Comment by NetMageSCW
2 months ago
But that means a port of Linux can’t be to RISC-V, it has to be to a specific implementation of RISC-V, or if sufficient (which seems still debatable) to a specific common RISC-V profile.
2 months ago
But that means a port of Linux can’t be to RISC-V, it has to be to a specific implementation of RISC-V, or if sufficient (which seems still debatable) to a specific common RISC-V profile.
>which seems still debatable
In what way are RISC-V profiles debatable? Canonical is spearheading the RVA23-as-a-default movement and so far, it seems that there are no heavy objections towards that effort (beyond the usual "Canonical sucks" shtick that you see in every discussion involving Canonical)
You can target the minimum instruction set and it'll run everywhere. Albeit very slowly. Perhaps you use a fat binary to get reasonable performance in most cases.
This isn't easy but it can be done (and it is being done on x86, despite constantly evolving variations of AVX).
Interestingly, RISC-V vector extensions are variable length.
So, you can compile your RISC-V software to require the equivalent of AVX and it will run on whatever size vectors the hardwre supports.
So, on x86-64, if I write AVX2 software and run it on AVX512 capable hardware, I am leaving performance on the table. But if I write software that uses AVX512, it will not run on hardware that does not support those extensions (flags).
On RISC-V, the same binary that uses 256 bit vectors on hardware that only supports that will use 512 bit vectors on hardware that supports it, or even 1024 bit vectors on hardware like the A100 cores of the SpacemiT K3.
So, I guess X86-64 is is the RyanAir of processors.
(Personal opinion) I get the impression that RISC-V-related discussions often lack of awareness of prior work/alternatives. A large amount of (x86) software actually uses our Highway library to run on whatever size vectors and instructions the CPU offers.
This works quite well in practice. As to leaving performance on the table, it seems RVV has some egregious performance differences/cliffs. For example, should we use vrgather (with what LMUL), or interesting workarounds such as widening+slide1, to implement a basic operation such as interleaving two vectors?
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