Re: Calling Convention for Vector ?


Jim Wilson
 

On Thu, Dec 26, 2019 at 2:01 PM Earl Killian <earl.killian@arilinc.com> wrote:
Vectors are passed in memory and returned in memory. Vectors are arbitrary length, whereas the vector registers are fixed length, and can only be used to temporarily hold a portion of a memory vector. Thus it doesn’t make sense to pass or return things in vector registers, or to have the registers saved or restored as part of the calling convention.
Some code will not want args in vector regs, so that we don't have to
save/restore them around calls. Some code will want args in vector
regs, so that they can have subroutines that operate on vectors. If
you have already loaded part of a vector into a vector register, it is
silly to send it back to memory just so you can call a function that
reads it back in. It is better to leave it in a register to reduce
memory bandwidth. So we need two calling conventions. Or
alternatively, one calling convention with optional vector support
that can be enabled only when needed. If you look at ARM SVE, you
will see that this is what they have done.

I think this is more complicated for rvv though as we have LMUL up to
8, which means we need 16 registers worst case for two arguments,
which will have to be v8-15 or v16-v23 or v24-v31 because of alignment
issues. Plus we need v0 for an optional mask so we can't use v1-v7
for arguments. And vlen will have to be an implicit argument.
Someone will have to spend time doing experiments to see how well this
works in practice to make sure it is reasonable. And we will need a
reasonable compiler first before we can do experiments, which we don't
really have yet, and may not have for a while. Not to mention
hardware to test on. I think it will be a while before we can
formally specify a vector calling convention.

Jim

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