[Bug c/54907] 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

--- Comment #2 from yangzhe1990 at gmail dot com 2012-10-12 11:20:39 UTC ---
No, p is not modified twice.

p is modified once,
*p is modified once.

(In reply to comment #1)
> Not a bug p is modified twice without a seqence point the result is undefined
> 
> 
> 
> ________________________________
> From: yangzhe1990 at gmail dot com <gcc-bugzilla@gcc.gnu.org>
> To: gcc-bugs@gcc.gnu.org 
> Sent: Friday, 12 October 2012, 11:10
> Subject: [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
> 
> 
> 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: unassigned@gcc.gnu.org
>         ReportedBy: yangzhe1990@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.