[PATCH] D133668: [HLSL] Use _BitInt(16) for int16_t to avoid promote to int.

[Chris Bieneman via Phabricator via cfe-commits]

beanz added a comment.

In D133668#3847871 <https://reviews.llvm.org/D133668#3847871>, @rjmccall wrote:

> But that's purely on the implementation level, right?  Everything is implicitly vectorized and you're just specifying the computation of a single lane, but as far as that lane-wise computation is concerned, you just have expressions which produce scalar values.

Yes, with the caveat that the language doesn't dictate the maximum SIMD width, but some features have minimum widths. The language source (and IR) operate on one lane of scalar and vector values, but we do have cross-SIMD lane operations, and true scalar (uniform) values, so we have to model the full breadth of parallel fun...

> If you don't otherwise have 16-bit (or 8-bit?) types, and it's the type behavior you want, I'm fine with you just using `_BitInt`.  I just want to make sure I understand the situation well enough to feel confident that you've considered the alternatives.

We don't currently have 8-bit types (although I fully expect someone will want them because ML seems to love small data types). I suspect that the promoting integer behaviors for types smaller than int will likely never make sense for HLSL (or really any SPMD/implicit-SIMD) programming model).

My inclination is that we should define all our smaller than `int` fixed-size integer types as `_BitInt` to opt-out of promotion. Along with that I expect that we'll disable `short` under HLSL. We will still have `char`, but the intent for `char` is really for the _extremely_ limited cases where strings get used (i.e. `printf` for debugging).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D133668/new/

https://reviews.llvm.org/D133668