[Bug c/67764] New: -Wconversion generates false warnings for bitmask+cast expressions

[marcin.slusarz at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67764

            Bug ID: 67764
           Summary: -Wconversion generates false warnings for bitmask+cast
                    expressions
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marcin.slusarz at intel dot com
  Target Milestone: ---

void _setbit(unsigned char *b, unsigned int i)
{
        b[i / 8] |= (unsigned char)(1 << (i % 8));
}
void _setbit_works(unsigned char *b, unsigned int i)
{
        unsigned char ii = (unsigned char)(1 << (i % 8));
        b[i / 8] |= ii;
}

void _mask_stupid(unsigned char *b, unsigned int i)
{
        b[i] |= (unsigned char)i;
}

void _mask_stupid_works(unsigned char *b, unsigned int i)
{
        unsigned char ii = (unsigned char)i;
        b[i] |= ii;
}

$ gcc -c conversion_bug.c -Wconversion -o /dev/null

generates:

conversion_bug.c: In function ‘_setbit’:
conversion_bug.c:3:21: warning: conversion to ‘unsigned char’ from ‘int’ may
alter its value [-Wconversion]
         b[i / 8] |= (unsigned char)(1 << (i % 8));
                     ^
conversion_bug.c: In function ‘_mask_stupid’:
conversion_bug.c:13:17: warning: conversion to ‘unsigned char’ from ‘int’ may
alter its value [-Wconversion]
         b[i] |= (unsigned char)i;
                 ^


gcc-4.9.3 does *not* generate this warning, while gcc-5.2.1 does.


gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 5.2.1-17'
--with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --with-arch-32=i586 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --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 5.2.1 20150911 (Debian 5.2.1-17)