[Bug c++/43247] New: Icorrect optimization while declaring array[1]

[Bug c++/43247] New: Icorrect optimization while declaring array[1]

[ogoffart at kde dot org]

This code should print "WORKS!!!!" a couple of time. But with -O2, g++ optimize
the condition away.
It works with -O1

Tested with gcc 4.4.3 on linux x86_64 (Archlinux)


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

struct QVectorTypedData
{
    int array[1];
};

int main(int , char **)
{
    QVectorTypedData *d;
    d = static_cast<QVectorTypedData *>(::malloc(sizeof(QVectorTypedData)+
(45-1) * sizeof(int)));
    memset(d->array, 0, 45 * sizeof(int));
    int *array = d->array;
    int count = 0;
    for (int i = 0; i < 10; i++) {
        fprintf(stderr, "%d %d %d %d\n", i, (i>=3), (i<=8), (i>=3) && (i <=
8));
        if (i >= 3 && i <= 8) {
            fprintf(stderr, "WORKS!!!\n");
        }
        array[i] = 4;
    }
    return 0;
}


-- 
           Summary: Icorrect optimization while declaring array[1]
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ogoffart at kde dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug c++/43247] Icorrect optimization while declaring array[1]

[thiago at kde dot org]

------- Comment #1 from thiago at kde dot org  2010-03-03 14:41 -------
Problem also happens on:

gcc 4.4.3 on linux 32-bit
gcc 4.4.1 on linux ARM (armel gnueabi)

Also reproducible with -O1 -ftree-vrp.


-- 

thiago at kde dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |thiago at kde dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug c++/43247] Icorrect optimization while declaring array[1]

[thiago at kde dot org]

------- Comment #2 from thiago at kde dot org  2010-03-03 14:44 -------
Also:
-O1 -ftree-vrp -fno-cprop-registers -fno-defer-pop
-fno-guess-branch-probability -fno-if-conversion -fno-if-conversion2
-fno-ipa-pure-const -fno-ipa-reference -fno-merge-constants
-fno-omit-frame-pointer -fno-split-wide-types -fno-tree-ch -fno-tree-copy-prop
-fno-tree-copyrename -fno-tree-dce -fno-tree-dominator-opts -fno-tree-dse
-fno-tree-fre -fno-tree-sink -fno-tree-sra -fno-tree-ter

However, if I add -fno-tree-ccp, the program starts to work as expected again.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Icorrect optimization while declaring array[1]

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2010-03-03 16:08 -------
Confirmed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|c++                         |tree-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
      Known to fail|                            |4.3.0 4.3.4 4.4.3
      Known to work|                            |4.2.4 4.5.0
   Last reconfirmed|0000-00-00 00:00:00         |2010-03-03 16:08:00
               date|                            |
            Summary|Icorrect optimization while |[4.3/4.4 Regression]
                   |declaring array[1]          |Icorrect optimization while
                   |                            |declaring array[1]
   Target Milestone|---                         |4.3.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2010-03-26 18:46 -------
*** Bug 43537 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |juha dot kallioinen at nokia
                   |                            |dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[hjl dot tools at gmail dot com]

------- Comment #5 from hjl dot tools at gmail dot com  2010-03-26 18:54 -------
This is fixed by revision 151360:

http://gcc.gnu.org/ml/gcc-cvs/2009-09/msg00106.html

and was introduced by revision 118729:

http://gcc.gnu.org/ml/gcc-cvs/2006-11/msg00380.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[thiago at kde dot org]

------- Comment #6 from thiago at kde dot org  2010-03-26 21:46 -------
Is this fix going to be backported to the 4.4.x line?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2010-03-27 10:45 -------
I very much doubt that the cited revision fixed anything here.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[hjl dot tools at gmail dot com]

------- Comment #8 from hjl dot tools at gmail dot com  2010-03-27 14:24 -------
(In reply to comment #7)
> I very much doubt that the cited revision fixed anything here.
> 

If it is true, that only means that the bug is latent on trunk.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2010-05-22 18:13 -------
GCC 4.3.5 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.5                       |4.3.6


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[thiago at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

--- Comment #10 from Thiago Macieira <thiago at kde dot org> 2010-12-22 10:35:23 UTC ---
This is still not fixed. I can reproduce now with a different testcase, in
4.5.1. However, this time, the same code works fine in 4.4. The reason is again
accessing an array out-of-bounds for elements that we know to be there. Pay
attention to the way operator== is implemented in the following code.

