[Bug c/56463] New: infinite loop when having integer overflow in a simple accumulator

[nakacristo at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56463

             Bug #: 56463
           Summary: infinite loop when having integer overflow in a simple
                    accumulator
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: nakacristo@hotmail.com


A colleague has showed me a problem and I have reduced it to this code:

#include <stdio.h>
int main()
{
    int i,j;
    int a=0;
    for(i=0;i<20000;i++)
        for(j=0;j<600;j++)
            a=a+3435345234440;
    printf("%d\n",a);
    return 0;
}

With 'gcc test.c' or 'gcc test.c -O' works correctly, but with 'gcc test.c -O2'
fails. More exactly, fails with 'gcc test.c -O -fstrict-overflow -ftree-pre
-ftree-vrp'.

With the optimization it becomes an infinite loop
00000000004004f4 <main>:
  4004f4:    b8 58 02 00 00           mov    $0x258,%eax
  4004f9:    ba 58 02 00 00           mov    $0x258,%edx
  4004fe:    83 e8 01                 sub    $0x1,%eax
  400501:    0f 44 c2                 cmove  %edx,%eax
  400504:    eb f8                    jmp    4004fe <main+0xa>

The overflow could make the variable 'a' to take any value, but never to make
the loop infinite.


My gcc version: gcc version 4.6.3 (Gentoo 4.6.3 p1.9, pie-0.5.2)
I have checked that in gcc-4.5.3 works correctly.