[Docs][RISCV] Document RISC-V vector codegen (llvm#96740)

This is a revival of https://reviews.llvm.org/D142348, and attempts to
document how RVV semantics can be expressed in LLVM IR as well as how
codegen works in the backend.

Parts of this are taken from the original RFC
https://lists.llvm.org/pipermail/llvm-dev/2020-October/145850.html, but
I've largely rewritten this from the original differential revision to
exclude explaining the specification itself and instead just focus on
the LLVM specific bits. (I figured that there's better material
available elsewhere for learning about RVV itself)

I've also updated it to include as much as I know about fixed vector
codegen as well as the recent changes to vsetvli insertion.

Author: lukel97

llvm/docs/RISCV/RISCVVectorExtension.rst

@@ -0,0 +1,334 @@
+=========================
+ RISC-V Vector Extension
+=========================
+
+.. contents::
+   :local:
+
+The RISC-V target supports the 1.0 version of the `RISC-V Vector Extension (RVV) <https://github.com/riscv/riscv-v-spec/blob/v1.0/v-spec.adoc>`_.
+This guide gives an overview of how it's modelled in LLVM IR and how the backend generates code for it.
+
+Mapping to LLVM IR types
+========================
+
+RVV adds 32 VLEN sized registers, where VLEN is an unknown constant to the compiler. To be able to represent VLEN sized values, the RISC-V backend takes the same approach as AArch64's SVE and uses `scalable vector types <https://llvm.org/docs/LangRef.html#t-vector>`_.
+
+Scalable vector types are of the form ``<vscale x n x ty>``, which indicates a vector with a multiple of ``n`` elements of type ``ty``.
+On RISC-V ``n`` and ``ty`` control LMUL and SEW respectively.
+
+LLVM only supports ELEN=32 or ELEN=64, so ``vscale`` is defined as VLEN/64 (see ``RISCV::RVVBitsPerBlock``).
+Note this means that VLEN must be at least 64, so VLEN=32 isn't currently supported.
+
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+|                   | LMUL=⅛        | LMUL=¼         | LMUL=½           | LMUL=1            | LMUL=2            | LMUL=4            | LMUL=8            |
++===================+===============+================+==================+===================+===================+===================+===================+
+| i64 (ELEN=64)     | N/A           | N/A            | N/A              | <v x 1 x i64>     | <v x 2 x i64>     | <v x 4 x i64>     | <v x 8 x i64>     |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| i32               | N/A           | N/A            | <v x 1 x i32>    | <v x 2 x i32>     | <v x 4 x i32>     | <v x 8 x i32>     | <v x 16 x i32>    |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| i16               | N/A           | <v x 1 x i16>  | <v x 2 x i16>    | <v x 4 x i16>     | <v x 8 x i16>     | <v x 16 x i16>    | <v x 32 x i16>    |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| i8                | <v x 1 x i8>  | <v x 2 x i8>   | <v x 4 x i8>     | <v x 8 x i8>      | <v x 16 x i8>     | <v x 32 x i8>     | <v x 64 x i8>     |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| double (ELEN=64)  | N/A           | N/A            | N/A              | <v x 1 x double>  | <v x 2 x double>  | <v x 4 x double>  | <v x 8 x double>  |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| float             | N/A           | N/A            | <v x 1 x float>  | <v x 2 x float>   | <v x 4 x float>   | <v x 8 x float>   | <v x 16 x float>  |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+| half              | N/A           | <v x 1 x half> | <v x 2 x half>   | <v x 4 x half>    | <v x 8 x half>    | <v x 16 x half>   | <v x 32 x half>   |
++-------------------+---------------+----------------+------------------+-------------------+-------------------+-------------------+-------------------+
+
+(Read ``<v x k x ty>`` as ``<vscale x k x ty>``)
+
+
+Mask vector types
+-----------------
