[Bug other/52609] New: -Wstrict-aliasing / missed diagnostics

[Bug other/52609] New: -Wstrict-aliasing / missed diagnostics

[pluto at agmk dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52609

             Bug #: 52609
           Summary: -Wstrict-aliasing / missed diagnostics
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pluto@agmk.net
            Target: x86_64-gnu-linux


void bug1( unsigned* buffer, int index, float w )
{
        *(float*)(&buffer[index])=w;
}

void bug2( unsigned* buffer, int index, float w )
{
        float* ptr=(float*)&buffer[index];
        *ptr=w;
}

accessing unsigned* via float* looks buggy but

$ gcc48 -Wall -Wstrict-aliasing=3 -Wextra -fstrict-aliasing -O2 -S alias-bug.c

reports no warnings.

[Bug other/52609] -Wstrict-aliasing / missed diagnostics

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52609

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-03-17 21:57:19 UTC ---
>accessing unsigned* via float* looks buggy

It does not have to be if the original argument was originally of type float.
Aliasing is not about type of pointers but the type which is used to access and
such.

[Bug other/52609] -Wstrict-aliasing / missed diagnostics

[pluto at agmk dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52609

--- Comment #2 from Pawel Sikora <pluto at agmk dot net> 2012-03-18 09:32:03 UTC ---
(In reply to comment #1)
> >accessing unsigned* via float* looks buggy
> 
> It does not have to be if the original argument was originally of type float.
> Aliasing is not about type of pointers but the type which is used to access and
> such.

ok, here's an updated testcase:

$ cat alias-bug.c
unsigned buffer[1];

float bug1( unsigned u )
{
        buffer[0]=u;
        return *(float*)(&buffer[0]); // warning.
}

float bug2( unsigned u )
{
        buffer[0]=u;
        float* ptr=(float*)&buffer[0];
        return *ptr; // missed strict aliasing warning.
}

gcc repots warning only for bug1() and misses warning for bug2():

$ gcc -Wall -Wstrict-aliasing=3 -O2 alias-bug.c -c
alias-bug.c: In function 'bug1':
alias-bug.c:6:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]

[Bug other/52609] -Wstrict-aliasing / missed diagnostics

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52609

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-19 10:54:24 UTC ---
(In reply to comment #2)
> (In reply to comment #1)
> > >accessing unsigned* via float* looks buggy
> > 
> > It does not have to be if the original argument was originally of type float.
> > Aliasing is not about type of pointers but the type which is used to access and
> > such.
> 
> ok, here's an updated testcase:
> 
> $ cat alias-bug.c
> unsigned buffer[1];
> 
> float bug1( unsigned u )
> {
>         buffer[0]=u;
>         return *(float*)(&buffer[0]); // warning.
> }
> 
> float bug2( unsigned u )
> {
>         buffer[0]=u;
>         float* ptr=(float*)&buffer[0];
>         return *ptr; // missed strict aliasing warning.
> }
> 
> gcc repots warning only for bug1() and misses warning for bug2():

There is a duplicate bug for this somewhere.  The warning machinery
only looks at a single stmt.

[Bug other/52609] -Wstrict-aliasing / missed diagnostics

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52609

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dilyan.palauzov at aegee
                   |                            |dot org

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-10-23 18:47:08 UTC ---
*** Bug 55040 has been marked as a duplicate of this bug. ***