[Bug c/53369] New: Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[Bug c/53369] New: Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[rahulnitk2004 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53369

             Bug #: 53369
           Summary: Integral promotion with ~ operator - Different
                    behavior for signed and unsigned char.
    Classification: Unclassified
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rahulnitk2004@gmail.com


Created attachment 27413
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27413
The preprocessed file

I face a problem when i use bitwise ~ operator on signed char. With unsigned
char, it works as per the K&R ANSI C specification.

Here is the code snippet.

    signed char a = 1 << 7;
    unsigned char b = 1 << 7;

    printf("%hd  %hu\n", ~a, ~b);

Output for the program is shown as: 127  65407.

As per the integral promotion concept, a char variable whether signed or
unsigned will be converted to int before performing an operation on it.
So before performing (~a), a will be converted to int and will have 32 bits
(In my machine int is 32 bit). So ~a should result in 0XFFFFFF7F. So when i
print ~a it should result in 0XFF7F as format specifier is "%hd". Instead it is
only 127(0X7F). 
Similarly for unsigned char b, ~b should result in 0XFFFFFF7F and output should
be 65407(0XFF7F) as i used "%hu" as format specifier. That is printed
correctly. 

I am using GCC version 4.5.2 on Ubuntu.

Any suggestions?

Thanks
Rahul
---------------------------------------------------------------------------

Code:

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

int main(void)
{
    signed char a = 1 << 7;
    unsigned char b = 1 << 7;

    printf("%hd  %hu\n", ~a, ~b);

    return(EXIT_SUCCESS);
}

Compilation command used: gcc -Wall test.c (No warnings).

Output: 127  65407

------------------------------------------------------------------------------

Other Deatils:

------------------------------------------------------------------------------
rahul@ubuntu:~/sample$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.5.2-8ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.5 --enable-shared --enable-multiarch
--with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib/x86_64-linux-gnu
--enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-plugin --enable-gold --enable-ld=default
--with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686
--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 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)
------------------------------------------------------------------------------

rahul@ubuntu:~/sample$ gcc -v -save-temps -Wall test.c
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.5.2-8ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.5 --enable-shared --enable-multiarch
--with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib/x86_64-linux-gnu
--enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-plugin --enable-gold --enable-ld=default
--with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686
--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 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-mtune=generic' '-march=x86-64'
 /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/cc1 -E -quiet -v test.c
-D_FORTIFY_SOURCE=2 -mtune=generic -march=x86-64 -Wall -fpch-preprocess
-fstack-protector -o test.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/include
 /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-mtune=generic' '-march=x86-64'
 /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/cc1 -fpreprocessed test.i
-quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -Wall
-version -fstack-protector -o test.s
GNU C (Ubuntu/Linaro 4.5.2-8ubuntu4) version 4.5.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.5.2, GMP version 4.3.2, MPFR version 3.0.0-p8,
MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (Ubuntu/Linaro 4.5.2-8ubuntu4) version 4.5.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.5.2, GMP version 4.3.2, MPFR version 3.0.0-p8,
MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 9755ab75799195519479ef699703b13b
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-mtune=generic' '-march=x86-64'
 as -V -Qy --64 -o test.o test.s
GNU assembler version 2.21.0 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.21.0.20110327
COMPILER_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-mtune=generic' '-march=x86-64'
 /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/collect2 --build-id
--eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -z relro
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../crt1.o
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../crti.o
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/crtbegin.o
-L/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2
-L/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../..
-L/usr/lib/x86_64-linux-gnu test.o -lgcc --as-needed -lgcc_s --no-as-needed -lc
-lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/crtend.o
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../crtn.o
-------------------------------------------------------------------------------

[Bug c/53369] Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[rahulnitk2004 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53369

--- Comment #1 from Rahul Ramachandran <rahulnitk2004 at gmail dot com> 2012-05-16 00:03:58 UTC ---
Created attachment 27414
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27414
The assemply file

[Bug c/53369] Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[rahulnitk2004 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53369

Rahul Ramachandran <rahulnitk2004 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #27414|0                           |1
        is obsolete|                            |

--- Comment #2 from Rahul Ramachandran <rahulnitk2004 at gmail dot com> 2012-05-16 00:05:02 UTC ---
Created attachment 27415
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27415
The assemply file

[Bug c/53369] Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53369

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

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-05-16 00:11:53 UTC ---
    signed char a = 1 << 7;
    unsigned char b = 1 << 7;

    printf("%hd  %hu\n", ~a, ~b);


let's see ~a is really ~(int)a.  Likewise ~b is really ~(int)b.
#define printf __builtin_printf
int main(void)
{    
    signed char a = 1 << 7;
    unsigned char b = 1 << 7;

    printf("%x  %x\n", ~a, ~b);
}

--- CUT ----
7f  ffffff7f


This is the correct behavior.  As (int)(unsigned char)a does a zero extend as
the unsigned char fits directly in an int.

Think of this way.  The value from unsigned char and signed char are unchanged
when promoted into int.  So with the first one you get -128 and the second case
you get 128.

and then you take the ~ and you get 127 and (~(1<<7)) .

[Bug c/53369] Integral promotion with ~ operator - Different behavior for signed and unsigned char.

[rahulnitk2004 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53369

--- Comment #4 from Rahul Ramachandran <rahulnitk2004 at gmail dot com> 2012-05-16 09:09:07 UTC ---
So basically char value -128 is represented in integer as
11111111-11111111-11111111-10000000 and +128 as 10000000. So taking the
compliment will result in 0X7F and 0XFFFFFF7F.

Thanks for the explanation.

Rahul