Missing compiler intrinsics

[calebzulawski]

I think it would be helpful to collect a list of all compiler intrinsics (the extern "platform-intrinsics" ones) that need to be added.

• [x] simd_neg
• [ ] simd_overflowing_add etc
• [x] simd_trunc
• [x] simd_round
• [x] ~simd_lt_packed etc (all other packed intrinsics can be emulated with this one to start)~
• [ ] simd_sat_shl <-- only supported in recent LLVM
• [ ] simd_abs
• [ ] simd_sext (or some kind of simd_cast for integer -> mask)
• [x] simd_bitmask/simd_select_bitmask (these already exist, but they should be modified to accept arrays of u8
• [ ] simd_bswap

[workingjubilee]

On one hand, logically there isn't any reason it should yield the same results, as integers are simpler types. On the other hand, https://github.com/rust-lang/stdsimd/issues/97 still gives me the heebie-jeebies about adding simd_abs as an intrinsic.

What would simd_lt_packed be equivalent to?

[calebzulawski]

I think I previously decided (but never updated this issue) that we don't need an intrinsics for operating on packed bit masks. I think we only need two intrinsics, one for sign extending a packed bit mask to a full width mask, and one for truncating a full width mask to a bit mask.

[calebzulawski]

Actually, looking at the rustc codegen, I think simd_bitmask might be the truncation I'm talking about.