为什么编译器对原始/std数组使用XMM寄存器，而对向量不使用?

更新:似乎这是MSVC的错误，但它也发生在ICC版本14中，最高优化(/o3)打开。

UPDATE2:使用ICC关闭优化后，我得到:

159000 <<li> std::数组/gh>
• raw array 117,000
162313 <<li>向量/gh>

我正在使用下面的代码比较std::array与raw数组与std::vector的性能。我已经测试了使用MSVC 2012编译器和英特尔编译器vs 14，在Win 7 64上，64位编译。CPU是Intel第三代。

结果是(一致地):

• std::数组~ 35600
• raw array ~35,600
向量~ 40000

当我检查汇编时，编译器为std::array和原始数组选择XMM寄存器，因此可能会发生某种SIMD处理?然而，对于std::vector，使用常规的r8-r15寄存器。

假设我上面是正确的，为什么XMM寄存器不用于std::vector?

下面是完整的测试代码(您需要增加默认堆栈保留大小):

#include <iostream>
#include <vector>
#include <array>

const unsigned int noElements = 10000000;
const unsigned int noIterations = 500;
void testVector(){
    volatile unsigned long long sum = 0;
    unsigned long long start = 0;
    unsigned long long finish = 0;
    unsigned int x;
    unsigned int y;
    std::vector<unsigned int> vec;
    vec.resize(noElements);
    start = __rdtscp(&x);
    for(int i=0; i<noIterations; i++){
        for(int i=0; i<noElements; i++){
            vec[i] = i;
        }
        for(int i=0; i<noElements; i++){
            sum += (3 * vec[i]);
        }
    }
    finish = __rdtscp(&y);
    std::cout << "std::vector:t" << (finish - start)/1000000 << std::endl;
}

void testRawArray(){
    volatile unsigned long long sum = 0;
    unsigned long long start = 0;
    unsigned long long finish = 0;
    unsigned int x;
    unsigned int y;
    unsigned int myRawArray[noElements];
    start = __rdtscp(&x);
    for(int i=0; i<noIterations; i++){
        for(int i=0; i<noElements; i++){
            myRawArray[i] = i;
        }
        for(int i=0; i<noElements; i++){
            sum += (3 * myRawArray[i]);
        }
    }
    finish = __rdtscp(&y);
    std::cout << "raw array: t" << (finish - start)/1000000 << std::endl;
}
void testStdArray(){
    volatile unsigned long long sum = 0;
    unsigned long long start = 0;
    unsigned long long finish = 0;
    unsigned int x;
    unsigned int y;
    std::array<unsigned int, noElements> myStdArray;
    start = __rdtscp(&x);
    for(int i=0; i<noIterations; i++){
        for(int i=0; i<noElements; i++){
            myStdArray[i] = i;
        }
        for(int i=0; i<noElements; i++){
            sum += (3 * myStdArray[i]);
        }
    }
    finish = __rdtscp(&y);
    std::cout << "std::array: t" << (finish - start)/1000000 << std::endl;
}

int main(){
    testStdArray();
    testRawArray();
    testVector();
}

fayard@speed:Desktop$ uname -a
Darwin speed.home 13.2.0 Darwin Kernel Version 13.2.0: Thu Apr 17 23:03:13 PDT 2014; root:xnu-2422.100.13~1/RELEASE_X86_64 x86_64
fayard@speed:Desktop$ g++-4.9 --version
g++-4.9 (Homebrew gcc49 4.9.0 --enable-all-languages) 4.9.0
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
fayard@speed:Desktop$ g++-4.9 -std=c++11 -Ofast main-array.cpp -o main
fayard@speed:Desktop$ ./main
std::array:  1891738
raw array:   1889974
std::vector: 1891721
fayard@speed:Desktop$ icpc --version
icpc (ICC) 14.0.3 20140415
Copyright (C) 1985-2014 Intel Corporation.  All rights reserved.
fayard@speed:Desktop$ icpc -std=c++11 -Ofast main-array.cpp -o main
fayard@speed:Desktop$ ./main
std::array:  1896141
raw array:   1886859
std::vector: 2135880

for(int i=0; i<noElements; i++){
    vec[i] = i;
}

main-array.cpp(23): (col. 9) remark: loop was not vectorized: existence of vector dependence