普通循环未使用 gcc 4.8.5 自动矢量化

Question

我正在尝试了解有关 gcc 中的自动矢量化的更多信息。在我的项目中，我必须使用 gcc 4.8.5，并且我看到一些循环没有矢量化。因此，我创建了一个小示例来播放并了解为什么它们不是。

我感兴趣的是 gcc 不对循环进行矢量化，并想了解如何对其进行矢量化。不幸的是我不是很熟悉 GCC 的输出信息。

a) 我希望这个循环会被矢量化为一个简单的案例

b) 有什么我遗漏的小事吗？

非常感谢大家...

小例子是：

#include <iostream>
#include <vector>

using namespace std;

class test
{

public:
    test();
    ~test();
    void calc_test();
};

test::test()
{
}

test::~test()
{
}

void
test::calc_test(void)
{
vector<int> ffs_psd(10000,5.0);
vector<int> G_qh_sp(10000,1.0);
vector<int> G_qv_sp(10000,3.0);
vector<int> B_erm_qh(10000,50.0);
vector<int> B_erm_qv(10000,2.0);


for ( uint ang=0; ang < 6808; ang++)
{
   ffs_psd[0] += (G_qh_sp[ang] * B_erm_qh[ang])  +  (G_qv_sp[ang] * B_erm_qv[ang]);      
}

}

int main(int argc, char * argv[])
{
  test m_test;
  m_test.calc_test();
}

我用gcc 4.8.5编译:

c++ -O3 -ftree-vectorize -fopt-info-vec-missed -ftree-vectorizer-verbose=5 -std=c++11 test.cpp

我从编译器得到的输出是：

test.cpp:34: note: ===vect_slp_analyze_bb===

test.cpp:34: note: === vect_analyze_data_refs ===

test.cpp:34: note: get vectype with 4 units of type value_type
test.cpp:34: note: vectype: vector(4) int
test.cpp:34: note: get vectype with 4 units of type value_type
test.cpp:34: note: vectype: vector(4) int
test.cpp:34: note: get vectype with 4 units of type value_type
test.cpp:34: note: vectype: vector(4) int
test.cpp:34: note: get vectype with 4 units of type value_type
test.cpp:34: note: vectype: vector(4) int
test.cpp:34: note: get vectype with 4 units of type value_type
test.cpp:34: note: vectype: vector(4) int
test.cpp:34: note: === vect_pattern_recog ===
test.cpp:34: note: vect_is_simple_use: operand _27
test.cpp:34: note: def_stmt: _27 = (long unsigned int) ang_212;

test.cpp:34: note: type of def: 3.
test.cpp:34: note: vect_is_simple_use: operand ang_212
test.cpp:34: note: def_stmt: ang_212 = PHI <ang_43(78), 0(76)>

test.cpp:34: note: type of def: 2.
test.cpp:34: note: vect_is_simple_use: operand 4
test.cpp:34: note: vect_recog_widen_mult_pattern: detected: 
test.cpp:34: note: get vectype with 4 units of type uint
test.cpp:34: note: vectype: vector(4) unsigned int
test.cpp:34: note: get vectype with 2 units of type long unsigned int
test.cpp:34: note: vectype: vector(2) long unsigned int
test.cpp:34: note: patt_2 = ang_212 w* 4;

test.cpp:34: note: pattern recognized: patt_2 = ang_212 w* 4;

test.cpp:34: note: vect_is_simple_use: operand _29
test.cpp:34: note: def_stmt: _29 = *_67;

test.cpp:34: note: type of def: 3.
test.cpp:34: note: vect_is_simple_use: operand _34
test.cpp:34: note: def_stmt: _34 = *_69;

test.cpp:34: note: type of def: 3.
test.cpp:34: note: === vect_analyze_dependences ===
test.cpp:34: note: can't determine dependence between *_67 and MEM[(value_type &)__first_111]
test.cpp:34: note: can't determine dependence between *_68 and MEM[(value_type &)__first_111]
test.cpp:34: note: can't determine dependence between *_69 and MEM[(value_type &)__first_111]
test.cpp:34: note: can't determine dependence between *_70 and MEM[(value_type &)__first_111]
test.cpp:34: note: === vect_analyze_data_refs_alignment ===
test.cpp:34: note: vect_compute_data_ref_alignment:
test.cpp:34: note: SLP: step doesn't divide the vector-size.
test.cpp:34: note: Unknown alignment for access: *__first_125
test.cpp:34: note: vect_compute_data_ref_alignment:
test.cpp:34: note: SLP: step doesn't divide the vector-size.
test.cpp:34: note: Unknown alignment for access: *__first_153
test.cpp:34: note: vect_compute_data_ref_alignment:
test.cpp:34: note: SLP: step doesn't divide the vector-size.
test.cpp:34: note: Unknown alignment for access: *__first_139
test.cpp:34: note: vect_compute_data_ref_alignment:
test.cpp:34: note: SLP: step doesn't divide the vector-size.
test.cpp:34: note: Unknown alignment for access: *__first_167
test.cpp:34: note: vect_compute_data_ref_alignment:
test.cpp:34: note: can't force alignment of ref: MEM[(value_type &)__first_111]
test.cpp:34: note: === vect_analyze_data_ref_accesses ===
test.cpp:34: note: not consecutive access MEM[(value_type &)__first_111] = _41;

test.cpp:34: note: === vect_analyze_slp ===
test.cpp:34: note: Failed to SLP the basic block.
test.cpp:34: note: not vectorized: failed to find SLP opportunities in basic block.

