问题
I have a column vector A which is 10 elements long. I have a matrix B which is 10 by 10. The memory storage for B is column major. I would like to overwrite the first row in B with the column vector A.
Clearly, I can do:
for ( int i=0; i < 10; i++ )
{
B[0 + 10 * i] = A[i];
}
where I've left the zero in 0 + 10 * i to highlight that B uses column-major storage (zero is the row-index).
After some shenanigans in CUDA-land tonight, I had a thought that there might be a CPU function to perform a strided memcpy?? I guess at a low-level, performance would depend on the existence of a strided load/store instruction, which I don't recall there being in x86 assembly?
回答1:
Short answer: The code you have written is as fast as it's going to get.
Long answer: The memcpy function is written using some complicated intrinsics or assembly because it operates on memory operands that have arbitrary size and alignment. If you are overwriting a column of a matrix, then your operands will have natural alignment, and you won't need to resort to the same tricks to get decent speed.
来源：https://stackoverflow.com/questions/6013779/is-there-a-standard-strided-version-of-memcpy