Provide AoS access to SoA

问题

I have data laid out in memory in a Structure of Arrays (SoA) or Sturcture of Pointers (SoP) form, and have a way to access that data as though it were laid out in Array of Structure (AoS) form -- code given below.

However, I am not too happy about use of struct AoS_4_SoP -- although this struct appears to use templates, it is not really generic since, for example, foo and bar are hard-coded inside it.

Two questions/requests:

1) For read-write performance, is AoS access provided as good as the direct SoA access?

2) What would a more generic scheme be? (I have seen quamrana's code here, but it hasn't helped.)

struct  SoP{      // Structure of Pointers
   int    *foo{ nullptr };
   double *bar{ nullptr };
   SoP( int *xi, double *xd ):foo(xi), bar(xd){};
};

struct SoR{       // Structure of References
   int    &foo;
   double &bar;
   SoR( int &xi, double &xd ):foo(xi), bar(xd){};
};

template< typename T,  typename S >
struct AoS_4_SoP {
    AoS_4_SoP( T *x ) : p( x ){};
    T    *p;

          S  operator[](std::size_t idx) const { return { p->foo[idx], p->bar[idx] }; }
    const S  operator[](std::size_t idx) const { return { p->foo[idx], p->bar[idx] }; }
};

Here's a main() showing the use of the above:

int main()
{
    std::vector< int    > ibuf{ 11, 22, 33, 44 };
    std::vector< double > dbuf{ 0.11, 0.22, 0.33, 0.44 };;

    SoP  x_sop( ibuf.data(),  dbuf.data() );

    ibuf.at(2)  = 333;
    std::cout << "Access via SoP syntax:\n      "
              << x_sop.foo[2]
              << "        "
              << x_sop.bar[2] << std::endl;

    AoS_4_SoP<SoP, SoR> xacc( &x_sop );

    std::cout << "Access via AoS syntax:\n      "
              << xacc[2].foo
              << "        "
              << xacc[2].bar << std::endl;

    // show write access via SoA syntax
    ibuf.at(2)   = 3333;
    dbuf.at( 2 ) = 0.333333;  // will get overwritten below
    xacc[2].bar = 0.3333;

    std::cout << "Values written via SoP, read via SoP:\n      "
              << x_sop.foo[2]
              << "       "
              << x_sop.bar[2] << std::endl;

    // show write access via AoS syntax
    xacc[2].foo = 333333;
    dbuf.at( 2 ) = 0.3333333333;  // will get overwritten below
    xacc[2].bar = 0.333333;

    std::cout << "Values written via AoS, read via AoS:\n      "
              << xacc[2].foo
              << "     "
              << xacc[2].bar << std::endl;
}

Above code can be compiled via:

// x86_64-w64-mingw32-g++.exe -D_WIN64 -Wall -Wextra -Werror -std=c++11 -O3 -static-libgcc -static-libstdc++ aossoa.cc -o aossoa.exe

and results in the following output:

Access via SoP syntax:
      333        0.33
Access via AoS syntax:
      333        0.33
Values written via SoP, read via SoP:
      3333       0.3333
Values written via AoS, read via AoS:
      333333     0.333333

回答1:

I think this template will work.

template<class T, class U, class D, class S>
struct Accessor {
    T* p;
    U* (T::*pFirst);
    D* (T::*pSecond);
    S operator[](size_t index) {
        return {(p->*pFirst)[index], (p->*pSecond)[index]};
    }
    Accessor(T* p_, U * (T::*pF), D * (T::*pS)): p(p_), pFirst(pF), pSecond(pS) {}
};

void main() {
    std::vector< int    > ibuf{ 11, 22, 33, 44 };
    std::vector< double > dbuf{ 0.11, 0.22, 0.33, 0.44 };;

    SoP  x_sop(ibuf.data(),  dbuf.data());

    Accessor<SoP, int, double, SoR> aos(&x_sop, &SoP::foo, &SoP::bar);

    aos[0].foo;
}

Now the template Accessor knows nothing about the names of the members of T.

At least it compiles under VS2015

回答2:

Here's my adaptation of quamrana's solution for the "inverse" use case (SoA access for AoS):

template<class T, class S, class M0, class M1>
struct Accessor2{
    T*  p;
    M0  m0;
    M1  m1;

          S operator[](std::size_t idx)       { return { m0[idx], m1[idx] }; }
    const S operator[](std::size_t idx) const { return { m0[idx], m1[idx] }; }

    Accessor2(T* x, M0 p0, M1 p1): p(x), m0(p0), m1(p1){}
};

template< typename T,  typename S >
struct AoS_4_SoP : public Accessor2<T, S, decltype(T::foo), decltype(T::bar)>
{
#ifndef COMPILER_CAN_INFER_PARENT_CLASS_TEMPLATE_SPECIFICATION
    AoS_4_SoP(T *x) : Accessor2<T, S, decltype(T::foo), decltype(T::bar)>
                      (x, x->foo, x->bar ){}
#else
    AoS_4_SoP(T *x):Accessor2(x, x->foo, x->bar ){}
#endif
};

Regarding the #ifndef COMPILER_CAN_INFER_PARENT_CLASS_TEMPLATE_SPECIFICATION, the compiler I am using viz, g++ 6.2.0 (x86_64_6.2.0_posix_seh_rt_v5_rev1/mingw64/bin/x86_64-w64-mingw32-g++.exe) is unable to infer the parent class' template specification. But, as Lightness Races in Orbit says on this page: the base's injected-class-name ought to be sufficient information for the compiler.

来源：https://stackoverflow.com/questions/42184288/provide-aos-access-to-soa