[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

------- Additional Comments From davidel at xmailserver dot org  2004-10-06 18:12 -------
Created an attachment (id=7296)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=7296&action=view)
Test program


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] New: Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

When compiling the simple program below using -O1 and -O{2,3}, you will get
different results.

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

#define __HI(x) ((int *) &(x))[1]

double test(double v) {
        int hi;
        hi = __HI(v);
        return hi + v;
}

int main(int ac, char **av) {
        double v = 2.30912;
        if (ac > 1)
                v = atof(av[1]);
        printf("val=%lf\n", test(v));
        return 0;
}


Looking at the code generation you will notice that the offset of the "fildl" is
clearly wrong, being it accessing the uninitialized part of the stack (it should
have been "12(%ebp)"):

test:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        fildl   -4(%ebp)
        faddl   8(%ebp)
        leave
        ret

-- 
           Summary: Bad code generation (using -O2 and -O3) on x86 target
                    when using the listed snippet
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: davidel at xmailserver dot org
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: 3.3.2
GCC target triplet: 3.3.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

------- Additional Comments From davidel at xmailserver dot org  2004-10-06 18:13 -------
Created an attachment (id=7298)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=7298&action=view)
Intermediate output


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

------- Additional Comments From davidel at xmailserver dot org  2004-10-06 18:13 -------
Created an attachment (id=7297)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=7297&action=view)
Code generated


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-06 18:20 -------
You are violating aliasing rules.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

------- Additional Comments From davidel at xmailserver dot org  2004-10-06 18:30 -------
Note that this code works with -O1, and it is part on the NetLib math library
that worked for ages. Now, there are many ways to achieve the same thing, but if
aliasing rules wants to be enforced, the should be enforced with every
optimization level, and not break with -O2/3 and work just fine with -O1. A lot
of existing C code alias values in this or similar way, and if this breaks other
stuff will too. 


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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-06 18:34 -------
Yes but -O2 enables -fstrict-aliasing which means follow the ANSI/ISO C
aliasing rules which you/the program is violating.
Use either -fno-strict-aliasing or an union which GCC defines (unlike the ANSI/
ISO C standard where it is undefined too).

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-10-06 18:41 -------
Andrew is right: some codes break because only higher optimization 
levels do certain aggressive transformations, and work when lower 
optimizations are used. There is not much we can do about it, apart 
from asking our users to fix their codes. The manual contains a 
section on aliasing problems, and how to avoid them. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[davidel at xmailserver dot org]

------- Additional Comments From davidel at xmailserver dot org  2004-10-06 18:50 -------
Oh, I agree. But in this case a clear error instead of a warning would be
better. Error better fits the bad code generation, instead of a warning.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-06 19:00 -------
It cannot be an error since this only undefined at runtime and not undefined
at compile time.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-10-06 19:03 -------
Well, to be more precise: if we knew how to formulate the conditions under 
which we are sure that an aliasing violation happens, we'd be happy to  
provide clearer compiler diagnostics. However, this has been discussed many 
times on the mailing lists, and somehow nobody ever came up with a real 
good way to determine wether the aliasing rules are violated or not. 
 
So for the moment: we're sorry, but we don't know how to do that. Your 
code is invoking undefined behavior at runtime, and the standards allow 
the compiler to let this slip by with no diagnostic required. 
 
W. 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-05 08:59 -------
Reopening to ...

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865

[Bug c/17865] Bad code generation (using -O2 and -O3) on x86 target when using the listed snippet

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-05 08:59 -------
Mark as a dup of bug 21920.

*** This bug has been marked as a duplicate of 21920 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17865