#539 Warning from the compiler on two nested loops

[Jean-Claude Arbaut]

The same problem happens with gcc -O2 and gfortran -O2, respectively with the programs:

#include <stdio.h>

int main() {
    int n = 1626, i, j, s = 0;
    for(i = 1; i <= n; i++) {
        for(j = 1; j <= n; j++) {
            s += i * j;
        }
    }
    printf("%d\n", s);
    return 0;
}

program bug
    integer :: n = 1626, i, j, s = 0
    do i = 1, n
        do j = 1, n
            s = s + i * j
        end do
    end do
    print *, s
end program

With gcc -O2, I get the message

bug.c: In function 'main':
cc1.exe: warning: iteration 1623u invokes undefined behavior [-Waggressive-loop-optimizations]
bug.c:5:5: note: containing loop
     for(i = 1; i <= n; i++) {
     ^

With gfortran -O2, I get instead

f951.exe: Warning: iteration 1623 invokes undefined behavior [-Waggressive-loop-optimizations]
bug.f90:3:0:

     do i = 1, n
 ^
note: containing loop

This warning seem to appear only for n >= 1626, and only with -O2 or higher.
I got this behaviour with both the x86-win32-dwarf and x86-64-win32-seh compilers (in the "Personal Builds/mingw-builds/" directory), for versions 4.9.3, 5.2.0 and 5.3.0 in the sourceforge repository, but not with the version 4.8.5.

[Ozkan Sezer]

Hmm, s overflows (-2147432845) at i==57 and j==1130, but yes gcc4.8 doesn't warn.

[Jean-Claude Arbaut]

Interesting, with "unsigned" instead of "int" in the C program, I get no warning. So actually, the compiler is clever enough to see there will eventually be an undefined behaviour due to integer overflow. Thanks.

[NightStrike]

This should probably be closed to a gcc PR.
On May 11, 2016 6:17 AM, "Jean-Claude Arbaut" telephore@users.sf.net
wrote:

Interesting, with "unsigned" instead of "int" in the C program, I get no
warning. So actually, the compiler is clever enough to see there will
eventually be an undefined behaviour due to integer overflow. Thanks.

---

• [bugs:#539] https://sourceforge.net/p/mingw-w64/bugs/539/ Warning from
  the compiler on two nested loops*

Status: open
Group: v1.0 (example)
Created: Wed May 11, 2016 09:36 AM UTC by Jean-Claude Arbaut
Last Updated: Wed May 11, 2016 09:54 AM UTC
Owner: nobody

The same problem happens with gcc -O2 and gfortran -O2, respectively with
the programs:

include <stdio.h></stdio.h>

int main() {
int n = 1626, i, j, s = 0;
for(i = 1; i <= n; i++) {
for(j = 1; j <= n; j++) {
s += i * j;
}
}
printf("%d\n", s);
return 0;}

program bug
integer :: n = 1626, i, j, s = 0
do i = 1, n
do j = 1, n
s = s + i * j
end do
end do
print *, s
end program

With gcc -O2, I get the message

bug.c: In function 'main':cc1.exe: warning: iteration 1623u invokes undefined behavior [-Waggressive-loop-optimizations]bug.c:5:5: note: containing loop
for(i = 1; i <= n; i++) {
^

With gfortran -O2, I get instead

f951.exe: Warning: iteration 1623 invokes undefined behavior [-Waggressive-loop-optimizations]
bug.f90:3:0:

 do i = 1, n

^
note: containing loop

This warning seem to appear only for n >= 1626, and only with -O2 or
higher.
I got this behaviour with both the x86-win32-dwarf and x86-64-win32-seh
compilers (in the "Personal Builds/mingw-builds/" directory), for versions
4.9.3, 5.2.0 and 5.3.0 in the sourceforge repository, but not with the
version 4.8.5.

---

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