+
+Mask vectors are physically represented using a layout of densely packed bits in a vector register.
+They are mapped to the following LLVM IR types:
+
+- ``<vscale x 1 x i1>``
+- ``<vscale x 2 x i1>``
+- ``<vscale x 4 x i1>``
+- ``<vscale x 8 x i1>``
+- ``<vscale x 16 x i1>``
+- ``<vscale x 32 x i1>``
+- ``<vscale x 64 x i1>``
+
+Two types with the same SEW/LMUL ratio will have the same related mask type.
+For instance, two different comparisons one under SEW=64, LMUL=2 and the other under SEW=32, LMUL=1 will both generate a mask ``<vscale x 2 x i1>``.
+
+Representation in LLVM IR
+=========================
+
+Vector instructions can be represented in three main ways in LLVM IR:
+
+1. Regular instructions on both scalable and fixed-length vector types
+
+   .. code-block:: llvm
+
+       %c = add <vscale x 4 x i32> %a, %b
+       %f = add <4 x i32> %d, %e
+
+2. RISC-V vector intrinsics, which mirror the `C intrinsics specification <https://github.com/riscv-non-isa/rvv-intrinsic-doc>`_
+
+   These come in unmasked variants:
+
+   .. code-block:: llvm
+
+       %c = call @llvm.riscv.vadd.nxv4i32.nxv4i32(
+              <vscale x 4 x i32> %passthru,
+	      <vscale x 4 x i32> %a,
+	      <vscale x 4 x i32> %b,
+	      i64 %avl
+	    )
+
+   As well as masked variants:
+
+   .. code-block:: llvm
+
+       %c = call @llvm.riscv.vadd.mask.nxv4i32.nxv4i32(
+              <vscale x 4 x i32> %passthru,
+	      <vscale x 4 x i32> %a,
+	      <vscale x 4 x i32> %b,
+	      <vscale x 4 x i1> %mask,
+	      i64 %avl,
+	      i64 0 ; policy (must be an immediate)
+	    )
+
+   Both allow setting the AVL as well as controlling the inactive/tail elements via the passthru operand, but the masked variant also provides operands for the mask and ``vta``/``vma`` policy bits.
+
+   The only valid types are scalable vector types.
+
+3. :ref:`Vector predication (VP) intrinsics <int_vp>`
+
+   .. code-block:: llvm
+
+       %c = call @llvm.vp.add.nxv4i32(
+	      <vscale x 4 x i32> %a,
+	      <vscale x 4 x i32> %b,
+	      <vscale x 4 x i1> %m
+	      i32 %evl
+	    )
+
+   Unlike RISC-V intrinsics, VP intrinsics are target agnostic so they can be emitted from other optimisation passes in the middle-end (like the loop vectorizer). They also support fixed-length vector types.
+
+   VP intrinsics also don't have passthru operands, but tail/mask undisturbed behaviour can be emulated by using the output in a ``@llvm.vp.merge``.
+   It will get lowered as a ``vmerge``, but will be merged back into the underlying instruction's mask via ``RISCVDAGToDAGISel::performCombineVMergeAndVOps``.
+
+
+The different properties of the above representations are summarized below:
+
++----------------------+--------------+-----------------+----------+------------------+----------------------+-----------------+
+|                      | AVL          | Masking         | Passthru | Scalable vectors | Fixed-length vectors | Target agnostic |
++======================+==============+=================+==========+==================+======================+=================+
+| LLVM IR instructions | Always VLMAX | No              | None     | Yes              | Yes                  | Yes             |
++----------------------+--------------+-----------------+----------+------------------+----------------------+-----------------+
+| RVV intrinsics       | Yes          | Yes             | Yes      | Yes              | No                   | No              |
++----------------------+--------------+-----------------+----------+------------------+----------------------+-----------------+
+| VP intrinsics        | Yes (EVL)    | Yes             | No       | Yes              | Yes                  | Yes             |
++----------------------+--------------+-----------------+----------+------------------+----------------------+-----------------+
+
+SelectionDAG lowering
