[Bug c/45164] New: -O3 optimization problematic on "long long" types near 64 bit wraparound

[Bug c/45164] New: -O3 optimization problematic on "long long" types near 64 bit wraparound

[robots at marcabel dot com]

Here is a small test case, with output and version information in the comments.

There are enough ways of stating this problem that it was not evident to me
within the existing bug list.  One skilled in the art would know if this is a
duplicate.

I'm not sure where to upload my .i file, so I will try to attach it a few
minutes after I post this.

-------

/*
 *  Illustration of GCC bug
 *  Marc W. Abel, August 2, 2010
 *
 *  gcc version information follows
 *  ------------------------------------
 *  Using built-in specs.
 *  Target: x86_64-linux-gnu
 *  Configured with: ../src/configure -v --with-pkgversion='Ubuntu
4.4.3-4ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc
--disable-werror --with-arch-32=i486 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
 *  Thread model: posix
 *  gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) 
 *  ------------------------------------
 *
 *  Invocation and correct output (using -O2):
 *
 *  $ gcc -O2 o3bug.c && ./a.out | head -n 6
 *  7ffffffffffffffd fdffffffffffff7f
 *  7ffffffffffffffe feffffffffffff7f
 *  7fffffffffffffff ffffffffffffff7f
 *  8000000000000000 0000000000000080
 *  8000000000000001 0100000000000080
 *  8000000000000002 0200000000000080
 *
 *  Invocation and incorrect output (using -O3):
 *
 *  $ gcc -O3 o3bug.c && ./a.out | head -n 6
 *  7ffffffffffffffd fdffffffffffff7f
 *  7ffffffffffffffe feffffffffffff7f
 *  7fffffffffffffff ffffffffffffff7f
 *  8000000000000000 100ffffffffffff7f
 *  8000000000000001 101ffffffffffff7f
 *  8000000000000002 102ffffffffffff7f
 */

#include <stdio.h>

/*
 *  Convert an integer into an eight-byte format for persistance, LSB first.
 */
static void int_to_persist(unsigned char *eight_bytes, long long n)
{
        int i;

        for (i=0; i<8; i++, n >>= 8)
                eight_bytes[i] = n & 255u;
}

int main(int argc, char *argv[])
{
        unsigned char bytes[8];
        int i;
        long long m;

        for (m = 0x7ffffffffffffffdLL; ; ++m) {
                printf("%llx ", m);

                int_to_persist(bytes, m);
                for (i=0; i<8; i++)
                        printf("%02x", bytes[i]);
                printf("\n");
        }
}


-- 
           Summary: -O3 optimization problematic on "long long" types near
                    64 bit wraparound
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: robots at marcabel dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45164

[Bug c/45164] -O3 optimization problematic on "long long" types near 64 bit wraparound

[robots at marcabel dot com]

------- Comment #1 from robots at marcabel dot com  2010-08-02 16:47 -------
Created an attachment (id=21371)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21371&action=view)
Preprocessor output / compiler input


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45164

[Bug c/45164] -O3 optimization problematic on "long long" types near 64 bit wraparound

[robots at marcabel dot com]

------- Comment #2 from robots at marcabel dot com  2010-08-02 16:48 -------
Created an attachment (id=21372)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21372&action=view)
Source code to produce

in case it doesn't paste well from my initial report.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45164

[Bug c/45164] -O3 optimization problematic on "long long" types near 64 bit wraparound

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2010-08-02 16:48 -------
Signed overflow is undefined; use either unsigned or -fwrapv to get the
behavior you want.


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45164

[Bug c/45164] -O3 optimization problematic on "long long" types near 64 bit wraparound

[robots at marcabel dot com]

------- Comment #4 from robots at marcabel dot com  2010-08-02 16:54 -------
Confirmed resolved invalid.  Thank you!


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45164