Specify the Calling Convention for Fixed-Length Vectors

[kito-cheng]

Previously, there was no mention of fixed-length vectors in the psABI, however fortunately, GCC and Clang are implemented same ABI rule for fixed-length vectors, so this PR is document the de factor behavior for that.

• Fixed-length vector are treat as aggregates.
• The aligment requirement is equivalent to the alignment requirement of its elemental type.

[kito-cheng]

cc. @palmer-dabbelt @JeffreyALaw @preames @topperc @rofirrim @lhtin

[lhtin]

• Fixed-length vector are treat as aggregates.

This proposal is great and avoids complicating the passing of fixed-length vectors. But the problem is that it doesn't take advantage of vector registers. Are there any more details about this issue?

GCC and Clang are implemented same ABI rule for fixed-length vectors

Also I tried, GCC currently passes through reference even if the vector size is less than 2*XLEN, which is not according to this proposal. Here is the example: https://godbolt.org/z/3ooovcz7c

[kito-cheng]

Also I tried, GCC currently passes through reference even if the vector size is less than 2*XLEN, which is not according to this proposal. Here is the example: https://godbolt.org/z/3ooovcz7c

That's bug, try to drop v from the -march and you will see different way to pass the arguments

[kito-cheng]

This proposal is great and avoids complicating the passing of fixed-length vectors. But the problem is that it doesn't take advantage of vector registers. Are there any more details about this issue?

We need separated calling convention for that case, because not every system has v, and here is the working draft for that one:

https://github.com/riscv-non-isa/riscv-elf-psabi-doc/tree/fixed-length-vector-cc