[llvm-dev] [riscv] How do I use the RISC-V Vector extension instructions in LLVM IR?

Fri Nov 6 01:01:30 PST 2020

Hi Sam,

Thank you very much for your reply!

I checked out the patch from Roger's RFC. It provides initial support 
for IR intrinsics for the vector load, store and integer add 
instructions, as well as the necessary infrastructure to issue vsetvli 
instructions. Currently it generates assembly code where every vector 
instruction is preceded by a dedicated vsetvli to ensure that sew, lmul 
and vl have the correct values for the operation. The RFC mentions that 
a later pass should remove redundant vsetvli, however that is not part 
of the current patch.

The main reason why I was trying to use intrinsics for the RISC-V vector 
instructions was that I was hoping to use LLVM's instruction scheduler 
for vector instructions. So after applying the patch, I went ahead and 
added some basic scheduling definitions for the vector instruction class 
templates to RISCVSchedule.td and RISCVInstrInfoV.td and created a new 
SchedMachineModel for a simple in-order vector processor with separate 
vector LSU and vector ALU, for which all vector instructions occupy the 
respective unit for 4 cycles and have a latency of 7 cycles (see [1] for 
my patches).

I then wrote LLVM IR code with a series of vector load instructions, 
followed by a series of vector add and finally vector store instructions 
[2] and compiled it with:

llc -mtriple riscv32 -mattr=+experimental-v -O3 -enable-misched 
-enable-post-misched -mcpu=vectorproc-rv32 -o test.S test.ll

I was hoping that the instruction scheduler would interleave some of the 
vector load and add instructions in order to better utilize the two 
functional units of my vector processor model, but all vector 
instructions remained in the initial order [3].

Maybe the interleaved vsetvli instructions prevent the scheduler from 
changing the order of the vector instructions. Also, I am not sure if 
the scheduling definitions that I added are even remotely correct and 
whether this could ever work. I would be very grateful if you could take 
a short look at this and maybe point me in the right direction to get 
this working.

Thanks a lot,
Michael

[1] https://github.com/michael-platzer/llvm-project/tree/riscv-v-sched
[2] https://gist.github.com/michael-platzer/420d8fde744fa3d8476f0f591c72932b
[3] https://gist.github.com/michael-platzer/a37d4e42af3eda0b169fe5dd72da4921

On 10/29/20 8:19 PM, Sam Elliott wrote:
> Michael,
>
> Apologies for taking so long to get back to you.
>
> Trunk LLVM does not contain support for code generating the V 
> extension yet. The experimental-v support you see there is only 
> MC-layer support, where you can use the V instructions in assembly 
> (including inline assembly). There is currently (today) no way of 
> turning LLVM IR vector intrinsics into RISC-V V extension instructions.
>
> Hanna Kruppe did have a fork of LLVM where she and others were working 
> on prototype support for the V extension. The work on support for the 
> V extension has continued, but not via upstreaming the exact changes 
> in that prototype - instead there is an RFC about code generation 
> support: 
> http://lists.llvm.org/pipermail/llvm-dev/2020-October/145850.html which 
> includes links to proposed upstream patches implementing the RFC's 
> approach.
>
> I'm not sure the status of that RFC with respect to intrinsics for 
> operations like vsetvl - but it seems likely to me that such an 
> operation will be managed at a lower level than LLVM intrinsics. I 
> hope someone like Roger or Evandro can chime in with their intentions 
> with respect to this behaviour.
>
> Sam
>
>
>> On 29 Sep 2020, at 12:30 pm, Michael Platzer via llvm-dev 
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hi Everyone,
>>
>> I am wondering how to use RISC-V V (Vector) extension instructions in 
>> LLVM IR. In 2019 Kruppe and Espasa gave a talk [1] overviewing the 
>> Vector extension and on slide 16 [2] they show LLVM IR samples which 
>> use the vector instructions through intrinsic functions, such as:
>>
>> %vl = call i32 @llvm.riscv.vsetvl(i32 %n)
>>
>> At the time of the talk (April 2019) LLVM support for the V extension 
>> was developed out-of-tree at https://github.com/hanna-kruppe/rvv-llvm 
>> . However, that repository is archived now and the README file 
>> indicates that it is outdated since support for the RISC-V V 
>> extension is now developed upstream. I assume that this means that 
>> the features are now available from LLVM master.
>>
>> However, when I pull the current master and build it and try to 
>> compile the sample code with llc (specifying the target with 
>> --mtriple=riscv32-unkown-none-rv32imv ), I get following error:
>>
>> error: ../llvm-project/build/bin/llc: test.ll:4:18: error: use of 
>> undefined value '@llvm.riscv.vsetvl'
>>
>> It seems that the V extension is available, since `llc -march=riscv32 
>> -mattr=help` lists it:
>>
>> Available features for this target:
>>   ...
>>   experimental-v           - 'V' (Vector Instructions).
>>
>> Do I have to explicitly enable intrinsics for target features that 
>> are marked as experimental? Are these vector intrinsics shown in the 
>> slides even present in the upstream version? If yes, how do I use 
>> them? If no, how do I then use vector instructions in LLVM IR?
>>
>> Any hints would be greatly appreciated!
>>
>> Thank you,
>> Michael
>>
>> [1] 
>> https://llvm.org/devmtg/2019-04/slides/TechTalk-Kruppe-Espasa-RISC-V_Vectors_and_LLVM.pdf
>> [2] 
>> https://llvm.org/devmtg/2019-04/slides/TechTalk-Kruppe-Espasa-RISC-V_Vectors_and_LLVM.pdf#page=16
>>
>> P.S: I asked this question on StackOverflow first ( 
>> https://stackoverflow.com/q/64099125/1404847 ), but am now asking on 
>> this list since I did not get a reply.
>>
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>
> --
> Sam Elliott
> Software Team Lead
> Senior Software Developer - LLVM and OpenTitan
> lowRISC CIC
>
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