Re: [PATCH] RISC-V: Remove unit-stride store from ta attribute

[Palmer Dabbelt <palmer@dabbelt.com>]

On Fri, 16 Dec 2022 12:01:04 PST (-0800), jeffreyalaw@gmail.com wrote:
>
>
> On 12/14/22 04:36, juzhe.zhong@rivai.ai wrote:
>> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
>>
>> Since store instructions doesn't care about tail policy, we remove
>> vste from "ta" attribute. Hence, we could have more fusion chances
>> and better optimization.
>>
>> gcc/ChangeLog:
>>
>>          * config/riscv/vector.md: Remove vste.
> Just to confirm that I understand the basic model.  Vector stores only
> update active elements, thus they don't care about tail policy, right?
>
> Assuming that's the case, then this is OK.

That had been my assumption as well, but I don't see that explicitly 
called out in the ISA manual.  I see

   Masked vector stores only update active memory elements.

where "active" is defined as

    * The _body_ elements are those whose element index is greater than or equal
    to the initial value in the `vstart` register, and less than the current
    vector length setting in `vl`. The body can be split into two disjoint subsets:

    ** The _active_ elements during a vector instruction's execution are the
    elements within the body and where the current mask is enabled at that element
    position.  The active elements can raise exceptions and update the destination
    vector register group.

but I don't see anything about the unmasked stores.  The blurb about 
tail elements only applies to registers groups, not memory addresses, so 
I think that's kind of a grey area there too.  I was pretty sure the intent
here was to have tail elements not updated in memory, so hopefully I'm just
missing something in the spec.

I open an issue to check: https://github.com/riscv/riscv-v-spec/issues/846