Re: [boost] BOOST_NO_CXX11_CONSTEXPR and msvc-14.0

Subject: Re: [boost] BOOST_NO_CXX11_CONSTEXPR and msvc-14.0
From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2015-07-21 20:51:25

[Glen Fernandes]
> Only what was reported for the RC:

Was this reported on Connect? I didn't see any bug reports filed against the STL.

> 1. For x86 and x64: std::alignment_of<std::nullptr_t>::value is 1, yet
> compiler aligns at 4
> 2. For x86: std::alignment_of<T Class::*>::value is 4, yet compiler aligns at 8
>     For x64: std::alignment_of<T Class::*>::value is 8, yet compiler aligns at 4
> 3. For x86: std::alignment_of<T (Class::*)()>::value is 4, yet
> compiler aligns at 8
>     For x64: std::alignment_of<T (Class::*)()>::value is 8, yet
> compiler aligns at 4

I am observing discrepancies with nullptr_t on x86/x64 and PMDs on x64 only, but not with PMFs. I've filed DevDiv#1202506 "alignof reports inaccurate values" with the repro at the end of this mail.

If you can demonstrate x86 PMD failure, or x86/x64 PMF failure, I'll add that to the bug report.

> I haven't checked the RTM yet, but apparently VC12's std::alignment_of
> for the relevant types reported the same value that the compiler chose
> to align objects of those types.

Yeah, that's because 2013 used library-only tech, and 2015 uses alignof as required/depicted by the Standard.

Thanks,
STL

C:\Temp>type meow.cpp
#ifdef _MSC_VER
    #pragma warning(disable: 4127) // conditional expression is constant
#endif

#include <stddef.h>

template <typename T> struct RemoveReference {
    typedef T type;
};

template <typename T> struct RemoveReference<T&> {
    typedef T type;
};

template <typename T> struct RemoveReference<T&&> {
    typedef T type;
};

template <typename T> struct GetAlign {
    T    elem0;
    char elem1;
    T    elem2;
};

template <typename T> struct AlignmentOf {
    using U = typename RemoveReference<T>::type;

    static const size_t value = sizeof(GetAlign<U>) - 2 * sizeof(U);
};

struct Meow { };

#include <cstddef>
#include <iostream>
using namespace std;

template <typename T> void print(const char * const name) {
    cout << name << endl;
    cout << "AlignmentOf<T>::value = " << AlignmentOf<T>::value << endl;
    cout << "           alignof(T) = " << alignof(T);

    if (AlignmentOf<T>::value != alignof(T)) {
        cout << " <== FAIL";
    }

    cout << endl << endl;
}

#define PRINT(X) print<X>(#X)

int main() {
    PRINT(char);
    PRINT(short);
    PRINT(int);
    PRINT(long);
    PRINT(long long);

    PRINT(float);
    PRINT(double);

    PRINT(int *);

    using PMD = int Meow::*;
    using PMF = int (Meow::*)(int);

    PRINT(nullptr_t);
    PRINT(PMD);
    PRINT(PMF);
}

[x86]
C:\Temp>cl /EHsc /nologo /W4 /MTd meow.cpp
meow.cpp

C:\Temp>meow
char
AlignmentOf<T>::value = 1
           alignof(T) = 1

short
AlignmentOf<T>::value = 2
           alignof(T) = 2

int
AlignmentOf<T>::value = 4
           alignof(T) = 4

long
AlignmentOf<T>::value = 4
           alignof(T) = 4

long long
AlignmentOf<T>::value = 8
           alignof(T) = 8

float
AlignmentOf<T>::value = 4
           alignof(T) = 4

double
AlignmentOf<T>::value = 8
           alignof(T) = 8

int *
AlignmentOf<T>::value = 4
           alignof(T) = 4

nullptr_t
AlignmentOf<T>::value = 4
           alignof(T) = 1 <== FAIL

PMD
AlignmentOf<T>::value = 4
           alignof(T) = 4

PMF
AlignmentOf<T>::value = 4
           alignof(T) = 4

[x64]
C:\Temp>cl /EHsc /nologo /W4 /MTd meow.cpp
meow.cpp

C:\Temp>meow
char
AlignmentOf<T>::value = 1
           alignof(T) = 1

short
AlignmentOf<T>::value = 2
           alignof(T) = 2

int
AlignmentOf<T>::value = 4
           alignof(T) = 4

long
AlignmentOf<T>::value = 4
           alignof(T) = 4

long long
AlignmentOf<T>::value = 8
           alignof(T) = 8

float
AlignmentOf<T>::value = 4
           alignof(T) = 4

double
AlignmentOf<T>::value = 8
           alignof(T) = 8

int *
AlignmentOf<T>::value = 8
           alignof(T) = 8

nullptr_t
AlignmentOf<T>::value = 8
           alignof(T) = 1 <== FAIL

PMD
AlignmentOf<T>::value = 4
           alignof(T) = 8 <== FAIL

PMF
AlignmentOf<T>::value = 8
           alignof(T) = 8