std::vector<int> numbers;

for (int i = 0; i < 50; ++i) numbers.push_back(i);

int sum = 0;

//vectorization
for (auto number = numbers.begin(); number != numbers.end(); ++number) sum += *number;

//vectorization
for (auto number = numbers.begin(); number != numbers.end(); ++number) {
    auto && ref = *number;
    sum += ref;
}

//definition of range based for loops from http://en.cppreference.com/w/cpp/language/range-for
//vectorization
for (auto __begin = numbers.begin(),__end = numbers.end();
    __begin != __end; ++__begin) {
    auto && ref = *__begin;
    sum += ref;
}

//no vectorization :(
for (auto number : numbers) sum += number;

//no vectorization :(
for (auto& number : numbers) sum += number;

//no vectorization :(
for (const auto& number : numbers) sum += number;

//no vectorization :(
for (auto&& number : numbers) sum += number;

printf("%f\n",sum);

看到拆卸,循环的标准都被矢量化：

00BFE9B0  vpaddd      xmm1,xmm1,xmmword ptr [eax]  
00BFE9B4  add         ecx,4  
00BFE9B7  add         eax,10h  
00BFE9BA  cmp         ecx,edx  
00BFE9BC  jne         main+140h (0BFE9B0h)

但是基于循环的范围不是：

00BFEAC6  add         esi,dword ptr [eax]  
00BFEAC8  lea         eax,[eax+4]  
00BFEACB  inc         ecx  
00BFEACC  cmp         ecx,edi  
00BFEACE  jne         main+256h (0BFEAC6h)

有没有什么原因为什么编译器无法向这些循环进行向量化？

我真的想使用新的语法,但是放大矢量化太差了.

我刚刚看到了this question,所以我试过/ Qvec-report：2标志,给出了另一个原因：

loop not vectorized due to reason '1200'

那是：

Loop contains loop-carried data dependences that prevent vectorization. Different iterations of
the loop interfere with each other such that vectorizing the loop would produce wrong answers,and
the auto-vectorizer cannot prove to itself that there are no such data dependences.

这是同样的错误吗？ (我也尝试过最后一个vc编译器“2013年11月CTP”)

我应该在MS连接上报告吗？

编辑

Du评论,我做了同样的测试与一个raw int数组而不是一个向量,所以没有迭代器类涉及,只是原始的指针.

现在除了两个“模拟的基于范围的”循环之外,所有循环都被矢量化.

编译器说这是由于’501’原因：

Induction variable is not local; or upper bound is not loop-invariant.

我不知道发生了什么事情

const size_t size = 50;
int numbers[size];

for (size_t i = 0; i < size; ++i) numbers[i] = i;

int sum = 0;

//vectorization
for (auto number = &numbers[0]; number != &numbers[0] + size; ++number) sum += *number;

//vectorization
for (auto number = &numbers[0]; number != &numbers[0] + size; ++number) {
    auto && ref = *number;
    sum += ref;
}

//definition of range based for loops from http://en.cppreference.com/w/cpp/language/range-for
//NO vectorization ?!
for (auto __begin = &numbers[0],__end = &numbers[0] + size;
    __begin != __end; ++__begin) {
    auto && ref = *__begin;
    sum += ref;
}

//NO vectorization ?!
for (auto __begin = &numbers[0],__end = &numbers[0] + size;
    __begin != __end; ++__begin) {
    auto && ref = *__begin;
    sum += ref;
}

//vectorization ?!
for (auto number : numbers) sum += number;

//vectorization ?!
for (auto& number : numbers) sum += number;

//vectorization ?!
for (const auto& number : numbers) sum += number;

//vectorization ?!
for (auto&& number : numbers) sum += number;

printf("%f\n",sum);