[Bug c++/49510] New: bitshift warnings

[Bug c++/49510] New: bitshift warnings

[claytongdavis at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49510

           Summary: bitshift warnings
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: claytongdavis@gmail.com


This is a request to improve the warnings for bitshifts where the right operand
is negative or >= the size of the left operand.  I am aware that this is
undefined behavior, but a warning would be nice in a case like the one included
below.  GCC implements a cyclic bitshift instead.

$ more gcc_bitshift.C 
#include <iostream>

int main()
{
  int shift = 32;
  std::cout<<"0xFFFFFFFF>>32 = "<<(0xFFFFFFFF>>shift)<<std::endl;
  std::cout<<"1<<32 = "<<(1<<shift)<<std::endl;

  shift = -1;
  std::cout<<"0xFFFFFFFF>>(-1) = "<<(0xFFFFFFFF>>shift)<<std::endl;
  std::cout<<"1<<(-1) = "<<(1<<shift)<<std::endl;
}

$ g++ -Wall -Wextra gcc_bitshift.C 

$ ./a.out 
0xFFFFFFFF>>32 = 4294967295
1<<32 = 1
0xFFFFFFFF>>(-1) = 1
1<<(-1) = -2147483648

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
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)

[Bug c++/49510] bitshift warnings

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49510

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.06.23 11:42:52
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-23 11:42:52 UTC ---
Confirmed.

[Bug c++/49510] bitshift warnings

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49510

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
   Target Milestone|---                         |4.9.0
         Resolution|---                         |FIXED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm, I don't think we can provide this for compile time warnings.
We do provide runtime error/warnings with -fsanitize=undefined



/app/example.cpp:7:46: runtime error: shift exponent 32 is too large for 32-bit
type 'unsigned int'
/app/example.cpp:8:28: runtime error: shift exponent 32 is too large for 32-bit
type 'int'
/app/example.cpp:11:48: runtime error: shift exponent -1 is negative
/app/example.cpp:12:30: runtime error: shift exponent -1 is negative

If the shifter was that constant we do provide compile time warnings already
(and have for ever, in 4.1.2 for sure):
<source>: In function 'int main()':
<source>:7:46: warning: right shift count >= width of type
[-Wshift-count-overflow]
    7 |   std::cout<<"0xFFFFFFFF>>32 = "<<(0xFFFFFFFF>>32)<<std::endl;
      |                                    ~~~~~~~~~~^~~~
<source>:8:28: warning: left shift count >= width of type
[-Wshift-count-overflow]
    8 |   std::cout<<"1<<32 = "<<(1<<32)<<std::endl;
      |                           ~^~~~
<source>:11:48: warning: right shift count is negative [-Wshift-count-negative]
   11 |   std::cout<<"0xFFFFFFFF>>(-1) = "<<(0xFFFFFFFF>>-1)<<std::endl;
      |                                      ~~~~~~~~~~^~~~
<source>:12:30: warning: left shift count is negative [-Wshift-count-negative]
   12 |   std::cout<<"1<<(-1) = "<<(1<<-1)<<std::endl;
      |                             ~^~~~


-fsanitize=undefined was added in GCC 4.9.0. I don't see how we can warn
without knowing if the code will be executed.