[PATCH] D156319: [RISCV] Relax ta/ma policy into tu/mu in InsertVSETVLI

[Wang Pengcheng via Phabricator via llvm-commits]

wangpc added subscribers: JojoR, zixuan-wu.
wangpc added a comment.

In D156319#4536353 <https://reviews.llvm.org/D156319#4536353>, @luke wrote:

> In D156319#4535747 <https://reviews.llvm.org/D156319#4535747>, @craig.topper wrote:
>
>> In D156319#4535006 <https://reviews.llvm.org/D156319#4535006>, @luke wrote:
>>
>>> This reduces the number of vsetvli toggles and improves code size at least, but I can't comment on the effect of undisturbed on register renaming etc.
>>
>> This can create false dependencies on out of order CPUs that will prevent instructions from being executed until the previous writer of their destination register has completed.
>
>
>
> In D156319#4535952 <https://reviews.llvm.org/D156319#4535952>, @wangpc wrote:
>
>> I like the idea, but as @craig.topper says, this can be negative for OoO processors.
>> What about making this feature a SubtargetFeature and then processors can enable it depending on their implementation?
>> (and there won't be huge changes in tests)
>
> I thought there might be a caveat as such, thanks for the explanation. I don't have any specific subtarget in mind for this, so I'm happy to remove it off of the review queue for now. We can always pick it up later if there's an in order processor that could utilise it

As far as I know, THead XuanTie C908 core ([1, 2, 3]) is in-order and there is no register renaming I think.
CC @zixuan-wu @JojoR to see if they are interested in this.

1. XuanTie C908: High-performance RISC-V Processor Catered to AIoT Industry <https://www.t-head.cn/product/%E7%8E%84%E9%93%81C908?spm=a2ouz.12986968.0.0.69212cbdEIxg0J>
2. Product Introduction of C908, Chinese <https://riscv.org/blog/2022/11/xuantie-c908-high-performance-risc-v-processor-catered-to-aiot-industry-chang-liu-alibaba-cloud/>
3. T-Head XuanTie C908 RISC-V core targets AIoT applications <https://www.cnx-software.com/2022/11/04/t-head-xuantie-c908-risc-v-core-targets-aiot-applications/>


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