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Daniel Lemire's blog
AVX-512 gotcha: avoid compressing words to memory with AMD Zen 4 processors

The recent AMD processors (Zen 4) provide extensive support for the powerful AVX-512 instructions. AVX-512 (Advanced Vector Extensions 512) is an extension to the x86 instruction set architecture (ISA) introduced by Intel. These instructions enhance the capabilities of processors by allowing for more data to be processed in parallel. You can process registers made of 64 bytes!

One of the neat trick is that given a mask, you can ‘compress’ words: Suppose that you have a vector made of thirty-two 16-bit words, and you want to only keep the second one and third one, then you can use the vpcompressw instruction and the mask 0b110. It will produce a register where the second and third words are placed in first and second position.

An even nicer trick is that you can use this instruction to write just these two words out to memory. You can invoke this functionality with the _mm_mask_compressstoreu_epi16 function intrinsic.

This works well on recent Intel processors, but not so well on AMD Zen 4 processors.

We have a fast function in the simdjson library to minify a file (remove unnecessary spaces).

https://github.com/simdjson/simdjson/pull/2335

source

www.group-telegram.com/us/easton_channel.com/31366

381 viewsEaston's RSS Bot, Feb 14 at 21:31

Daniel Lemire's blog
AVX-512 gotcha: avoid compressing words to memory with AMD Zen 4 processors

The recent AMD processors (Zen 4) provide extensive support for the powerful AVX-512 instructions. AVX-512 (Advanced Vector Extensions 512) is an extension to the x86 instruction set architecture (ISA) introduced by Intel. These instructions enhance the capabilities of processors by allowing for more data to be processed in parallel. You can process registers made of 64 bytes!

One of the neat trick is that given a mask, you can ‘compress’ words: Suppose that you have a vector made of thirty-two 16-bit words, and you want to only keep the second one and third one, then you can use the vpcompressw instruction and the mask 0b110. It will produce a register where the second and third words are placed in first and second position.

An even nicer trick is that you can use this instruction to write just these two words out to memory. You can invoke this functionality with the _mm_mask_compressstoreu_epi16 function intrinsic.

This works well on recent Intel processors, but not so well on AMD Zen 4 processors.

We have a fast function in the simdjson library to minify a file (remove unnecessary spaces).

https://github.com/simdjson/simdjson/pull/2335

source

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