https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124618
Bug ID: 124618
Summary: aggressive-loop-optimizations false positive UB
warning
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: andrea.agostini.sax at gmail dot com
Target Milestone: ---
Created attachment 64007
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=64007&action=edit
preprocessed
As per title. The warning is not triggered with GCC 14, but it was there in GCC
13 too. I compiled locally with `-v -save-temps -std=c++20 -Wall -O3` and GCC
13.3: the resulting preprocessed file is attached, and the output is below.
Repro (https://godbolt.org/z/hn7jYodq3):
```
#include <cstddef>
#include <memory>
#include <new>
// int vector with capacity == 8
struct smallvec_int
{
alignas(int) std::byte storage_[sizeof(int) * 8]; // no-init
std::size_t size_ = 0;
int* data() noexcept
{
return std::launder(reinterpret_cast<int*>(storage_));
}
smallvec_int(std::initializer_list<int> init)
{
for (const auto& elem : init)
{
std::construct_at(data() + size_, elem);
++size_;
}
}
void assign(std::initializer_list<int> init)
{
if (init.size() > 8)
throw std::bad_alloc();
const std::size_t old_size = size_;
const std::size_t new_size = init.size();
const std::size_t min_size = std::min(old_size, new_size);
std::size_t i = 0;
for (; i < min_size; ++i)
{
data()[i] = init.begin()[i];
}
for (; i < new_size; ++i)
{
std::construct_at(data() + size_, init.begin()[i]);
++size_;
}
for (; i < old_size; ++i)
{
std::destroy_at(data() + i);
}
size_ = new_size;
}
};
//----------------------------------------------------------------------------
int main() {
smallvec_int v = {1, 2, 3};
v.assign({1, 2});
}
```
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/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
13.3.0-6ubuntu2~24.04.1' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --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-libstdcxx-backtrace
--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-13-EldibY/gcc-13-13.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-EldibY/gcc-13-13.3.0/debian/tmp-gcn/usr
--enable-offload-defaulted --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 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04.1)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-Wall' '-O3'
'-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
/usr/libexec/gcc/x86_64-linux-gnu/13/cc1plus -E -quiet -v -imultiarch
x86_64-linux-gnu -D_GNU_SOURCE repro.cpp -mtune=generic -march=x86-64
-std=c++20 -Wall -O3 -fpch-preprocess -D_FORTIFY_SOURCE=3
-fasynchronous-unwind-tables -fstack-protector-strong -Wformat-security
-fstack-clash-protection -fcf-protection -o a-repro.ii
ignoring duplicate directory "/usr/include/x86_64-linux-gnu/c++/13"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/13/include-fixed/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/13/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/13
/usr/include/x86_64-linux-gnu/c++/13
/usr/include/c++/13/backward
/usr/lib/gcc/x86_64-linux-gnu/13/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-Wall' '-O3'
'-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
/usr/libexec/gcc/x86_64-linux-gnu/13/cc1plus -fpreprocessed a-repro.ii
-D_FORTIFY_SOURCE=3 -quiet -dumpdir a- -dumpbase repro.cpp -dumpbase-ext .cpp
-mtune=generic -march=x86-64 -O3 -Wall -std=c++20 -version
-fasynchronous-unwind-tables -fstack-protector-strong -Wformat-security
-fstack-clash-protection -fcf-protection -o a-repro.s
GNU C++20 (Ubuntu 13.3.0-6ubuntu2~24.04.1) version 13.3.0 (x86_64-linux-gnu)
compiled by GNU C version 13.3.0, GMP version 6.3.0, MPFR version
4.2.1, MPC version 1.3.1, isl version isl-0.26-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 7896445e4990772fdae9dc0659a99266
In member function ‘void smallvec_int::assign(std::initializer_list<int>)’,
inlined from ‘int main()’ at repro.cpp:52:13:
repro.cpp:36:55: warning: iteration 4611686018427387903 invokes undefined
behavior [-Waggressive-loop-optimizations]
36 | std::construct_at(data() + size_, init.begin()[i]);
| ^
repro.cpp:34:14: note: within this loop
34 | for (; i < new_size; ++i)
| ~~^~~~~~~~~~
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-Wall' '-O3'
'-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
as -v --64 -o a-repro.o a-repro.s
GNU assembler version 2.42 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.42
COMPILER_PATH=/usr/libexec/gcc/x86_64-linux-gnu/13/:/usr/libexec/gcc/x86_64-linux-gnu/13/:/usr/libexec/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/13/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/13/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-Wall' '-O3'
'-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
/usr/libexec/gcc/x86_64-linux-gnu/13/collect2 -plugin
/usr/libexec/gcc/x86_64-linux-gnu/13/liblto_plugin.so
-plugin-opt=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
-plugin-opt=-fresolution=a.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 --build-id
--eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie -z now -z relro
/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/13/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/13
-L/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/13/../../../../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/13/../../.. a-repro.o -lgcc --push-state
--as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s
--pop-state /usr/lib/gcc/x86_64-linux-gnu/13/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++20' '-Wall' '-O3'
'-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
