Reenable C backend for non-SIMD platforms

[markos]

This is rather important. Many platforms don't have SIMD at all. Original pre-Intel Hyperscan did include a C backend, we plan to reenable that. This would also be a good indication of how much faster a SIMD port is for each platform.

[mr-c]

@markos Will you be using SIMDe for the C backend? Please let us know if we are missing any needed intrinsics and I'll try to fast track them.

[markos]

This is not decided yet. But it's a suggestion worth investigating. I will be looking at the whole SIMD approach soon, so SIMDe is an obvious choice. The biggest problem I've had with similar approaches is that they are too x86-centric and that is bad for every other platform (eg emulating movemasks on non-x86).

[markos]

Furthermore, I should clarify that the C backend will be 2-fold. First a C backend for SIMD, but also reinstating pure scalar algorithms. Both have to be done.

[markos]

@mr-c I'd like to evaluate SIMDe as it's next on my to-do list. You mentioned it also has a C backend. Could you please point me to how to enable it? Also, one aspect that we would like to have is emulation of wider vectors, eg. emulate 256-bit/512-bit SIMD on NEON or VSX. Is that available?

[mr-c]

@markos Yes, SIMDe works with C/C++ codebases. See https://github.com/simd-everywhere/simde#usage
When I'm adapting a typical x86-64 SIMD using codebase for Debian, I use these notes https://wiki.debian.org/SIMDEverywhere#Approach

SIMDe implementation of 256 & 512 bit AVX/AVX2/AVX512 x86-64 intrinsics often (but not always) have optimized NEON and VSX versions that are selected automatically when compiling on those platforms.

[markos]

I wasn't clear enough, I'm looking for a C backend, in order to run SIMDe code on a non-SIMD platform -or one that does not currently have SIMD support enabled. Essentially a way to emulate SIMD using plain C. This would enable vectorscan to run on platforms without current SIMD support.

Reason: to enable running on very new architectures without SIMD support.

[mr-c]

Maybe a definition will help:

"SIMDe" == the SIMD Everywhere drop in header-only library that implements SIMD intrinsics (SSE*, AVX*, NEON, etc..) on various architectures.

SIMD Everywhere is the "way to emulate SIMD using plain C" that you are asking for :-)

[markos]

that is great to hear! So how do I enable just the C backend? Assuming I have a system with non-supported SIMD, or one with supported SIMD but I disable that support. If I integrate vectorscan with simde, is there a define that forces the C backend? Essentially that's what I'm asking.

[mr-c]

The easy way enable portability with SIMDe is to replace include <x86intrin.h> with

#define SIMDE_ENABLE_NATIVE_ALIASES=1
#include <simde/x86/avx512.h>

and add SIMDe to your include path.

To force the usage of the non-optimized implementations, you can define SIMDE_NO_NATIVE prior to the import (or on the compiler command line)

[markos]

@mr-c so, this proved to be easier than I expected, I now have a working SIMDe-based backend that I'm testing on a platform without supported SIMD (Loongson64). Actually running the unit tests now, if everything works, I'll test the SIMDe backend on other arches as well and compare performance.

[markos]

Ok, it's slower of course, but out of 20k unit tests, only 4 failures, not bad at all! And easy to fix from what it seems. Thanks for the suggestions.