[Bug c/54907] New: post increasing a value pointed by p in subexpression of an expression modifying p saves the increased value in the wrong place

[yangzhe1990 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907



             Bug #: 54907

           Summary: post increasing a value pointed by p in subexpression

                    of an expression modifying p saves the increased value

                    in the wrong place

    Classification: Unclassified

           Product: gcc

           Version: 4.7.1

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: yangzhe1...@gmail.com





#include <stdio.h>



int main() {

    char s[] = "axxxxx";

    char *p = s;



    printf("s = %s in the beginning.\n"

           "p is pointed at the %d-th char.\n", s, p - s);

    //p = p + (*p)++ * 3 + 2 - 'a' * 3; // (1)

    p += (*p)++ * 3 + 2 - 'a' * 3; // (2)

    printf("p is moved ahead by %d steps\n", p - s);

    printf("s = %s after the operation.\n", s);

    return 0;

}



The expected result is "bxxxxx". But the output is "axbxxx".



Maybe in the wrong code, when it saves the value, it lookups the address again

by *p, but p is modified in the expression.



As discussed in stackoverflow,

http://stackoverflow.com/questions/12823663/would-p-p-p-3-c-cause-an-undefined-behavior?answertab=votes#tab-top

most people think it's a bug of gcc.



Bug found in gcc 4.4.6, 4.7.1, g++ 4.4.6. g++ 4.7.1 produces the correct

result.