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

[Michael Platzer via llvm-dev]

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that it is outdated since support for the RISC-V V extension is now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 via llvm-dev]

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

--

Sam Elliott

Software Team Lead

Senior Software Developer - LLVM and OpenTitan

lowRISC CIC

[Michael Platzer via llvm-dev]

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