Comment by camel-cdr

2 days ago

I really like what you are doing here, the state of inline assembly is a similar travesty to the state of guided codegen/autovec.

On concern I have is how this maps to ARM64 syntax, because ARM64 is massively overloading all mnemonics.

For example:

    ld1d z0.h, p0/z, [x1, x2, lsl 3]
    ld1d z0.h, p0/z, [x1, z0.h, lsl 3]

Have extremely different performance characteristics, yet would map to the same code:

    ld1d dst, p0/z, [base + idx<<3]

Imo this makes reading the assembly quite bothersome. I'm already not a fan of ARM64 doing the mnemonic overloading, but at least you can figure out the operation by looking at the same line further to the right.

Also, maybe I missed it, but how are you dealing with things like the /z modifier, pre/post-increment load/store and load pair? Or things like TBL/ST4/LD4?

Oh and how are the types going to work for RVV, where the type can't be determined at compile-time in all situations?

1 comment

camel-cdr

gingerBill 1 day ago

I haven't fully thought out that syntax yet, but it's a problem with AVX-512 in terms of its predicate operands too.

My hunch would be the following:

        sve_ld1d_scalar :: asm(base: [^]u64, idx: i64, #mask pred: u16) -> (dst: #simd[vscale * 4]f32) [
                dst  = %z0,
                pred = #predicate(indirect=zeroing) %p0,
                dsth: u16 = dst,
        ] {
                ld1d dsth, pred, [base + idx<<3]
        }

        sve_ld1d_gather :: asm(base: [^]u64, #mask pred: u16) -> (dst: #simd[vscale * 4]f32) [
                dst  = %z0,
                pred = #predicate(indirect=zeroing) %p0,
                dsth: u16 = dst,
        ] {
                ld1d dsth, pred, [base + dsth<<3]
        }

So the parameter is marked as a predicate with zeroing or whatever, and then `pred` is just a normal operand as the binding section specifies everything.

This is not current behaviour yet but it I am considering it when I need to specify this for even AVX-512 and RISC-V behaviour (which has multiple different possibilities).