https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88895
Bug ID: 88895
Summary: unnecessary warnings in unreachable code (shift)
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: scott.a.mayo at gmail dot com
Target Milestone: ---
Created attachment 45450
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45450&action=edit
processed .cpp
The compiler complains about shift operations that are undefined, when the code
in question is going to be unreachable anyway, at points that should be obvious
even without optimization. Basically, g++ is warning about undefined shifts too
early; it should wait until code generation and see what unreachable code is
weeded out.
The attachment shows a standard idiom for packing arbitrary integer types into
buffer in big endian fashion, regardless of the endianness of the platform. g++
generates the correct code (which melts down to a single instruction!) but
along the way it's squawking.
I realize it's easier to warn earlier in the process, but I'm trying to enforce
0 tolerance for warnings, and having to turn off this warning is onerous since
the code is a template, wants to live in a header, so turning off the warning
the obvious way also turns it off in places it may be wanted. This is a big
deal in safety-critical code.
g++:
gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04)
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.3.0-27ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--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 --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Command line:
g++ -O3 -Wall StopWhining.cpp
Warnings:
StopWhining.cpp: In instantiation of ‘void PackRule::pack(void*, T) const [with
T = unsigned int]’:
StopWhining.cpp:75:34: required from here
StopWhining.cpp:33:63: warning: right shift count >= width of type
[-Wshift-count-overflow]
case 8: tobyte[sizeof(T) - 8] = (unsigned char)(v >> 56);
//waterfall
~~~^~~~~~
StopWhining.cpp:34:63: warning: right shift count >= width of type
[-Wshift-count-overflow]
case 7: tobyte[sizeof(T) - 7] = (unsigned char)(v >> 48);
~~~^~~~~~
StopWhining.cpp:35:63: warning: right shift count >= width of type
[-Wshift-count-overflow]
case 6: tobyte[sizeof(T) - 6] = (unsigned char)(v >> 40);
~~~^~~~~~
StopWhining.cpp:36:63: warning: right shift count >= width of type
[-Wshift-count-overflow]
case 5: tobyte[sizeof(T) - 5] = (unsigned char)(v >> 32);
~~~^~~~~~
StopWhining.cpp: In instantiation of ‘T PackRule::unpack(const void*, T*) const
[with T = unsigned int]’:
StopWhining.cpp:77:21: required from here
StopWhining.cpp:58:63: warning: left shift count >= width of type
[-Wshift-count-overflow]
case 8: v |= (0xffU & (T)frombyte[sizeof(T) - 8]) << 56;
//waterfall
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
StopWhining.cpp:59:63: warning: left shift count >= width of type
[-Wshift-count-overflow]
case 7: v |= (0xffU & (T)frombyte[sizeof(T) - 7]) << 48;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
StopWhining.cpp:60:63: warning: left shift count >= width of type
[-Wshift-count-overflow]
case 6: v |= (0xffU & (T)frombyte[sizeof(T) - 6]) << 40;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
StopWhining.cpp:61:63: warning: left shift count >= width of type
[-Wshift-count-overflow]
case 5: v |= (0xffU & (T)frombyte[sizeof(T) - 5]) << 32;
.ii attached
