Question about loop unrolling in rte_ring datastructure.

[Aditya Ambadipudi <Aditya.Ambadipudi@arm.com>]

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Hello all.

My name is Aditya Ambadipudi. I am not the sharpest tool in the shed.

I was reading through the rte_ring datastructure. And I have two questions about the optimizations that are being made there.


  1.  Loop unrolling:
https://github.com/DPDK/dpdk/blob/main/lib/ring/rte_ring_elem_pvt.h#L28-L35
Why are we unrolling these loops manually. GCC will generate SIMD instructions for these loops automatically. Irrespective of wheither or not we unroll the loops

Unrolled loop: https://godbolt.org/z/n97noqYn7
Regular loop:https://godbolt.org/z/h6G9o9773

This is true of both x86 and ARM.

  2.  Normalizing to few fixed types:

It looks like we separate out enqueue/dequeue operations into 3 functions. One for each element size 32, 64, 128.

Again I am not clear on why we are doing this. Both 128 & 64 are multiples of 32. Why can't we just normalize everything to 32?

I feel like this is in some shape or form related to loop unrolling. But I am not able to figure it out on my own.

I am working on a patch that is closely related to this. And I would greatly appreciate any assistance anyone can provide on this.

Thank you,
Aditya Ambadipudi
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