+=====================
+
+For most regular **scalable** vector LLVM IR instructions, their corresponding SelectionDAG nodes are legal on RISC-V and don't require any custom lowering.
+
+.. code-block::
+
+   t5: nxv4i32 = add t2, t4
+
+RISC-V vector intrinsics also don't require any custom lowering.
+
+.. code-block::
+
+   t12: nxv4i32 = llvm.riscv.vadd TargetConstant:i64<10056>, undef:nxv4i32, t2, t4, t6
+
+Fixed-length vectors
+--------------------
+
+Because there are no fixed-length vector patterns, fixed-length vectors need to be custom lowered and performed in a scalable "container" type:
+
+1. The fixed-length vector operands are inserted into scalable containers with ``insert_subvector`` nodes. The container type is chosen such that its minimum size will fit the fixed-length vector (see ``getContainerForFixedLengthVector``).
+2. The operation is then performed on the container type via a **VL (vector length) node**. These are custom nodes defined in ``RISCVInstrInfoVVLPatterns.td`` that mirror target agnostic SelectionDAG nodes, as well as some RVV instructions. They contain an AVL operand, which is set to the number of elements in the fixed-length vector.
+   Some nodes also have a passthru or mask operand, which will usually be set to ``undef`` and all ones when lowering fixed-length vectors.
+3. The result is put back into a fixed-length vector via ``extract_subvector``.
+
+.. code-block::
+
+       t2: nxv2i32,ch = CopyFromReg t0, Register:nxv2i32 %0
+       t6: nxv2i32,ch = CopyFromReg t0, Register:nxv2i32 %1
+     t4: v4i32 = extract_subvector t2, Constant:i64<0>
+     t7: v4i32 = extract_subvector t6, Constant:i64<0>
+   t8: v4i32 = add t4, t7
+
+   // is custom lowered to:
+
+       t2: nxv2i32,ch = CopyFromReg t0, Register:nxv2i32 %0
+       t6: nxv2i32,ch = CopyFromReg t0, Register:nxv2i32 %1
+       t15: nxv2i1 = RISCVISD::VMSET_VL Constant:i64<4>
+     t16: nxv2i32 = RISCVISD::ADD_VL t2, t6, undef:nxv2i32, t15, Constant:i64<4>
+   t17: v4i32 = extract_subvector t16, Constant:i64<0>
+
+VL nodes often have a passthru or mask operand, which are usually set to ``undef`` and all ones for fixed-length vectors.
+
+The ``insert_subvector`` and ``extract_subvector`` nodes responsible for wrapping and unwrapping will get combined away, and eventually we will lower all fixed-length vector types to scalable. Note that fixed-length vectors at the interface of a function are passed in a scalable vector container.
+
+.. note::
+
+   The only ``insert_subvector`` and ``extract_subvector`` nodes that make it through lowering are those that can be performed as an exact subregister insert or extract. This means that any fixed-length vector ``insert_subvector`` and ``extract_subvector`` nodes that aren't legalized must lie on a register group boundary, so the exact VLEN must be known at compile time (i.e., compiled with ``-mrvv-vector-bits=zvl`` or ``-mllvm -riscv-v-vector-bits-max=VLEN``, or have an exact ``vscale_range`` attribute).
+
+Vector predication intrinsics
+-----------------------------
+
+VP intrinsics also get custom lowered via VL nodes.
+
+.. code-block::
+
+   t12: nxv2i32 = vp_add t2, t4, t6, Constant:i64<8>
+
+   // is custom lowered to:
+
+   t18: nxv2i32 = RISCVISD::ADD_VL t2, t4, undef:nxv2i32, t6, Constant:i64<8>
+
+The VP EVL and mask are used for the VL node's AVL and mask respectively, whilst the passthru is set to ``undef``.
+
+Instruction selection
+=====================
+
+``vl`` and ``vtype`` need to be configured correctly, so we can't just directly select the underlying vector ``MachineInstr``. Instead pseudo instructions are selected, which carry the extra information needed to emit the necessary ``vsetvli``\s later.
+
+.. code-block::
+
+   %c:vrm2 = PseudoVADD_VV_M2 %passthru:vrm2(tied-def 0), %a:vrm2, %b:vrm2, %vl:gpr, 5 /*sew*/, 3 /*policy*/
+
+Each vector instruction has multiple pseudo instructions defined in ``RISCVInstrInfoVPseudos.td``.
