Re: Re: [PATCH] RISC-V: Basic VLS code gen for RISC-V

["juzhe.zhong@rivai.ai" <juzhe.zhong@rivai.ai>]

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In the future, we will definitely mixing VLA and VLS-vlmin together in a codegen and it will not cause any issues.
For VLS-vlmin, I prefer it is used in length style auto-vectorization (I am not sure since my SELECT_VL patch is not
finished, I will check if can work when I am working in SELECT_VL patch).

>> In general I don't have a good overview of which optimizations we gain by
>> such an approach or rather which ones are prevented by VLA altogether?
These patches VLS modes can help for SLP auto-vectorization.



juzhe.zhong@rivai.ai
 
From: Robin Dapp
Date: 2023-05-30 17:05
To: juzhe.zhong@rivai.ai; Richard Biener; Kito.cheng
CC: rdapp.gcc; gcc-patches; palmer; kito.cheng; jeffreyalaw; pan2.li
Subject: Re: [PATCH] RISC-V: Basic VLS code gen for RISC-V
>>> but ideally the user would be able to specify -mrvv-size=32 for an
>>> implementation with 32 byte vectors and then vector lowering would make use
>>> of vectors up to 32 bytes?
> 
> Actually, we don't want to specify -mrvv-size = 32 to enable vectorization on GNU vectors.
> You can take a look this example:
> https://godbolt.org/z/3jYqoM84h <https://godbolt.org/z/3jYqoM84h> 
> 
> GCC need to specify the mrvv size to enable GNU vectors and the codegen only can run on CPU with vector-length = 128bit.
> However, LLVM doesn't need to specify the vector length, and the codegen can run on any CPU with RVV  vector-length >= 128 bits.
> 
> This is what this patch want to do.
> 
> Thanks.
I think Richard's question was rather if it wasn't better to do it more
generically and lower vectors to what either the current cpu or what the
user specified rather than just 16-byte vectors (i.e. indeed a fixed
vlmin and not a fixed vlmin == fixed vlmax).
 
This patch assumes everything is fixed for optimization purposes and then
switches over to variable-length when nothing can be changed anymore.  That
is, we would work on "vlmin"-sized chunks in a VLA fashion at runtime?
We would need to make sure that no pass after reload makes use of VLA
properties at all.
 
In general I don't have a good overview of which optimizations we gain by
such an approach or rather which ones are prevented by VLA altogether?
What's the idea for the future?  Still use LEN_LOAD et al. (and masking)
with "fixed vlmin"?  Wouldn't we select different IVs with this patch than
what we would have for pure VLA?
 
Regards
Robin