我正在尝试了解更多关于gcc中的自动矢量化的信息。在我的项目中，我必须使用gcc 4.8.5，我看到一些循环没有矢量化。因此，我创建了一个小例子来播放，看看为什么它们不是。
我感兴趣的是gcc不会对循环进行矢量化，并了解如何对其进行矢量化。不幸的是，我对gcc的输出消息不太熟悉。
a( 我预计这个循环将被矢量化为一个琐碎的情况
b( 我缺少什么琐碎的东西吗？
提前非常感谢大家。。。
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:编译它
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：床垫后回答如下：
@马特：
非常感谢你的回答。我不知道矢量没有对齐。这些信息非常有用，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，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.
组装输出是(希望我复制粘贴正确的部分，因为我的组装知识不是很好(：
.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 $32, %rax
cmpq $27232, %rax
jne .L16
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。即使类已对齐，std::vector的标准分配器也不能保证对齐。例如，std::vector<__m64> A创建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，最简单的改变是使用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;
}
循环产生以下内容：
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
用GCC 4.8.3 -std=c++11 -Wall -Wextra -pedantic-errors -O2 -ftree-vectorize -march=native实现Godbolt
另一种选择是将boost::alignment::aligned_allocator与向量一起使用。
最后，您可以编写自己的allocator，vector可以使用它来正确地对齐事物。这里有一篇文章解释了分配器的需求。这里还有一个关于同样基本事物的SO问题。