[Bug c/66777] New: faggressive-loop-optimizations behavior.

[Bug c/66777] New: faggressive-loop-optimizations behavior.

[dongkyun.s at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66777

            Bug ID: 66777
           Summary: faggressive-loop-optimizations behavior.
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dongkyun.s at samsung dot com
  Target Milestone: ---

The following program seems to have optimized with -O2
(-faggressive-loop-optimizations).

#include <stdio.h>

int main(void)
{
        int a[5];
        int i;

        for (i = 0; a[i] && i < 5; i++)
                printf("%d\n", a[i]);

        return 0;
}

1) .s with -O2 option (-faggressive-loop-optimizations)
...
.L4
      r02     mo3w r0, #:lower16:.LC0
      movt    r0, #:upper16:.LC0
      bl      printf
      ldr     r1, [r4, #4]!
      cmp     r1, #0         // a[i]
      bne     .L4
...

2) .s without -On option (-fno-aggressive-loop-optimizations)
...
.L4
...
       ldr     r2, [fp, #-8]
       mvn     r3, #23
       mov     r2, r2, asl #2
       sub     r1, fp, #4
       add     r2, r2, r1
       add     r3, r2, r3
       ldr     r3, [r3, #0]
       cmp     r3, #0     // a[i]
       beq     .L3
       ldr     r3, [fp, #-8]
       cmp     r3, #4     // i < 5
       ble     .L4
.L3
...

< -faggressive-loop-optimizations >
This assumes that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The bounds for
the number of iterations of a loop are used to guide loop unrolling and peeling
and loop exit test optimizations. This option is enabled by default. 

The problem with 1) option (-faggressive-loop-optimizations) is that the
developer cannot recognize the program have bug which can cause NULL pointer
exception.

Is there some warning option releated to this optimization ?

[Bug c/66777] faggressive-loop-optimizations behavior.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66777

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
-Waggressive-loop-optimizations, but it doesn't warn for me in this case.  Of
course we warn about

t.c: In function ‘main’:
t.c:8:26: warning: ‘a[0]’ is used uninitialized in this function
[-Wuninitialized]
             for (i = 0; a[i] && i < 5; i++)
                          ^
>From gcc-bugs-return-491557-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Jul 06 12:12:41 2015
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To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/66759] [6 Regression] ICE in generic-match.c on 456.hmmer
Date: Mon, 06 Jul 2015 12:12:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf759

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.

[Bug c/66777] faggressive-loop-optimizations behavior.

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66777

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-07-07
                 CC|                            |ebotcazou at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> -Waggressive-loop-optimizations, but it doesn't warn for me in this case.

Note that it's enabled by default.  And it only warns for simple cases, in
particular not if you have a && in the exit condition.

[Bug c/66777] faggressive-loop-optimizations behavior.

[dongkyun.s at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66777

dongkyun.s at samsung dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from dongkyun.s at samsung dot com ---
Verified and confirmed that this was caused by code bug (not by optimization
problem).