[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: unassig...@gcc.gnu.org

        ReportedBy: nakacri...@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.