Adding support for the ARM64 FEAT_RPRFM instruction to Go's Assembler

[Oleg Aleksandrov]

I'm working on adding support for the ARM64 FEAT_RPRFM (Range Prefetch Memory) instruction to Go's assembler. Currently, it's not supported.  Instruction documentation.

I have a question about the order of arguments in the Go assembler syntax.

According to the ARM specification, the instruction format is:

RPRFM (<rprfop>|#<imm6>), <Xm>, [<Xn|SP>]

Where:  

• (<rprfop>|#<imm6>) - prefetch operation (or immediate).  
• <Xm> - general-purpose register holding metadata.  
• [<Xn|SP>] - base address register (or stack pointer).

I'm considering two possible orders for Go's assembler:

Option 1: 
RPRFM Rm, Rn, <rprfop|imm6>  

• Rm - metadata register  
• Rn - address register  

Option 2: 
RPRFM Rn, Rm, <rprfop|imm6>  

•  Rn - address register
•  Rm - metadata register

Question:  
Which of these two options better fits the existing design patterns of Go's assembler for ARM64?  
Or is there a more idiomatic way to support this instruction?

[Jorropo]

I am not aware of an existing convention for prefetches.

Copy range instructions (like AMD64's REP MOVS) use DST, SRC, length order.

Dropping the destination since prefetch doesn't write anywhere does it ?
I would use address, length order.

FWIW it is how slices and strings are represented in the compiler so the order isn't completely made up.

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