Package: gcc-8
Version: 8.3.0-6
Severity: important
File: /usr/bin/gcc
Dear Maintainer,
I have encountered a bug on the simple code below:
--
#include <stdio.h>
int fits(int x, int n)
{
int TMin_n = -(1 << (n-1));
int TMax_n = (1 << (n-1)) - 1;
return x >= TMin_n && x <= TMax_n;
}
int main(void)
{
int cnt;
int x = 1<<31;
for(cnt = 1; cnt<=32; cnt++)
{
if (fits(x,cnt))
{
printf("%d : ok\n", cnt);
}
else
{
printf("%d : ko\n", cnt);
}
}
return 0;
}
--
When compiled without optimization, the code produces a correct
output (comparison is always false except for n=32):
--
sfaucou@sfaucou-laptop:~/Temp$ gcc bug.c
sfaucou@sfaucou-laptop:~/Temp$ ./a.out
1 : ko
2 : ko
3 : ko
4 : ko
5 : ko
6 : ko
7 : ko
8 : ko
9 : ko
10 : ko
11 : ko
12 : ko
13 : ko
14 : ko
15 : ko
16 : ko
17 : ko
18 : ko
19 : ko
20 : ko
21 : ko
22 : ko
23 : ko
24 : ko
25 : ko
26 : ko
27 : ko
28 : ko
29 : ko
30 : ko
31 : ko
32 : ok
--
When compiling with optimization, it produces a wrong output
(comparison is evaluated to false for n=32):
--
sfaucou@sfaucou-laptop:~/Temp$ gcc -O bug.c
sfaucou@sfaucou-laptop:~/Temp$ ./a.out
1 : ko
2 : ko
3 : ko
4 : ko
5 : ko
6 : ko
7 : ko
8 : ko
9 : ko
10 : ko
11 : ko
12 : ko
13 : ko
14 : ko
15 : ko
16 : ko
17 : ko
18 : ko
19 : ko
20 : ko
21 : ko
22 : ko
23 : ko
24 : ko
25 : ko
26 : ko
27 : ko
28 : ko
29 : ko
30 : ko
31 : ko
32 : ko
--
From what I have seen from the generated code, the optimized
version computes the result at compile-time and simply prints the
message "ko".
I have tested the code with various optimization options (-O, -O1,
-O2, -O3), for ia32 and x86-64 ISA. I did also reproduce the bug
on machine running Ubuntu (same version of gcc).
Syste information as produced by reportbug are given below.
Let me know if you need further information or if I can help in
some other way.
Best regards,
Sébastien.
-- System Information:
Debian Release: 10.2
APT prefers stable-updates
APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
Kernel: Linux 4.19.0-6-amd64 (SMP w/4 CPU cores)
Kernel taint flags: TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8),
LANGUAGE= (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled
Versions of packages gcc-8 depends on:
ii binutils 2.31.1-16
ii cpp-8 8.3.0-6
ii gcc-8-base 8.3.0-6
ii libc6 2.28-10
ii libcc1-0 8.3.0-6
ii libgcc-8-dev 8.3.0-6
ii libgcc1 1:8.3.0-6
ii libgmp10 2:6.1.2+dfsg-4
ii libisl19 0.20-2
ii libmpc3 1.1.0-1
ii libmpfr6 4.0.2-1
ii libstdc++6 8.3.0-6
ii zlib1g 1:1.2.11.dfsg-1
Versions of packages gcc-8 recommends:
ii libc6-dev 2.28-10
Versions of packages gcc-8 suggests:
pn gcc-8-doc <none>
pn gcc-8-locales <none>
ii gcc-8-multilib 8.3.0-6
pn libasan5-dbg <none>
pn libatomic1-dbg <none>
pn libgcc1-dbg <none>
pn libgomp1-dbg <none>
pn libitm1-dbg <none>
pn liblsan0-dbg <none>
pn libmpx2-dbg <none>
pn libquadmath0-dbg <none>
pn libtsan0-dbg <none>
pn libubsan1-dbg <none>
-- no debconf information
