[Bug c++/83116] New: Statement with no effect causes wrong code of static object constexpr method

[gcc at gajoch dot pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83116

            Bug ID: 83116
           Summary: Statement with no effect causes wrong code of static
                    object constexpr method
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at gajoch dot pl
  Target Milestone: ---

Created attachment 42685
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42685&action=edit
Preprocessed source

Following testcase:

$ cat main.cpp 
#include <cstdio>

struct foo {
    constexpr foo() : size_(0) {
        (void)size();
    }
    constexpr int size() {
        return size_;
    }
    int size_;
};

int main() {
    static foo var;
    var.size_ = 5;
    std::printf("%d %d", var.size_, var.size());
    return 0;
}

$ g++ -Wall -Wextra -pedantic -O0 main.cpp -o main && ./main
5 5

$ g++ -Wall -Wextra -pedantic -O1 main.cpp -o main && ./main
5 0

(Same happens on -Og, -O2 and -O3)

Tested on:
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.2.0 (GCC)


Correct behavior is seen using gcc-5.4, clang and msvc.
Removing constructor body gives correct code - this is stripped version,
originally there was assert(this->size() >= 0);
UBSAN gives no results.

This bug occurs also on 6.1.0, 6.2.0, 6.3.0, 7.1.0, 7.2.0 and today's trunk
(8.0.0 20171122), as tested on compiler explorer: https://godbolt.org/g/hr4Nq4