+There is a variant of each pseudo for each possible LMUL, as well as a masked variant. So a typical instruction like ``vadd.vv`` would have the following pseudos:
+
+.. code-block::
+
+   %rd:vr = PseudoVADD_VV_MF8 %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_MF4 %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_MF2 %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_M1 %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm2 = PseudoVADD_VV_M2 %passthru:vrm2(tied-def 0), %rs2:vrm2, %rs1:vrm2, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm4 = PseudoVADD_VV_M4 %passthru:vrm4(tied-def 0), %rs2:vrm4, %rs1:vrm4, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm8 = PseudoVADD_VV_M8 %passthru:vrm8(tied-def 0), %rs2:vrm8, %rs1:vrm8, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_MF8_MASK %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_MF4_MASK %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_MF2_MASK %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vr = PseudoVADD_VV_M1_MASK %passthru:vr(tied-def 0), %rs2:vr, %rs1:vr, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm2 = PseudoVADD_VV_M2_MASK %passthru:vrm2(tied-def 0), %rs2:vrm2, %%rs1:vrm2, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm4 = PseudoVADD_VV_M4_MASK %passthru:vrm4(tied-def 0), %rs2:vrm4, %rs1:vrm4, mask:$v0, %avl:gpr, sew:imm, policy:imm
+   %rd:vrm8 = PseudoVADD_VV_M8_MASK %passthru:vrm8(tied-def 0), %rs2:vrm8, %rs1:vrm8, mask:$v0, %avl:gpr, sew:imm, policy:imm
+
+.. note::
+
+   Whilst the SEW can be encoded in an operand, we need to use separate pseudos for each LMUL since different register groups will require different register classes: see :ref:`rvv_register_allocation`.
+
+
+Pseudos have operands for the AVL and SEW (encoded as a power of 2), as well as potentially the mask, policy or rounding mode if applicable.
+The passthru operand is tied to the destination register which will determine the inactive/tail elements.
+
+For scalable vectors that should use VLMAX, the AVL is set to a sentinel value of ``-1``.
+
+There are patterns for target agnostic SelectionDAG nodes in ``RISCVInstrInfoVSDPatterns.td``, VL nodes in ``RISCVInstrInfoVVLPatterns.td`` and RVV intrinsics in ``RISCVInstrInfoVPseudos.td``.
+
+Mask patterns
+-------------
+
+For masked pseudos the mask operand is copied to the physical ``$v0`` register during instruction selection with a glued ``CopyToReg`` node:
+
+.. code-block::
+
+     t23: ch,glue = CopyToReg t0, Register:nxv4i1 $v0, t6
+   t25: nxv4i32 = PseudoVADD_VV_M2_MASK Register:nxv4i32 $noreg, t2, t4, Register:nxv4i1 $v0, TargetConstant:i64<8>, TargetConstant:i64<5>, TargetConstant:i64<1>, t23:1
+
+The patterns in ``RISCVInstrInfoVVLPatterns.td`` only match masked pseudos to reduce the size of the match table, even if the node's mask is all ones and could be an unmasked pseudo.
+``RISCVFoldMasks::convertToUnmasked`` will detect if the mask is all ones and convert it into its unmasked form.
+
+.. code-block::
+
+   $v0 = PseudoVMSET_M_B16 -1, 32
+   %rd:vrm2 = PseudoVADD_VV_M2_MASK %passthru:vrm2(tied-def 0), %rs2:vrm2, %rs1:vrm2, $v0, %avl:gpr, sew:imm, policy:imm
+
+   // gets optimized to:
+
+   %rd:vrm2 = PseudoVADD_VV_M2 %passthru:vrm2(tied-def 0), %rs2:vrm2, %rs1:vrm2, %avl:gpr, sew:imm, policy:imm
+
+.. note::
+
+   Any ``vmset.m`` can be treated as an all ones mask since the tail elements past AVL are ``undef`` and can be replaced with ones.
+
+.. _rvv_register_allocation:
+
+Register allocation
+===================
+
+Register allocation is split between vector and scalar registers, with vector allocation running first:
+
+.. code-block::
+
+  $v8m2 = PseudoVADD_VV_M2 $v8m2(tied-def 0), $v8m2, $v10m2, %vl:gpr, 5, 3
+
+.. note::
+
