https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97222
Bug ID: 97222
Summary: GCC discards attributes aligned and may_alias for
typedefs passed as template arguments
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mte.zych at gmail dot com
Target Milestone: ---
Hello!
GCC discards aligned attribute, applied on a typdef, when it's passed as a
template argument.
Compiler Expolorer:
GCC -> https://godbolt.org/z/bj8v1T
C++ Source Code:
#include <iostream>
typedef float vec __attribute__((vector_size(8)));
typedef float fp __attribute__((aligned(16)));
template <typename t> struct identity { typedef t type; };
int main ()
{
std::cout << sizeof(typename identity<vec>::type) << std::endl;
std::cout << sizeof(vec ) << std::endl;
std::cout << alignof(typename identity<fp>::type) << std::endl;
std::cout << alignof(fp ) << std::endl;
}
Program Output:
8
8
4
16
Compilation Log:
warning: ignoring attributes on template argument 'fp' {aka 'float'}
[-Wignored-attributes]
13 | std::cout << alignof(typename identity<fp>::type) << std::endl;
| ^
The above program shows that alignment of the fp typedef changes, after it's
been
passed through the identity meta-function - it's 4-bytes instead of expected
16-bytes.
What's interesting is not all type attributes are discarded - the type
attribute vector_size
is preserved after being passed through the identity meta-function.
This behavior is required, since removal of the vector_size attribute
would be a semantic change of the vec type, affecting even its size,
because the vec type would represent a single float, instead of a vector of 2
floats.
The same could be said about the aligned attribute - discarding it is also a
semantic change,
since alignment is a fundamental property of a type, affecting among others
code generation,
that is, two types are not equivalent if they have different alignment.
This is the reason why I argue that, passing a typedef as a template argument
should preserve its aligned attribute, instead of discarding it.
Moreover, the Intel C++ compiler implements this behavior correctly.
Compiler Expolorer:
ICC -> https://godbolt.org/z/9vr9se
Program Output:
8
8
16
16
The issue described above doesn't apply only to the aligned type attribute,
but also to the may_alias type attribute.
Compiler Expolorer:
GCC -> https://godbolt.org/z/6EqsnP
C++ Source Code:
typedef int integer __attribute__((may_alias));
template <typename t> struct identity { typedef t type; };
int main ()
{
typename identity<integer>::type i;
}
Compilation Log:
warning: ignoring attributes on template argument 'integer' {aka 'int'}
[-Wignored-attributes]
7 | typename identity<integer>::type i;
| ^
Again, discarding attribute may_alias is a semantic change,
because aliasing rules can affect code generation.
Why this issue is important?
Well, because it prevents generic programming, via C++ templates, using x86
SIMD types.
If we would look at definitions of x86 SIMD types, we will notice that
they are essentially typedefs with attributes vector_size and may_alias applied
on them:
- immintrin.h
typedef float __m256 __attribute__((__vector_size__(32),
__may_alias__));
- emmintrin.h
typedef long long __m128i __attribute__((__vector_size__(16),
__may_alias__));
typedef double __m128d __attribute__((__vector_size__(16),
__may_alias__));
- xmmintrin.h
typedef float __m128 __attribute__((__vector_size__(16),
__may_alias__));
Note that, the may_alias attributes is required and cannot be removed:
- /usr/lib/gcc/x86_64-linux-gnu/10/include/immintrin.h
/* The Intel API is flexible enough that we must allow aliasing with other
vector types, and their scalar components. */
Compiler Expolorer:
GCC -> https://godbolt.org/z/vz4fWK
C++ Source Code:
#include <immintrin.h>
template <typename t> struct identity { typedef t type; };
int main ()
{
typename identity<__m128>::type fvec4;
}
Compilation Log:
warning: ignoring attributes on template argument '__m128'
[-Wignored-attributes]
8 | typename identity<__m128>::type fvec4;
| ^
What's the root cause of this problem?
Well, the problem is in C++ a typedef is just an alias (a new name) for the old
type,
that is, it does *not* introduce a new type.
Implementing support for attributes vector_size, aligned and may_alias
in C++ typedefs requires an opaque/strong typedef,
introducing a brand new type and storing information about applied attributes.
typedef float fp __attribute__((aligned(16)));
Think about it - a typedef introducing the fp type has to create a new type,
because even though both fp and float types represent floating point numbers
identically,
the fp type is *not* the float type, because these types have different
alignment requirements.
Note that, the Intel C++ Compiler does not introduce new types for typedefs,
which have attributes aligned or may_alias applied on them.
Compiler Explorer:
ICC -> https://godbolt.org/z/MjdMqx
C++ Source Code:
#include <iostream>
typedef int vectorized_int __attribute__((vector_size(8)));
typedef int aligned_int __attribute__((aligned(16)));
typedef int aliasing_int __attribute__((may_alias));
int main ()
{
std::cout << typeid( int).name() << std::endl;
std::cout << typeid(vectorized_int).name() << std::endl;
std::cout << typeid( aligned_int).name() << std::endl;
std::cout << typeid( aliasing_int).name() << std::endl;
}
Program Output:
i
Dv2_i
i
i
However, this behavior leads to a contradiction,
in which there can exists a single type, which has 2 different alignment
requirements.
Compiler Explorer:
ICC -> https://godbolt.org/z/4o9o3M
C++ Source Code:
template <class, class> struct is_same { static const auto value =
false; };
template <class T> struct is_same <T, T> { static const auto value =
true; };
typedef float fp __attribute__((aligned(16)));
template <typename first_type, typename second_type>
struct check_same
{
static_assert(is_same<first_type, second_type>::value , "");
static_assert( sizeof(first_type) == sizeof(second_type), "");
static_assert(alignof(first_type) == alignof(second_type), "");
};
int main ()
{
check_same<int, signed int> { };
check_same< fp, float> { };
}
Compilation Log:
error: static assertion failed
static_assert(alignof(first_type) == alignof(second_type), "");
^
detected during instantiation of class
"check_same<first_type, second_type> [with first_type=fp={float},
second_type=float]"
To avoid these kind of issues, GCC could replicate the behavior of the
vector_size attribute,
that is, introduce a brand new type and store information about applied
attributes.
Thank you, Mateusz Zych
PS. I want to thank Ivo Kabadshow from JSC for helping me with preparing these
code samples!
