Bug#887354: g++-6: False positive sequence point warning on pre-increment
Package: g++-6
Version: 6.3.0-18
Severity: minor
Tags: upstream
Dear Maintainer,
*** Reporter, please consider answering these questions, where appropriate ***
* What led up to the situation?
* What exactly did you do (or not do) that was effective (or
ineffective)?
* What was the outcome of this action?
* What outcome did you expect instead?
*** End of the template - remove these template lines ***
int main()
{
unsigned i = 0;
i = ++i + 1;
return 0;
}
The above snippet produces the following warning.
main.cpp:4:7: warning: operation on ‘i’ may be undefined [-Wsequence-point]
i = ++i + 1;
~~^~~~~~~~~
DR 637 [1] clarifies this is well-defined, hence the warning is false.
The issue was noticed using g++ 6.3.0. At the time of this writing it
can be reproduced using 6.3, 7.2 and trunk on godbolt.org. clang
3.9.1, 4.0.1 and 5.0.0 tested on the same online service do not emit
any warnings for this snippet with -Weverything.
g++ verbose output is included according to upstream bug reporting
guidelines.
[1] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#637
*** g++.out
Using built-in specs.
COLLECT_GCC=g++
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' --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)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++14' '-Wall' '-Wextra' '-Wpedantic' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/6/cc1plus -E -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE main.cpp -mtune=generic -march=x86-64 -std=c++14 -Wall -Wextra -Wpedantic -fpch-preprocess -o main.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/6"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/6
/usr/include/x86_64-linux-gnu/c++/6
/usr/include/c++/6/backward
/usr/lib/gcc/x86_64-linux-gnu/6/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/6/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++14' '-Wall' '-Wextra' '-Wpedantic' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/6/cc1plus -fpreprocessed main.ii -quiet -dumpbase main.cpp -mtune=generic -march=x86-64 -auxbase main -Wall -Wextra -Wpedantic -std=c++14 -version -o main.s
GNU C++14 (Debian 6.3.0-18) version 6.3.0 20170516 (x86_64-linux-gnu)
compiled by GNU C version 6.3.0 20170516, GMP version 6.1.2, MPFR version 3.1.5, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++14 (Debian 6.3.0-18) version 6.3.0 20170516 (x86_64-linux-gnu)
compiled by GNU C version 6.3.0 20170516, GMP version 6.1.2, MPFR version 3.1.5, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: ec86852f7c0e5cbf6cd53f3ae50625fd
main.cpp: In function ‘int main()’:
main.cpp:4:7: warning: operation on ‘i’ may be undefined [-Wsequence-point]
i = ++i + 1;
~~^~~~~~~~~
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++14' '-Wall' '-Wextra' '-Wpedantic' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
as -v --64 -o main.o main.s
GNU assembler version 2.28 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.28
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/6/:/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/6/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/6/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++14' '-Wall' '-Wextra' '-Wpedantic' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/6/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/6/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper -plugin-opt=-fresolution=main.res -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --sysroot=/ --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/6/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/6 -L/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/6/../../../../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/6/../../.. main.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/6/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++14' '-Wall' '-Wextra' '-Wpedantic' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
-- System Information:
Debian Release: 9.3
APT prefers stable
APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
Kernel: Linux 4.9.0-5-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8), LANGUAGE=en_US:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
Versions of packages g++-6 depends on:
ii gcc-6 6.3.0-18
ii gcc-6-base 6.3.0-18
ii libc6 2.24-11+deb9u1
ii libgmp10 2:6.1.2+dfsg-1
ii libisl15 0.18-1
ii libmpc3 1.0.3-1+b2
ii libmpfr4 3.1.5-1
ii libstdc++-6-dev 6.3.0-18
ii zlib1g 1:1.2.8.dfsg-5
g++-6 recommends no packages.
Versions of packages g++-6 suggests:
pn g++-6-multilib <none>
pn gcc-6-doc <none>
ii libstdc++6-6-dbg 6.3.0-18
-- no debconf information
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