I was writing a magic squares program for my sons homework and somewhat
surprised/annoyed that gcc-4.1.1 and 4.0.3 have a horrible code generation bug.
Compiled with -O or -O2 on gcc-4.0.3 the following code infinitely loops.
gcc-4.1.1 with no optimisation works, but -O2 fails. Local variable "n" in
main() is modified by the try function when it cannot be.
# include <stdio.h>
# include <string.h>
int nums[] = {15, 9, 3, 7, 13, 17, 11, 5, 19};
void try_sq(int n);
int main(int argc, char **argv)
{
int n;
for (n = 0; n < 999999999; n++) {
if ((n % 10000000) == 0)
printf("%d\n", n);
try_sq(n);
}
return 0;
}
void try_sq(int n)
{ int sq[9], sq2[9];
int i, s;
memcpy(sq, nums, sizeof nums);
for (i = 0; i < 9; i++) {
int d = n % 10;
n /= 10;
if (sq[d] == 0)
return;
sq2[i] = sq[d];
sq[d] = 0;
}
/***********************************************/
/* Rows. */
/***********************************************/
s = sq2[0] + sq2[1] + sq2[2];
if (s != 33)
return;
s = sq2[3] + sq2[4] + sq2[5];
if (s != 33)
return;
s = sq2[6] + sq2[7] + sq2[8];
if (s != 33)
return;
/***********************************************/
/* Columns. */
/***********************************************/
s = sq2[0] + sq2[3] + sq2[6];
if (s != 33)
return;
s = sq2[1] + sq2[4] + sq2[8];
if (s != 33)
return;
s = sq2[2] + sq2[5] + sq2[8];
if (s != 33)
return;
/***********************************************/
/* Diagonal. */
/***********************************************/
s = sq2[0] + sq2[4] + sq2[8];
if (s != 33)
return;
s = sq2[2] + sq2[4] + sq2[6];
if (s != 33)
return;
printf("%2d %2d %2d\n", sq2[0], sq2[1], sq2[2]);
printf("%2d %2d %2d\n", sq2[3], sq2[4], sq2[5]);
printf("%2d %2d %2d\n", sq2[6], sq2[7], sq2[28]);
printf("\n===\n");
}
--
Summary: code generation / optimisation bug
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: fox at crisp dot demon dot co dot uk
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29406
