对齐指针未对齐访问? [英] Unaligned access on aligned pointer?
问题描述
我有此功能代码:
bool interpolate(const Mat &im, float ofsx, float ofsy, float a11, float a12, float a21, float a22, Mat &res)
{
bool ret = false;
// input size (-1 for the safe bilinear interpolation)
const int width = im.cols-1;
const int height = im.rows-1;
// output size
const int halfWidth = res.cols >> 1;
const int halfHeight = res.rows >> 1;
float *out = res.ptr<float>(0);
const float *imptr = im.ptr<float>(0);
for (int j=-halfHeight; j<=halfHeight; ++j)
{
const float rx = ofsx + j * a12;
const float ry = ofsy + j * a22;
#pragma omp simd
for(int i=-halfWidth; i<=halfWidth; ++i, out++)
{
float wx = rx + i * a11;
float wy = ry + i * a21;
const int x = (int) floor(wx);
const int y = (int) floor(wy);
if (x >= 0 && y >= 0 && x < width && y < height)
{
// compute weights
wx -= x; wy -= y;
int rowOffset = y*im.cols;
int rowOffset1 = (y+1)*im.cols;
// bilinear interpolation
*out =
(1.0f - wy) * ((1.0f - wx) * imptr[rowOffset+x] + wx * imptr[rowOffset+x+1]) +
( wy) * ((1.0f - wx) * imptr[rowOffset1+x] + wx * imptr[rowOffset1+x+1]);
} else {
*out = 0;
ret = true; // touching boundary of the input
}
}
}
return ret;
}
这是使用icpc
和以下选项编译的代码的一部分:
Which is part of code compiled with icpc
and the following options:
INTEL_OPT=-O3 -simd -xCORE-AVX2 -parallel -qopenmp -fargument-noalias -ansi-alias -no-prec-div -fp-model fast=2 -fma -align -finline-functions
INTEL_PROFILE=-g -qopt-report=5 -Bdynamic -shared-intel -debug inline-debug-info -qopenmp-link dynamic -parallel-source-info=2 -ldl
优化报告文件(.optr
)中有以下几行:
In the optimization report file (.optr
) there are these lines:
remark #15389: vectorization support: reference *out has unaligned access [ /home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Descriptors/hesaff/helpers.cpp(259,14) ]
remark #15389: vectorization support: reference *out has unaligned access [ /home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Descriptors/hesaff/helpers.cpp(263,14) ]
remark #15389: vectorization support: reference *out has unaligned access [ /home/luca/Dropbox/HKUST/CloudCache/cloudcache/CloudCache/Descriptors/hesaff/helpers.cpp(263,14) ]
第259行和第263行是明显更改了out
值的行.
Lines 259 and 263 are the ones where obviously out
value is changed.
据我所知,这意味着out
是未对齐的(此处有更多信息[此处).
To my knowledge, this means that the out
is unaligned (more info here[https://software.intel.com/en-us/articles/fdiag15126] and here ).
However, this is already weird, because as I alrady found out it here , cv::Mat
objects are already aligned. But this could be legit since the compiler could not know of the alignment...However the message appears if I specifically allocate out
as an aligned pointer if I add the following line:
out = (float*) _mm_malloc(res.cols*res.rows*sizeof(float), 32);
这应该足以告诉编译器数据已对齐.我错了吗?否则,我怎么能说out
已经对齐?
This should be enough to tell the compiler that the data are aligned. Am I wrong? Otherwise, how can I tell it that out
is already aligned?
推荐答案
编译器看到out++
.它不能保持对齐.
The compiler sees out++
. It can't stay aligned.
(也就是说,对于float*
访问,它保持对齐状态,但是该消息指的是SIMD矢量化,需要更严格的对齐方式)
(That is to say, it remains aligned for float*
access, but the message refers to SIMD vectorization which requires a stricter alignment)
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