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From: "claytongdavis at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/49510] New: bitshift warnings
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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)