编辑： 在 Matts 回答后：

@马特：

非常感谢您的回答。我不知道向量没有对齐。此信息非常有用，因为许多人认为循环将被矢量化是理所当然的，即使他们使用矢量作为容器也是如此。

不幸的是，即使您进行了更改，来自 gcc 的报告仍然没有矢量化（这次有不同的消息）：

test.cpp:47: note: misalign = 0 bytes of ref MEM[(value_type &)&ffs_psd]
test.cpp:47: note: not consecutive access _25 = MEM[(value_type &)&ffs_psd];

test.cpp:47: note: Failed to SLP the basic block.
test.cpp:47: note: not vectorized: failed to find SLP opportunities in basic block.

test.cpp:47: note: misalign = 0 bytes of ref MEM[(value_type &)&ffs_psd]
test.cpp:47: note: not consecutive access _25 = MEM[(value_type &)&ffs_psd];

test.cpp:47: note: Failed to SLP the basic block.
test.cpp:47: note: not vectorized: failed to find SLP opportunities in basic block.

汇编输出是（希望我复制粘贴正确的部分因为我的汇编知识不是很好）：

.L16
vmovdqa 40000(%rsp,%rax), %ymm1
vmovdqa 80000(%rsp,%rax), %ymm0
vpmulld 120000(%rsp,%rax), %ymm1, %ymm1
vpmulld 160000(%rsp,%rax), %ymm0, %ymm0
vpaddd  %ymm0, %ymm1, %ymm0
vpaddd  (%rsp,%rax), %ymm0, %ymm0
vmovdqa %ymm0, (%rsp,%rax)
addq    , %rax
cmpq    232, %rax
jne .L16

Answer 1

为了使用矢量化指令，操作数需要沿着正确的边界对齐。例如 __attribute__((aligned(32))) 或 __attribute__((aligned(16))) 等。即使 class 对齐，std::vector 的标准分配器也不保证对齐。例如 std::vector<__m64> A 创建一个 SSE 数据类型的向量，但它们可能不会对齐，因为 std::allocator 不会对齐所有内容。在我看来，最简单的更改是使用 std::array 和 __attribute__((aligned(32)))

#include <iostream>
#include <array>

using namespace std;

int main()
{
    array<int, 10000> ffs_psd __attribute__((aligned(32)));
    ffs_psd.fill(5);
    array<int, 10000> G_qh_sp __attribute__((aligned(32)));
    G_qh_sp.fill(1);
    array<int, 10000> G_qv_sp __attribute__((aligned(32)));
    G_qv_sp.fill(3);
    array<int, 10000> B_erm_qh __attribute__((aligned(32)));
    B_erm_qh.fill(50);
    array<int, 10000> B_erm_qv __attribute__((aligned(32)));
    B_erm_qv.fill(2);


    for ( uint ang=0; ang < 6808; ang++)
    {
        ffs_psd[0] += (G_qh_sp[ang] * B_erm_qh[ang])  +  (G_qv_sp[ang] * B_erm_qv[ang]);      
    }
    cout << ffs_psd[0] << endl;
}

循环产生这个：

vmovdqa ymm2, YMMWORD PTR [rsp+40000+rax]
vmovdqa ymm1, YMMWORD PTR [rsp+80000+rax]
vpmulld ymm2, ymm2, YMMWORD PTR [rsp+120000+rax]
vpmulld ymm1, ymm1, YMMWORD PTR [rsp+160000+rax]
add     rax, 32
vpaddd  ymm1, ymm2, ymm1
cmp     rax, 27232
vpaddd  ymm0, ymm0, ymm1
jne     .L13
vmovdqa xmm1, xmm0

与 GCC 4.8.3 -std=c++11 -Wall -Wextra -pedantic-errors -O2 -ftree-vectorize -march=native

一起在 Godbolt 上

另一种选择是将 boost::alignment::aligned_allocator 与您的向量一起使用。

最后，您可以编写自己的 allocator，vector 可以使用它来正确对齐事物。这是一篇文章，解释了关于相同基本事物的 allocator. Also here is a SO question 的要求。

普通循环未使用 gcc 4.8.5 自动矢量化

Trivial Loop not auto-vectorized with gcc 4.8.5

c++

gcc

auto-vectorization