+   Register allocation is split so that :ref:`RISCVInsertVSETVLI` can run after vector register allocation, but before scalar register allocation. It needs to be run before scalar register allocation as it may need to create a new virtual register to set the AVL to VLMAX.
+
+   Performing ``RISCVInsertVSETVLI`` after vector register allocation imposes fewer constraints on the machine scheduler since it cannot schedule instructions past ``vsetvli``\s, and it allows us to emit further vector pseudos during spilling or constant rematerialization.
+
+There are four register classes for vectors:
+
+- ``VR`` for vector registers (``v0``, ``v1,``, ..., ``v32``). Used when :math:`\text{LMUL} \leq 1` and mask registers.
+- ``VRM2`` for vector groups of length 2 i.e., :math:`\text{LMUL}=2` (``v0m2``, ``v2m2``, ..., ``v30m2``)
+- ``VRM4`` for vector groups of length 4 i.e., :math:`\text{LMUL}=4` (``v0m4``, ``v4m4``, ..., ``v28m4``)
+- ``VRM8`` for vector groups of length 8 i.e., :math:`\text{LMUL}=8` (``v0m8``, ``v8m8``, ..., ``v24m8``)
+
+:math:`\text{LMUL} \lt 1` types and mask types do not benefit from having a dedicated class, so ``VR`` is used in their case.
+
+Some instructions have a constraint that a register operand cannot be ``V0`` or overlap with ``V0``, so for these cases we also have ``VRNoV0`` variants.
+
+.. _RISCVInsertVSETVLI:
+
+RISCVInsertVSETVLI
+==================
+
+After vector registers are allocated, the ``RISCVInsertVSETVLI`` pass will insert the necessary ``vsetvli``\s for the pseudos.
+
+.. code-block::
+
+  dead $x0 = PseudoVSETVLI %vl:gpr, 209, implicit-def $vl, implicit-def $vtype
+  $v8m2 = PseudoVADD_VV_M2 $v8m2(tied-def 0), $v8m2, $v10m2, $noreg, 5, implicit $vl, implicit $vtype
+
+The physical ``$vl`` and ``$vtype`` registers are implicitly defined by the ``PseudoVSETVLI``, and are implicitly used by the ``PseudoVADD``.
+The ``vtype`` operand (``209`` in this example) is encoded as per the specification via ``RISCVVType::encodeVTYPE``.
+
+``RISCVInsertVSETVLI`` performs dataflow analysis to emit as few ``vsetvli``\s as possible. It will also try to minimize the number of ``vsetvli``\s that set VL, i.e., it will emit ``vsetvli x0, x0`` if only ``vtype`` needs changed but ``vl`` doesn't.
+
+Pseudo expansion and printing
+=============================
+
+After scalar register allocation, the ``RISCVExpandPseudoInsts.cpp`` pass expands the ``PseudoVSETVLI`` instructions.
+
+.. code-block::
+
+   dead $x0 = VSETVLI $x1, 209, implicit-def $vtype, implicit-def $vl
+   renamable $v8m2 = PseudoVADD_VV_M2 $v8m2(tied-def 0), $v8m2, $v10m2, $noreg, 5, implicit $vl, implicit $vtype
+
+Note that the vector pseudo remains as it's needed to encode the register class for the LMUL. Its AVL and SEW operands are no longer used.
+
+``RISCVAsmPrinter`` will then lower the pseudo instructions into real ``MCInst``\s.
+
+.. code-block:: nasm
+
+   vsetvli a0, zero, e32, m2, ta, ma
+   vadd.vv v8, v8, v10
+
+
+
+See also
+========
+
+- `[llvm-dev] [RFC] Code generation for RISC-V V-extension <https://lists.llvm.org/pipermail/llvm-dev/2020-October/145850.html>`_
+- `2023 LLVM Dev Mtg - Vector codegen in the RISC-V backend <https://youtu.be/-ox8iJmbp0c?feature=shared>`_
+- `2023 LLVM Dev Mtg - How to add an C intrinsic and code-gen it, using the RISC-V vector C intrinsics <https://youtu.be/t17O_bU1jks?feature=shared>`_
+- `2021 LLVM Dev Mtg “Optimizing code for scalable vector architectures” <https://youtu.be/daWLCyhwrZ8?feature=shared>`_

llvm/docs/UserGuides.rst

@@ -64,6 +64,7 @@ intermediate LLVM representation.
    Remarks
    RemoveDIsDebugInfo
    RISCVUsage
+   RISCV/RISCVVectorExtension
    SourceLevelDebugging
    SPIRVUsage
    StackSafetyAnalysis
@@ -284,3 +285,6 @@ Additional Topics
 
 :doc:`RISCVUsage`
    This document describes using the RISCV-V target.
+
+:doc:`RISCV/RISCVVectorExtension`
+   This document describes how the RISC-V Vector extension can be expressed in LLVM IR and how code is generated for it in the backend.