Switching between sibling/cousin registers via API calls

Project Infrastructure LLVM Dev List Archives
#1

I have a register hierarchy that looks like a binary tree.

v4 → {v2, v2} → {s, s}{s, s}(or, {x, y}, {z, w})

I have an instruction that can access the 2nd and/or 4th scalar and an

instruction that can access the 1st and/or 3rd scalar. What I need to do

is that given the first scalar, I need to be able to select the 2nd scalar,

or/and given the 3rd, select the 4th. I define a sibling register as a register

that has the same parent and a cousin as a register that has the same grandparent.

So ‘x’ is a cousin of ‘w’ because of the grandparent class and ‘x’ is a sibling

of ‘y’ because of the parent class.

I know I can move to parent/children registers via super/sub classes,

but is there any way in LLVM currently to easily select a cousin/sibling registers that

I am not seeing?

Currently what I’ve found to work is I do register - base_scalar_1 + base_scalar_2,

but this doesn’t seem like the best solution since this has an implicit

assumption on the ordering of the enumerated register values.

Any ideas?

Thanks,

Micah

#2

Hi Micah,

This sounds somewhat similar to what ARM uses for the stride-by-two vector load instructions. For example, Tuples2DSpc. While not exactly what you're looking for, perhaps something along those lines would work?

-Jim

#3

This looks interesting, my next question would be, how do you access them? For example, let us say I know that I am register that is within a Tuples2DSpc register class, how would I be able to select the opposite register?

I think the difference here is that I don’t want to use both of them at the same time, I want to use one register in one instruction, and its sibling register in the next instruction.