当我分析正在处理的代码时，一个特别的热点是以下循环：
for(int loc = start; loc<end; ++loc)
y[loc]+=a[offset+loc]*x[loc+d];
其中数组 y、a 和 x 没有重叠。在我看来，这样的循环应该很容易矢量化，但是当我使用带有选项“-O3 -ftree-vectorize -ftree-vectorizer-verbose=1”的 g++ 进行编译时，我没有任何迹象表明这个特定的循环已矢量化。但是，在上面的代码之前发生的循环：
for(int i=0; i<m; ++i)
y[i]=0;
确实根据输出进行矢量化。关于为什么第一个循环没有矢量化，或者我如何解决这个问题有什么想法吗？ （我并没有受过矢量化概念方面的教育，所以我可能错过了一些非常明显的东西）
根据 Oli 的建议，调高详细程度会产生以下注释（虽然我通常擅长阅读编译器警告/错误/输出，我不知道这意味着什么）：
./include/mv_ops.h:89: note: dependence distance = 0.
./include/mv_ops.h:89: note: accesses have the same alignment.
./include/mv_ops.h:89: note: dependence distance modulo vf == 0 between *D.50620_89 and *D.50620_89
./include/mv_ops.h:89: note: not vectorized: can't determine dependence between *D.50623_98 and *D.50620_89
One particular hot spot when I profile a code I am working on, is the following loop:
for(int loc = start; loc<end; ++loc)
y[loc]+=a[offset+loc]*x[loc+d];
where the arrays y, a, and x have no overlap. It seems to me that a loop like this should be easily vectorized, however when I compile using g++ with the options "-O3 -ftree-vectorize -ftree-vectorizer-verbose=1", I get no indication that this particular loop was vectorized. However, a loop occurring just before the code above:
for(int i=0; i<m; ++i)
y[i]=0;
does get vectorized according to the output. Any thoughts on why the first loop is not vectorized, or how I might be able to fix this? (I am not all that educated on the concept of vectorization, so I am likely missing something quite obvious)
As per Oli's suggestion, turning up the verbosity yields the following notes (while I am usually good at reading compiler warnings/errors/output, I have no idea what this means):
./include/mv_ops.h:89: note: dependence distance = 0.
./include/mv_ops.h:89: note: accesses have the same alignment.
./include/mv_ops.h:89: note: dependence distance modulo vf == 0 between *D.50620_89 and *D.50620_89
./include/mv_ops.h:89: note: not vectorized: can't determine dependence between *D.50623_98 and *D.50620_89
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您需要告诉编译器
x、y和a不重叠。在 C/C++ 术语中，这意味着通过使用限制（或__restrict）。当 gcc 假设没有别名时，它对优化非常积极，所以要小心。You need to tell the compiler that
x,y, andado not overlap. In C/C++ terms that means telling the compiler that those pointers do not alias by declaring them withrestrict(or__restrict). gcc is very aggressive about optimizations when it assumes no aliasing, so be careful.一种可能性是编译器无法保证不存在别名。换句话说，编译器如何确保
y、a和x不会以某种方式重叠？如果您提高详细程度，您可能会获得一些额外的信息。
One possibility is that the compiler can't guarantee that there are no aliases. In other words, how can the compiler be sure that
y,aandxdon't overlap in some way?If you turn the verbosity level up, you may get some extra info.