Comment by cogman10

2 months ago

> AND the software with no architecture-specific optimisations

The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V. I do not believe this is a lack of software optimization issue.

We are well past the days where hand written assembly gives much benefit, and modern compilers like gcc and llvm do nearly identical work right up until it comes to instruction emissions (including determining where SIMD instructions could be placed).

Unless these chips have very very weird performance characteristics (like the weirdness around x86's lea instruction being used for arithmetic) there's just not going to be a lot of missed heuristics.

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cogman10

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bobmcnamara  2 months ago

> The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V.

There's no carry bit, and no widening multiply(or MAC)

  • Findecanor  2 months ago

    RISC-V splits widening multiply out into two instructions: one for the high bits and one for the low. Just like 64-bit ARM does.

    Integer MAC doesn't exist, and is also hindered by a design decision not to require more than two source operands, so as to allow simple implementations to stay simple. The same reason also prevents RISC-V from having a true conditional move instruction: there is one but the second operand is hard-coded zero.

    FMAC exists, but only because it is in the IEEE 754 spec ... and it requires significant op-code space.

hrmtst93837  2 months ago

[flagged]

  • CyberDildonics  2 months ago

    The things you are talking about are taken care of by out of order execution and the CPU itself being smart about how it executes. Putting in prefetch instructions rarely beats the actual prefetcher itself. Compilers didn't end up generating perfect pentium asm either. OOO execution is what changed the game in not needing perfect compiler output any more.

  • cogman10  2 months ago

    While true, it's typically not going to be impactful on system performance.

    There's a reason, for example, why the linux distros all target a generic x86 architecture rather than a specific architecture.