https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85962
Bug ID: 85962
Summary: spurious warning on right shift constant > integer in
trivially dead code
Product: gcc
Version: 6.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: joshudson at gmail dot com
Target Milestone: ---
spurious warning on right shift constant > integer when code would be
eliminated by optimization step because it is only reachable when the warning
would not occur
Complete code snippet:
typedef unsigned long size_t;
typedef unsigned int uint32_t;
#define SIZE_MAX 0xFFFFFFFFFFFFFFFF
size_t vfunc(uint32_t n)
{
if (n > SIZE_MAX)
return n >> (sizeof(size_t) * 8);
else
return (size_t)n;
}
This senseless function is reduced from a real function in code that can't
assume the size of size_t is large enough to hold uint32_t. Note that n >
SIZE_MAX is not going to happen when compiling for 64 bits and will be
optimized away, but it raises a warning anyway:
eportbug.c: In function ‘vfunc’:
reportbug.c:8:12: warning: right shift count >= width of type
[-Wshift-count-overflow]
return n >> (sizeof(size_t) * 8);
If the code were not optimized away, the shift would not be out of range. I
compile with -O3 to get dead code elimination.
gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18+deb9u1'
--with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --program-prefix=x86_64-linux-gnu- --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 --enable-default-pie
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--with-arch-32=i686 --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 6.3.0 20170516 (Debian 6.3.0-18+deb9u1)
