为什么 GCC 不自动矢量化这个循环？

Question

我有以下 C 程序（我的实际用例的简化，表现出相同的行为）

#include <stdlib.h>
#include <math.h>
int main(int argc, char ** argv) {
    const float * __restrict__ const input = malloc(20000*sizeof(float));
    float * __restrict__ const output = malloc(20000*sizeof(float));

    unsigned int pos=0;
    while(1) {
            unsigned int rest=100;
            for(unsigned int i=pos;i<pos+rest; i++) {
                    output[i] = input[i] * 0.1;
            }

            pos+=rest;            
            if(pos>10000) {
                    break;
            }
    }
}

当我编译时，

 -O3 -g -Wall -ftree-vectorizer-verbose=5 -msse -msse2 -msse3 -march=native -mtune=native --std=c99 -fPIC -ffast-math

我得到输出，

main.c:10: note: not vectorized: unhandled data-ref 

其中 10 是内部 for 循环的行。当我查找为什么它可能会这样说时，它似乎在说指针可以使用别名，但它们不能出现在我的代码中，因为我有 __restrict 关键字。他们还建议包括 -msse 标志，但他们似乎也没有做任何事情。有什么帮助吗？

Answer 1

这确实看起来像一个错误。在下面的等效函数中，当针对 x86-64 目标进行编译时，foo() 已矢量化，但 bar() 则不是：

void foo(const float * restrict input, float * restrict output)
{
    unsigned int pos;
    for (pos = 0; pos < 10100; pos++)
        output[pos] = input[pos] * 0.1;
}

void bar(const float * restrict input, float * restrict output)
{
    unsigned int pos;
    unsigned int i;
    for (pos = 0; pos <= 10000; pos += 100)
        for (i = 0; i < 100; i++)
            output[pos + i] = input[pos + i] * 0.1;
}

添加 -m32< /code> 标志，改为编译 x86 目标，导致两个函数都被矢量化。

Answer 2

它不喜欢外循环格式，这会阻止它理解内循环。如果我只是将其折叠成一个循环，我就可以对其进行矢量化：（

#include <stdlib.h>
#include <math.h>
int main(int argc, char ** argv) {
    const float * __restrict__ input = malloc(20000*sizeof(float));
    float * __restrict__ output = malloc(20000*sizeof(float));

    for(unsigned int i=0; i<=10100; i++) {
            output[i] = input[i] * 0.1f;
    }
}

请注意，我并没有认真考虑如何将 pos+rest 限制正确地转换为单个 for 循环条件，这可能是错误的）

您可能可以通过将简化的内部循环放入使用指针和计数调用的函数中来利用这一点。即使再次内联它也可以正常工作。这是假设您删除了我刚刚简化的 while() 循环的部分内容，但您需要保留它们。

Answer 3

尝试：

const float * __restrict__ input = ...;
float * __restrict__ output = ...;

通过改变周围的事物来进行一些实验：

#include <stdlib.h>
#include <math.h>

int main(int argc, char ** argv) {

    const float * __restrict__ input = new float[20000];
    float * __restrict__  output = new float[20000];

    unsigned int pos=0;
    while(1) {
        unsigned int rest=100;
        output += pos;
        input += pos;
        for(unsigned int i=0;i<rest; ++i) {
            output[i] = input[i] * 0.1;
        }

        pos+=rest;
        if(pos>10000) {
            break;
        }
    }
}

g++ -O3 -g -Wall -ftree-vectorizer-verbose=7 -msse -msse2 -msse3 -c test.cpp

test.cpp:14: note: versioning for alias required: can't determine dependence between *D.4096_24 and *D.4095_21
test.cpp:14: note: mark for run-time aliasing test between *D.4096_24 and *D.4095_21
test.cpp:14: note: Alignment of access forced using versioning.
test.cpp:14: note: Vectorizing an unaligned access.
test.cpp:14: note: vect_model_load_cost: unaligned supported by hardware.
test.cpp:14: note: vect_model_load_cost: inside_cost = 2, outside_cost = 0 .
test.cpp:14: note: vect_model_simple_cost: inside_cost = 2, outside_cost = 0 .
test.cpp:14: note: vect_model_simple_cost: inside_cost = 2, outside_cost = 1 .
test.cpp:14: note: vect_model_simple_cost: inside_cost = 1, outside_cost = 0 .
test.cpp:14: note: vect_model_store_cost: inside_cost = 1, outside_cost = 0 .
test.cpp:14: note: cost model: Adding cost of checks for loop versioning to treat misalignment.

test.cpp:14: note: cost model: Adding cost of checks for loop versioning aliasing.

test.cpp:14: note: Cost model analysis:
  Vector inside of loop cost: 8
  Vector outside of loop cost: 6
  Scalar iteration cost: 5
  Scalar outside cost: 1
  prologue iterations: 0
  epilogue iterations: 0
  Calculated minimum iters for profitability: 2

test.cpp:14: note:   Profitability threshold = 3

test.cpp:14: note: Vectorization may not be profitable.
test.cpp:14: note: create runtime check for data references *D.4096_24 and *D.4095_21
test.cpp:14: note: created 1 versioning for alias checks.

test.cpp:14: note: LOOP VECTORIZED.
test.cpp:4: note: vectorized 1 loops in function.

Compilation finished at Wed Feb 16 19:17:59