-09-09 8:32 ` [Bug middle-end/106892] " rguenth at gcc dot gnu.org
2022-09-09 9:05 ` rguenth at gcc dot gnu.org
2022-09-09 9:31 ` rguenth at gcc dot gnu.org
2022-09-09 10:04 ` rguenth at gcc dot gnu.org
2022-09-09 12:43 ` rguenth at gcc dot gnu.org
2022-09-10 13:14 ` [Bug middle-end/106892] [11/12 " mikpelinux at gmail dot com
2022-09-12 7:53 ` marxin at gcc dot gnu.org
2022-10-11 12:06 ` cvs-commit at gcc dot gnu.org
2022-10-17 13:28 ` [Bug middle-end/106892] [11 " cvs-commit at gcc dot gnu.org
2022-10-17 13:29 ` rguenth at gcc dot gnu.org
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