https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83107
Bug ID: 83107
Summary: -Wshift-count-overflow does not report left shift
count >= width in some cases.
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: contact+gcc at clementfevrier dot fr
Target Milestone: ---
-Wshift-count-overflow does not report left shift count >= width in some cases.
If the result of a<<b is cast into a type that has a longer size than b, the
flag -Wshift-count-overflow will not display any warning.
I think it is a bug because, for example, (unsigned long int)1<<32 is just a
recast of the result of 1<<32.
1 is an int, so 1<<32 is also an int with the value 0, which is eventually
recast into an unsigned long int.
So -Wshift-count-overflow should report the count overflow.
The following code:
int main()
{
auto val1 = 1<<32;
auto val2 = (unsigned long int)1<<32;
}
display this message when compiled (g++ -std=gnu++11 main.cpp):
main.cpp: In function ‘int main()’:
main.cpp:3:17: warning: left shift count >= width of type
[-Wshift-count-overflow]
auto val1 = 1<<32;
^~
while I would expect to have in addition:
main.cpp:4:17: warning: left shift count >= width of type
[-Wshift-count-overflow]
auto val2 = (unsigned long int)1<<32;
^~
I tried with both gcc v5 and v8. Their respective "-save-temps" output follows
% gcc -v -save-temps -std=gnu++11 main.cpp
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='Ubuntu
5.4.0-6ubuntu1~16.04.5' --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 --disable-werror --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 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.5)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=gnu++11' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/5/cc1plus -E -quiet -v -imultiarch
x86_64-linux-gnu -D_GNU_SOURCE main.cpp -mtune=generic -march=x86-64
-std=gnu++11 -fpch-preprocess -fstack-protector-strong -Wformat
-Wformat-security -o main.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/5"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/5/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/5
/usr/include/x86_64-linux-gnu/c++/5
/usr/include/c++/5/backward
/usr/lib/gcc/x86_64-linux-gnu/5/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/5/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=gnu++11' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/5/cc1plus -fpreprocessed main.ii -quiet
-dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main -std=gnu++11
-version -fstack-protector-strong -Wformat -Wformat-security -o main.s
GNU C++11 (Ubuntu 5.4.0-6ubuntu1~16.04.5) version 5.4.0 20160609
(x86_64-linux-gnu)
compiled by GNU C version 5.4.0 20160609, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++11 (Ubuntu 5.4.0-6ubuntu1~16.04.5) version 5.4.0 20160609
(x86_64-linux-gnu)
compiled by GNU C version 5.4.0 20160609, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 86227c1230bcf689e6a1b7d559effd6b
main.cpp: In function ‘int main()’:
main.cpp:3:17: warning: left shift count >= width of type
[-Wshift-count-overflow]
auto val1 = 1<<32;
^
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=gnu++11' '-mtune=generic'
'-march=x86-64'
as -v --64 -o main.o main.s
GNU assembler version 2.26.1 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.26.1
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/5/:/usr/lib/gcc/x86_64-linux-gnu/5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/5/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/5/:/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/5/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=gnu++11' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/5/collect2 -plugin
/usr/lib/gcc/x86_64-linux-gnu/5/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
-plugin-opt=-fresolution=main.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --sysroot=/
--build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/5/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/5
-L/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/gcc/x86_64-linux-gnu/5/../../.. main.o -lgcc --as-needed -lgcc_s
--no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/x86_64-linux-gnu/5/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crtn.o
% gcc -v -save-temps main.cpp
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --prefix=/home/clement/bin
Thread model: posix
gcc version 8.0.0 20171023 (experimental) (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/cc1plus -E -quiet -v
-imultiarch x86_64-linux-gnu -D_GNU_SOURCE main.cpp -mtune=generic
-march=x86-64 -fpch-preprocess -o main.ii
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/x86_64-pc-linux-gnu
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/backward
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/include
/usr/local/include
/home/clement/bin/include
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/cc1plus -fpreprocessed
main.ii -quiet -dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main
-version -o main.s
GNU C++14 (GCC) version 8.0.0 20171023 (experimental) (x86_64-pc-linux-gnu)
compiled by GNU C version 8.0.0 20171023 (experimental), GMP version
6.1.0, MPFR version 3.1.4, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
GNU C++14 (GCC) version 8.0.0 20171023 (experimental) (x86_64-pc-linux-gnu)
compiled by GNU C version 8.0.0 20171023 (experimental), GMP version
6.1.0, MPFR version 3.1.4, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
Compiler executable checksum: 7a9ea9098b162b5d41035c367ad9dcf8
main.cpp: In function ‘int main()’:
main.cpp:3:17: warning: left shift count >= width of type
[-Wshift-count-overflow]
auto val1 = 1<<32;
^~
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
as -v --64 -o main.o main.s
GNU assembler version 2.26.1 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.26.1
COMPILER_PATH=/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/:/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/:/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/:/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/:/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/:/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../lib64/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-linux-gnu/:/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/collect2 -plugin
/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/liblto_plugin.so
-plugin-opt=/home/clement/bin/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
-plugin-opt=-fresolution=main.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2
/usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/crtbegin.o
-L/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0
-L/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../lib64
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../.. main.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/home/clement/bin/lib/gcc/x86_64-pc-linux-gnu/8.0.0/crtend.o
/usr/lib/x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=x86-64'
