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)