c++ - opencv CV_ENABLE_UNROLLED 这个特性有什么作用？

Question

我在 OpenCV 中看到它在许多地方使用了 #if CV_ENABLE_UNROLLED。例如

#if CV_ENABLE_UNROLLED
for( ; i <= width - 4; i += 4 )
{
    ST f = ky[0];
    const ST* S = (const ST*)src[0] + i, *S2;
    ST s0 = f*S[0] + _delta, s1 = f*S[1] + _delta,
        s2 = f*S[2] + _delta, s3 = f*S[3] + _delta;

    for( k = 1; k <= ksize2; k++ )
    {
        S = (const ST*)src[k] + i;
        S2 = (const ST*)src[-k] + i;
        f = ky[k];
        s0 += f*(S[0] + S2[0]);
        s1 += f*(S[1] + S2[1]);
        s2 += f*(S[2] + S2[2]);
        s3 += f*(S[3] + S2[3]);
    }

    D[i] = castOp(s0); D[i+1] = castOp(s1);
    D[i+2] = castOp(s2); D[i+3] = castOp(s3);
}
#endif

在filter.cpp

中

除了在循环中进行 4 次浮点运算意味着更少的条件检查次数之外，此功能还有其他优势吗？如果是，那是什么。如果不是，那么当把它放在宏中时，为什么我们不能一直使用它呢？

Answer 1

来自 OpenCV's revisions :

CV_ENABLE_UNROLLED (?? default)default) - Experimental feature, that activates some 4- or 8- unroll loops. It theoretically helps compiler optimize the code, and improves memory access. However, the coverage of this feature in code is still very limited.