An ex-ARM engineer critiques RISC-V

Re:An ex-ARM engineer critiques RISC-V by brandmeyer on Sunday November 01, 2020 @11:23AM (#24960461)

rotate-and-xor (and xor-and-rotate) are both common operations in ARX ciphers. They demand 4 macro-ops in RISC-V, but only one in ARMv8.

Bitfield insertion is only one instruction in most RISC ISAs, but 5 or more in RISC-V.

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Re:An ex-ARM engineer critiques RISC-V by Veedrac on Sunday November 01, 2020 @11:44AM (#24960636)

The (unfinalized) bitmanip extension has single-op rotates.

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Re:An ex-ARM engineer critiques RISC-V by CalChris on Sunday November 01, 2020 @12:23PM (#24960939)

I really don't like it. It seems like they copied x86 (bext, bdep) where they should have been plagiarizing armv8 (BFM, ...).

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Re:An ex-ARM engineer critiques RISC-V by brandmeyer on Sunday November 01, 2020 @12:38PM (#24961061)

I've watched that extension's development since it was little more than one smart guy's wish list. I don't think its fair to say that they copied any one architecture. The authors have put in a ton of time researching the tradeoffs and investigating the trade space over the years.

That said, until it gets ratified by the consortium and implemented in silicon its still just a (well-researched) wishlist.

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