ARm prfm 和 prfum 指令

Question

can someone explain what is the difference between the arm instructions prfm and prfum and usage of these ?

PRFUM https://developer.arm.com/documentation/dui0802/b/PRFUM

PRFM https://developer.arm.com/documentation/dui0801/g/A64-Data-Transfer-Instructions/PRFM--literal-

Answer 1

For questions like this, your primary source is the ARMv8 Architecture Reference manual. I happen to have the edition Ea at hand, so page numbers are going to refer to that edition.

On Page C6-1136, PRFM (immediate) is described. It allows to prefetch data that is at an offset that is a multiple of 8 in the range of 0 to +32760 relative to the value in a base register.

On Page C6-1142, PRFUM is described. It allows to prefetch data that is at any kind of offset (not just multiples of 8) in the range -256 to +255 relative to the value in a base register.

So if you need (for some reasons, like working with strings) to prefetch with byte accuracy, or you need to prefetch with a negative offset, you have to use PRFUM . On the other hand, if you want to prefetch with an offset of 256 or higher, you have to use PRFM .