Funnel shift - what is it?

问题:

When reading through CUDA 5.0 Programming Guide I stumbled on a feature called "Funnel shift" which is present in 3.5 compute-capable device, but not 3.0. It contains an annotation "see reference manual", but when I search for the "funnel shift" term in the manual, I don't find anything.

I tried googling for it, but only found a mention on http://www.cudahandbook.com, in the chapter 8:

8.2.3 Funnel Shift (SM 3.5)

GK110 added a 64-bit “funnel shift” instruction that may be accessed with the following intrinsics:

__funnelshift_lc(): returns most significant 32 bits of a left funnel shift.

__funnelshift_rc(): returns least significant 32 bits of a right funnel shift.

These intrinsics are implemented as inline device functions (using inline PTX assembler) in sm_35_intrinsics.h.

...but it still does not explain what the "left funnel shift" or "right funnel shift" is.

So, what is it and where does one need it?

回答1:

In the case of CUDA, two 32-bit registers are concatenated together into a 64-bit value; that value is shifted left or right; and the most significant (for a left shift) or least significant (for right shift) 32 bits are returned.

The intrinsics from sm_35_intrinsics.h are as follows:

unsigned int __funnelshift_lc(unsigned int lo, unsigned int hi, unsigned int shift);
unsigned int __funnelshift_rc(unsigned int lo, unsigned int hi, unsigned int shift);

According to Andy Glew (dead link removed), applications for funnel shift include fast misaligned memcpy; and as njuffa mentions in the comments above, it can be used to implement rotate if the two input words are the same.