[Bug c++/94939] New: [9.2.1] invalid code generation in ternary op with static class member (undefined behaviour nearby?)

[tobias.pankr...@ssw-trading.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94939

Bug ID: 94939
   Summary: [9.2.1] invalid code generation in ternary op with
static class member (undefined behaviour nearby?)
   Product: gcc
   Version: 9.2.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: tobias.pankr...@ssw-trading.com
  Target Milestone: ---

I've got the following classes, which are using the type_safe library
(https://github.com/foonathan/type_safe).

struct Old
{
inline constexpr Old();
constexpr Old(std::int64_t v)
: storage(v)
{}
std::int64_t storage = -1;
static const Old magic;
};

constexpr Old::Old() { storage = 0; }

constexpr Old Old::magic = Old(12);

struct OldDerived : Old
{
constexpr OldDerived(Old old)
: Old(old)
{}
constexpr OldDerived(std::int64_t v)
: Old(v)
{}
auto get() { return storage; }
static const OldDerived magic;
};

constexpr OldDerived OldDerived::magic = OldDerived(Old::magic);

struct New : public Old
{
New() = default;
};

template
struct conversions
{
using OldType = OldType_;
const OldType& toOld() const
{
return static_cast(
static_cast(type_safe::get(static_cast(*this;
}
};

template
struct one_more_type : conversions
{};

struct NewDerived
: type_safe::strong_typedef
, one_more_type
{
NewDerived() = default;
NewDerived(std::int64_t value)
: strong_typedef(New{{value}})
{}
};

As a colleague already pointed out, conversions::toOld() might have
an illegal cast in it and maybe that is all to this. What I observe is the
following code prints 'VALUE IS 0' (in production: VALUE IS ).

OldDerived
callee(bool b)
{
NewDerived a = NewDerived{11};
return b ? a.toOld() : OldDerived::magic;
}

__attribute_noinline__ void
caller_magic(bool b)
{
OldDerived value = callee(b);
std::cerr << "VALUE IS " << value.storage << std::endl;
}

void main()
{
caller_magic(true);
}

And if I do not use OldDerived::magic, the following code prints correctly
'VALUE IS 11'.

OldDerived
callee_lit(bool b)
{
NewDerived a = NewDerived{11};
return b ? a.toOld() : OldDerived(12);
}

__attribute_noinline__ void
caller_lit(bool b)
{
OldDerived value = callee_lit(b);
std::cerr << "VALUE IS " << value.storage << std::endl;
}

void main()
{
caller_lit(true);
}


Here is the code, including the parts from type_safe) inside the compiler
explorer: https://godbolt.org/z/TGB8Jg


Is it enough to fix the cast to make sure the code does what one expects?

[Bug c++/94939] [9.2.1] invalid code generation in ternary op with static class member (undefined behaviour nearby?)

[tobias.pankr...@ssw-trading.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94939

Tobias Pankrath  changed:

   What|Removed |Added

 Target||x86_64
   Host||Ubuntu 19.10
URL||https://godbolt.org/z/TGB8J
   ||g

--- Comment #1 from Tobias Pankrath  ---
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.2.1-9ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-9
--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
--with-target-system-zlib=auto --enable-multiarch --disable-werror
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.2.1 20191008 (Ubuntu 9.2.1-9ubuntu2)

[Bug c++/94939] [9.2.1] invalid code generation in ternary op with static class member (undefined behaviour nearby?)

[tobias.pankr...@ssw-trading.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94939

--- Comment #3 from Tobias Pankrath  ---
That was quick :-) Thank you for looking into it.

Is this enough? I pasted all type_safe code that is used for the example
directly in there: https://godbolt.org/z/TGB8Jg