explain SIMD terminology and commonly-confused terms#535
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| **Lane vs. field vs. element.** A *lane* is a position in the vector, numbered `0` to `N-1`, like an index into an array. An *element* is the value that currently occupies a lane. So "lane 2" is a slot, and "the element in lane 2" is its contents. *Field* is a third word you'll occasionally see — usually in older or vendor-specific writing — for the same idea; it carries no separate meaning here, and this guide sticks to *lane* for the position and *element* for the value. | ||
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| **Scalar vs. vector.** A *scalar* is a single value, the ordinary `f32` or `i32` you're already used to. A *vector* is a fixed-size group of those values that the CPU operates on together — an `f32x4` is four `f32` scalars packed into one register. The whole point of SIMD is to replace many scalar operations with one vector operation, so "scalar code" is the non-SIMD baseline you're comparing against. |
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on some architectures and target features (RISC-V V and Arm SVE), vectors are inherently variable-size (with the size determined by the programmer and/or the cpu, depending on the architecture) and portable-simd's fixed-size types are converted by the compiler to variable-size instructions when those are better. portable-simd doesn't yet have variable-size types.
programmerjake
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See #534 (comment) |
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Part of #29. Second PR in the series.
This PR broadens the beginner vocabulary and adds new terms, bringing the guide closer to how the finished documentation should look.
Addresses these checklist items:
New
Termsentries:Element,Splat,Mask,Horizontal, andIntrinsic(covering both vendorcore::archintrinsics and compiler/LLVM intrinsics, so readers can see what intrinsics actually look like). New "Words That Are Easy to Mix Up" section for the four distinctions above.Splat/Mask/Horizontalaren't on the checklist, but they're core beginner-facing vocabulary for this crate (splat appears in the README's hello-world, masks have a whole README section, and horizontal pairs with the existing "vertical" term), so I included them while we're here.