https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105876
Bug ID: 105876
Summary: use of -flto with g++ changes global extern const
std::string initialization in unexpected manner
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: lto
Assignee: unassigned at gcc dot gnu.org
Reporter: kevin.b.hendricks at icloud dot com
CC: marxin at gcc dot gnu.org
Target Milestone: ---
Created attachment 53101
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53101&action=edit
text copy of the above for extracting source code
Use of -flto with g++ causes extern const std::string variable to be
initialized after a static const class variable.
This behaviour is not present when -flto is not used. This same simple test
case works with and without -flto with clang++.
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/12.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-linker-build-id --enable-lto
--enable-multilib --enable-plugin --enable-shared --enable-threads=posix
--disable-libssp --disable-libstdcxx-pch --disable-werror
--with-build-config=bootstrap-lto --enable-link-serialization=1
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.1.0 (GCC)
uname -a
Linux kevin-virtualbox 5.15.41-1-MANJARO #1 SMP PREEMPT Wed May 18 09:20:01 UTC
2022 x86_64 GNU/Linux
Simple Test Case Sources
-----------------
--- sg_constants.h ---
#include <string>
extern const std::string MAIN_EXTERN_CONST_STRING;
--- sg_test.h ---
class sg_test
{
public:
sg_test();
};
--- main.cpp ---
#include <iostream>
#include <string>
#include "sg_constants.h"
#include "sg_test.h"
const std::string MAIN_EXTERN_CONST_STRING= "a_literal_string_value";
int main(int argc, char *argv[])
{
(void) argc; (void) argv;
std::cout << "In main MAIN_EXTERN_CONST_STRING is: " << "'" <<
MAIN_EXTERN_CONST_STRING << "'" << std::endl;
sg_test * psg = new sg_test();
(void) psg;
return 0;
}
--- sg_test.cpp ---
#include <iostream>
#include "sg_constants.h"
#include "sg_test.h"
static const std::string TEST_STATIC_CONST_STRING = MAIN_EXTERN_CONST_STRING;
sg_test::sg_test()
{
std::cout << "In sg_test object TEST_STATIC_CONST_STRING is: " << "'" <<
TEST_STATIC_CONST_STRING << "'" << std::endl;
std::cout << "In sg_test object MAIN_EXTERN_CONST_STRING is: " << "'" <<
MAIN_EXTERN_CONST_STRING << "'" << std::endl;
}
$ cat buildme.sh
g++ -I. -g -O2 -Wall -Wextra -c sg_test.cpp
g++ -I. -O2 -Wall -Wextra -c main.cpp
g++ -o no_lto_test *.o
rm -f *.o
g++ -I. -g -O2 -flto=auto -Wall -Wextra -c sg_test.cpp
g++ -I. -g -O2 -flto=auto -Wall -Wextra -c main.cpp
g++ -flto=auto -o lto_test *.o
echo "Without lto"
./no_lto_test
echo " "
echo "With lto"
./lto_test
$ ./buildme.sh
Without lto
In main MAIN_EXTERN_CONST_STRING is: 'a_literal_string_value'
In sg_test object TEST_STATIC_CONST_STRING is: 'a_literal_string_value'
In sg_test object MAIN_EXTERN_CONST_STRING is: 'a_literal_string_value'
With lto
In main MAIN_EXTERN_CONST_STRING is: 'a_literal_string_value'
In sg_test object TEST_STATIC_CONST_STRING is: ''
In sg_test object MAIN_EXTERN_CONST_STRING is: 'a_literal_string_value'
