[Bug c++/71384] New: Global constructors (init_array) emitted for trivial initialisation depending on source code ordering

["niels at penneman dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71384

            Bug ID: 71384
           Summary: Global constructors (init_array) emitted for trivial
                    initialisation depending on source code ordering
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: niels at penneman dot org
  Target Milestone: ---

GCC versions 4.7.2/5.3.0/5.3.1 are emitting a global constructor (to the
.init_array section) for the following piece of trivial code:

===

struct X
{
  constexpr X(int a);
  const int m_a;
};
X x(999);
inline constexpr X::X(int a):m_a(a) {}

===

Swapping the last two lines, i.e. placing the constructor implementation before
the declaration of 'x', causes 'x' to be emitted to the '.data' section with
value 999 instead. The latter is desirable as the generate binary is far
smaller and does not require global constructors.

How to reproduce:

g++ -std=c++11 -c -save-temps=obj global-constructor.cxx

Occurs regardless of whether you specify -O0, -O1, -O2, -O3. In a complete
program, -flto cannot resolve this. The issue occurs on both x86_64 and ARM, so
not in the backend. Also occurs with -std=c++14 and -std=c++17 on GCC 5.3.x.

Compiler versions used:

$ g++ -###
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/5.3.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-linker-hash-style=gnu --enable-plugin --enable-initfini-array
--disable-libgcj --with-isl --enable-libmpx --enable-gnu-indirect-function
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 5.3.1 20160406 (Red Hat 5.3.1-6) (GCC)

$ ${CROSS_COMPILE}g++ -###
Using built-in specs.
COLLECT_GCC=/home/niels/workspace/toolchains/build/arm-none-eabi/output/bin/arm-none-eabi-g++
COLLECT_LTO_WRAPPER=/home/niels/workspace/toolchains/build/arm-none-eabi/output/libexec/gcc/arm-none-eabi/5.3.0/lto-wrapper
Target: arm-none-eabi
Configured with:
/home/niels/workspace/toolchains/build/arm-none-eabi/work/src/gcc-5.3.0/configure
--build=x86_64-build_pc-linux-gnu --host=x86_64-build_pc-linux-gnu
--target=arm-none-eabi
--prefix=/home/niels/workspace/toolchains/build/arm-none-eabi/output
--with-local-prefix=/home/niels/workspace/toolchains/build/arm-none-eabi/output/arm-none-eabi/sysroot
--with-sysroot=/home/niels/workspace/toolchains/build/arm-none-eabi/output/arm-none-eabi/sysroot
--with-newlib --enable-threads=no --disable-shared --with-float=soft
--with-pkgversion='crosstool-NG crosstool-ng-1.22.0-134-ge1d494a'
--enable-__cxa_atexit --disable-libgomp --disable-libmudflap --disable-libssp
--disable-libquadmath --disable-libquadmath-support
--with-gmp=/home/niels/workspace/toolchains/build/arm-none-eabi/work/arm-none-eabi/buildtools
--with-mpfr=/home/niels/workspace/toolchains/build/arm-none-eabi/work/arm-none-eabi/buildtools
--with-mpc=/home/niels/workspace/toolchains/build/arm-none-eabi/work/arm-none-eabi/buildtools
--with-isl=/home/niels/workspace/toolchains/build/arm-none-eabi/work/arm-none-eabi/buildtools
--enable-lto --with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++ -lm'
--enable-target-optspace --disable-nls --disable-multilib
--enable-languages=c,c++
Thread model: single
gcc version 5.3.0 (crosstool-NG crosstool-ng-1.22.0-134-ge1d494a)

$ ${CROSS_COMPILE}g++ -###
Using built-in specs.
COLLECT_GCC=/opt/OSELAS.Toolchain-2012.12.1/arm-v5te-linux-gnueabi/gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/bin/arm-v5te-linux-gnueabi-g++
COLLECT_LTO_WRAPPER=/opt/OSELAS.Toolchain-2012.12.1/arm-v5te-linux-gnueabi/gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/bin/../libexec/gcc/arm-v5te-linux-gnueabi/4.7.2/lto-wrapper
Target: arm-v5te-linux-gnueabi
Configured with:
/home/mol/dude/tmp/OSELAS.Toolchain-2012.12.1/platform-arm-v5te-linux-gnueabi-gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/build-cross/gcc-4.7.2/configure
--build=x86_64-host-linux-gnu --host=x86_64-host-linux-gnu
--target=arm-v5te-linux-gnueabi
--with-sysroot=/home/mol/dude/tmp/OSELAS.Toolchain-2012.12.1/inst/opt/OSELAS.Toolchain-2012.12.1/arm-v5te-linux-gnueabi/gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/sysroot-arm-v5te-linux-gnueabi
--disable-multilib --with-float=soft --with-fpu=vfp --with-cpu=arm926ej-s
--enable-__cxa_atexit --disable-sjlj-exceptions --disable-nls
--disable-decimal-float --disable-fixed-point --disable-win32-registry
--enable-symvers=gnu --with-pkgversion=OSELAS.Toolchain-2012.12.1
--enable-threads=posix --with-system-zlib
--with-gmp=/home/mol/dude/tmp/OSELAS.Toolchain-2012.12.1/platform-arm-v5te-linux-gnueabi-gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/sysroot-host
--with-mpfr=/home/mol/dude/tmp/OSELAS.Toolchain-2012.12.1/platform-arm-v5te-linux-gnueabi-gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized/sysroot-host
--prefix=/home/mol/dude/tmp/OSELAS.Toolchain-2012.12.1/inst/opt/OSELAS.Toolchain-2012.12.1/arm-v5te-linux-gnueabi/gcc-4.7.2-glibc-2.16.0-binutils-2.22-kernel-3.6-sanitized
--enable-languages=c,c++ --enable-c99 --enable-long-long
--enable-libstdcxx-debug --enable-profile --enable-shared --disable-libssp
--enable-checking=release
Thread model: posix
gcc version 4.7.2 (OSELAS.Toolchain-2012.12.1)