[cfe-dev] [libc++] Implementing P0214R7

Tim Shen via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 5 17:54:02 PST 2017

Hi all,

A few weeks ago I chatted with Marshall about adding a P0214 implementation
to libc++, and it seemed possible.

We (Google) do have an implementation (Dimsum,
https://github.com/google/dimsum), but it's heavily extended, incomplete,
and non-conforming. To implement P0214 in libc++, it might be easier to
start from scratch.

Here is the plan:
(1) C++11 compatible scalar implementation for simd<> and related
(2) Add an ABI that uses vector registers. Basically, it means to store
data by __m128i/__m128 for x86.
(3) C++11 compatible simplistic implementation for simd_mask<T, ABI> and
`where expression` in terms of simd<U, ABI>. U is an unsigned integer with
the size sizeof(T).
(4) (optional and low priority) Implementation of [simd.math]. Start with a
scalar + for loop implementation.
(5) Optimization by specializing on different platforms and use of native
intrinsics. But for FWIW, modern compilers are good at auto-vectorizing
for-loops and scalar operations, so we might not need that many
(6) (libc++ unrelated) Rebase Dimsum onto libc++ and keep Dimsum evolving.
We'll propose some of the Dimsum's extensions to the TS.

Does it sound like a plan?

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