If I compile it with -O1, it prints "true" as it should. If I compile it with
-O2, it prints "false". If I compile it with -O1 -finline-small-functions
-finline -findirect-inlining -fstrict-overflow and compare the disassembly with
-O2 and a suitable list of -fno-*, the code is exactly identical, except for
some instructions that should perform the copy of half of m1's data into m3. So
in the end the comparison fails due to comparing to garbage.

=== code ===
#include <stdio.h>

template <int N, int M, typename T>
class QGenericMatrix
{
public:
    QGenericMatrix();
    QGenericMatrix(const QGenericMatrix<N, M, T>& other);
    explicit QGenericMatrix(const T *values);

    bool operator==(const QGenericMatrix<N, M, T>& other) const;
private:
    T m[N][M];    // Column-major order to match OpenGL.

    QGenericMatrix(int) {}       // Construct without initializing identity
matrix
};

template <int N, int M, typename T>
QGenericMatrix<N, M, T>::QGenericMatrix(const QGenericMatrix<N, M, T>& other)
{
    for (int col = 0; col < N; ++col)
        for (int row = 0; row < M; ++row)
            m[col][row] = other.m[col][row];
}

template <int N, int M, typename T>
QGenericMatrix<N, M, T>::QGenericMatrix(const T *values)
{
    for (int col = 0; col < N; ++col)
        for (int row = 0; row < M; ++row)
            m[col][row] = values[row * N + col];
}

template <int N, int M, typename T>
bool QGenericMatrix<N, M, T>::operator==(const QGenericMatrix<N, M, T>& other)
const
{
    for (int index = 0; index < N * M; ++index) {
        if (m[0][index] != other.m[0][index])
            return false;
    }
    return true;
}

typedef double qreal;
typedef QGenericMatrix<2, 2, qreal> QMatrix2x2;

int main(int , char**)
{
    qreal m1Data[] = {0.0, 0.0, 0.0, 0.0};
    QMatrix2x2 m1(m1Data);

    QMatrix2x2 m3 = m1;
    puts((m1 == m3) ? "true" : "false");
}
=== code ===

common args: -fno-exceptions -fno-rtti -fverbose-asm -march=core2 -mfpmath=sse
(though x87 math also shows the same problem)

prints "true" with: -O1 -finline-small-functions -finline -findirect-inlining
-fstrict-overflow

prints "false" with: -O2 -fno-align-functions -fno-align-jumps
-fno-align-labels -fno-caller-saves -fno-tree-switch-conversion -fno-tree-vrp
-fno-crossjumping -fno-cse-follow-jumps -fno-expensive-optimizations -fno-gcse
-fno-ipa-cp -fno-ipa-sra -fno-optimize-register-move
-fno-optimize-sibling-calls -fno-peephole2 -fno-regmove -fno-reorder-blocks
-fno-reorder-functions -fno-rerun-cse-after-loop -fno-schedule-insns2
-fno-strict-aliasing -fno-strict-aliasing -fno-thread-jumps
-fno-tree-builtin-call-dce -fno-tree-pre

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |
      Known to fail|                            |

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> 2010-12-22 17:48:55 UTC ---
>The reason is again accessing an array out-of-bounds for elements that we know to be there.

No that is undefined and different from the original testcase.

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[thiago at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

--- Comment #12 from Thiago Macieira <thiago at kde dot org> 2010-12-22 19:55:38 UTC ---
(In reply to comment #11)
> >The reason is again accessing an array out-of-bounds for elements that we know to be there.
> 
> No that is undefined and different from the original testcase.

Ok. Shall I open a new report with the new information?

[Bug tree-optimization/43247] [4.3/4.4 Regression] Incorrect optimization while declaring array[1]

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.6                       |4.4.7

--- Comment #13 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-27 12:14:08 UTC ---
4.3 branch is being closed, moving to 4.4.7 target.