Simd: Names of load instructions

[ngzhian]

I would like to discuss the names of load instructions (this came up in #237 and #271).

Currently, we have:

• v128.load
• load splats, v8x16.load_splat, v16x8.load_splat, v32x4.load_splat, v64x2.load_splat
• load extends, i16x8.load8x8_s ,i16x8.load8x8_u ,i32x4.load16x4_s ,i32x4.load16x4_u ,i64x2.load32x2_s ,i64x2.load32x2_u
• (proposed addition), v128.load32_zero, v128.load64_zero

Some issues:

1. There is some inconsistency in the prefix, v128 v.s. v8x16 v.s. i8x16.
2. The suffix (ignoring the sign extension) is also adhoc: in some cases, it has a shape (load8x8_s), in some cases it has the number of bytes loaded (in load32_zero), and sometimes it's just a word (splat).
3. This also makes writing the formal text a bit more involved since these are all separate syntax, even though they are all loads.

A more consistent naming convention will be:

• all instructions use v128.load prefix, this is the return type, it makes it clear that all the loads return a v128
• followed by number of bits to load, similar to MVP (i32.load8_s)
• followed by the "kind" of load, splat, extend, zero

With this convention, all the loads now look like:

• v128.load
• load splats, v128.load8_splat, v128.load16_splat, v128.load32_splat, v128.load64_splat
• load extends, v128.load64_extend8x8_s, v128.load64_extend8x8_u, v128.load64_extend16x4_s, v128.load64_extend16x4_u, v128.load64_extend32x2_s, v128.load64_extend32x2_s
• load extends, v128.load8x8_s, v128.load8x8_u, v128.load16x4_s, v128.load16x4_u, v128.load32x2_s, v128.load32x2_s
• (proposed addition), v128.load32_zero, v128.load64_zero

(The load extends are a bit wordy, maybe we can come up with a better name.)

cc @tlively @Maratyszcza

[binji]

I like that this is more explicit about the load size. It seems like it's a win for the splat/zero cases, but the extend case is less clear, IMO. Not sure quite how to improve, though.

[tlively]

Could we balance spec simplicity with readability by having the load-extends be v128.load<shape>_extend_{s,u}? Maybe we could say that 32, 64 and 8x8 are all members of some shape abstraction the syntax can use here?

Edit: In more detail, perhaps 32 and 64 could be aliases for the shapes 32x1 and 64x1?

[rossberg]

In the basic numeric load instructions we don't spell out "extend", so wouldn't the most symmetric names simply be v128.load<shape>_{s,u}?

[ngzhian]

Good point, so the names will be:

• v128.load
• load splats, v128.load8_splat, v128.load16_splat, v128.load32_splat, v128.load64_splat
• load extends, v128.load8x8_s, v128.load8x8_u, v128.load16x4_s, v128.load16x4_u, v128.load32x2_s, v128.load32x2_s
• (proposed addition), v128.load32_zero, v128.load64_zero

[tlively]

Those look great to me!

[binji]

Yes, these look pretty good to me too. I still think the extend instructions are not super clear, but that's OK. (still not as bad as i64.extend_i32_s vs. i64.extend32_s)