[Bug c/65357] New: aggressive loop optimization not correct

[Bug c/65357] New: aggressive loop optimization not correct

[weiming at ms1 dot url.com.tw]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

            Bug ID: 65357
           Summary: aggressive loop optimization not correct
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: weiming at ms1 dot url.com.tw

source:

#include <stdio.h>
#include <stdlib.h>

void    addlist(int n, int *list)
{
    int    i;
    for (i=0; i<n; i++)
    {
        *list++=*list+10;
        *list++=*list+11;
    }
}

int    tlist[10]={ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

void    main(void)
{
    int    i;
    addlist(5, tlist);
    for (i=0; i<10; i++)
    {
        fprintf(stdout, "%d: %d\n", i, tlist[i]);
    }
}

makefile:
tt.exe: tt.c
    gcc -O3 -o $@ $<

result of gcc 4.7.3:
0: 10
1: 12
2: 12
3: 14
4: 14
5: 16
6: 16
7: 18
8: 18
9: 20

result of gcc 4.8.4 and 4.9.2:
0: 11
1: 13
2: 13
3: 15
4: 15
5: 17
6: 17
7: 19
8: 19
9: 11

which is correct?

[Bug c/65357] aggressive loop optimization not correct

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |trippels at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #1 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
Both, because you invoke undefined behavior.
https://gcc.gnu.org/bugs/#nonbugs_c

[Bug c/65357] aggressive loop optimization not correct

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is addlist goes one past the end of the array tlist.

[Bug c/65357] aggressive loop optimization not correct

[weiming at ms1 dot url.com.tw]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

--- Comment #3 from Weiming Shih <weiming at ms1 dot url.com.tw> ---
I see. Thank you.

[Bug c/65357] aggressive loop optimization not correct

[weiming at ms1 dot url.com.tw]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

--- Comment #4 from Weiming Shih <weiming at ms1 dot url.com.tw> ---
How can I get a warning for such an undefined behavior?

[Bug c/65357] aggressive loop optimization not correct

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Perhaps by enabling warnings?  E.g. with -Wall you get all the important
warnings:
warning: operation on ‘list’ may be undefined [-Wsequence-point]
(twice) and
warning: return type of ‘main’ is not ‘int’ [-Wmain]
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Subject: [Bug target/65358] wrong parameter passing code with tail call optimization on arm
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide358

--- Comment #6 from Mikael Pettersson <mikpelinux at gmail dot com> ---
(In reply to Honggyu Kim from comment #4)

> Can I add this testcase with your modification as my first gcc contribution?
> :)

Sure, just attach it to this PR as a new test case.

[Bug c/65357] aggressive loop optimization not correct

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65357

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I missed there was a sequence point issue there too :).