https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110914
Bug ID: 110914
Summary: Optimization eliminating necessary assignment before
0-byte memcpy
Product: gcc
Version: 11.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: erosenberger at kinetica dot com
Target Milestone: ---
Given the attached source file, when compiled with "g++ -O2 test.cpp", no code
is generated for the line "val_int = 0;" in the Int8_buf constructor. In the
case where the memcpy on the following line is instructed to copy 0 bytes, this
results in val_int remaining uninitialized (and just containing whatever random
value was in memory previously).
In this test program, the result is that the output from main() is "49 49",
when it should be "49 0". (This is because the second 49 is the value remaining
in stack memory from the first call to foo(...) that is never initialized to 0
by the skipped line.)
If the skipped line is changed to initialize to a value other than 0, then
correct code is generated. If compiled with -O1, correct code is also
generated.
Using Compiler Explorer it appears that gcc 9 and earlier produce the correct
code, and gcc 10 is the first version to produce incorrect code.
gcc -v:
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.4.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--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 --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-XeT9lY/gcc-11-11.4.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-XeT9lY/gcc-11-11.4.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)
