CreateAsyncGroups.cpp

Go to the documentation of this file.

//===- CreateAsyncGroups.cpp - Create async device copies -----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
 
#include "mlir/Dialect/NVGPU/Transforms/Transforms.h"
 
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/Dialect/NVGPU/IR/NVGPUDialect.h"
#include "mlir/Dialect/NVGPU/Transforms/Utils.h"
#include "mlir/Dialect/Vector/IR/VectorOps.h"
#include "mlir/IR/BuiltinAttributes.h"
#include "mlir/IR/BuiltinTypes.h"
 
using namespace mlir;
 
/// Return "true" if the given vector transfer op is contiguous and suitable
/// for replacement with an async copy.
template <typename OpTy>
static bool isContiguousXferOp(OpTy op) {
  return op.getPermutationMap().isMinorIdentity() && op.isDimInBounds(0) &&
         op.hasPureBufferSemantics() &&
         cast<MemRefType>(nvgpu::getMemrefOperand(op).getType())
             .isLastDimUnitStride();
}
 
/// Return "true" if the given op is a contiguous and suitable
/// vector.transfer_write or vector.store op.
static bool isContiguousStore(Operation *write) {
  if (auto transferWrite = dyn_cast<vector::TransferWriteOp>(write))
    return isContiguousXferOp(transferWrite) && !transferWrite.getMask();
  // vector.store are always contiguous.
  return isa<vector::StoreOp>(write);
}
 
/// Return "true" if the given op is a contiguous and suitable
/// vector.transfer_read or vector.load op.
static bool isContiguousRead(Operation *read) {
  if (auto transferRead = dyn_cast<vector::TransferReadOp>(read))
    return isContiguousXferOp(transferRead);
  // vector.load are always contiguous.
  return isa<vector::LoadOp>(read);
}
 
namespace {
/// A vector.create_mask op and extract position.
struct TransferMask {
  vector::CreateMaskOp createMaskOp;
  SmallVector<int64_t> extractPosition;
};
} // namespace
 
/// If the given vector load op has a mask that is defined by
/// vector.create_mask, return that op.
static FailureOr<TransferMask> getMaskOp(Operation *loadOp) {
  auto transferRead = dyn_cast<vector::TransferReadOp>(loadOp);
  if (!transferRead || !transferRead.getMask())
    return TransferMask{{}, {}};
  assert(transferRead.getMask().getType().getRank() == 1 &&
         "expected 1-D mask");
 
  // Case 1: Mask is the result of a vector.create_mask.
  if (auto maskOp =
          transferRead.getMask().getDefiningOp<vector::CreateMaskOp>())
    return TransferMask{maskOp, {}};
 
  // Case 2: Mask is the result of a vector.extract(vector.create_mask).
  if (auto extractOp =
          transferRead.getMask().getDefiningOp<vector::ExtractOp>())
    if (auto maskOp =
            extractOp.getSource().getDefiningOp<vector::CreateMaskOp>())
      return TransferMask{maskOp,
                          SmallVector<int64_t>(extractOp.getStaticPosition())};
 
  // All other cases: not supported.
  return failure();
}
 
/// Build an SSA value that represents the number of read elements.
static Value buildNumReadElements(OpBuilder &b, Location loc,
                                  Operation *readOp) {
  FailureOr<TransferMask> transferMask = getMaskOp(readOp);
  assert(succeeded(transferMask) && "invalid transfer mask");
 
  // No mask => no num_read_elements.
  if (!transferMask->createMaskOp)
    return Value();
 
  // No extract: return size of "ones" segment in the mask.
  if (transferMask->extractPosition.empty()) {
    assert(transferMask->createMaskOp.getNumOperands() == 1 &&
           "expected single operand");
    return transferMask->createMaskOp.getOperand(0);
  }
 
  // vector.extract(vector.create_mask).
  // If extract_pos < num_ones, take number of elements from the least
  // significant dimension. (Do this for all dimensions and bit-AND the
  // conditions.)
  assert(transferMask->createMaskOp.getVectorType().getRank() -
                 transferMask->extractPosition.size() ==
             1 &&
         "expected N-D -> (N-1)-D extract");
  Value cond;
  // Note: There is one more `sz` than `pos`. The loop end with the last `pos`.
  for (auto [pos, sz] : llvm::zip(transferMask->extractPosition,
                                  transferMask->createMaskOp->getOperands())) {
    Value cmp =
        arith::CmpIOp::create(b, loc, arith::CmpIPredicate::slt,
                              arith::ConstantIndexOp::create(b, loc, pos), sz);
    if (!cond) {
      cond = cmp;
      continue;
    }
    cond = arith::AndIOp::create(b, loc, cmp, cond);
  }
  return arith::SelectOp::create(
      b, loc, cond, transferMask->createMaskOp->getOperands().back(),
      arith::ConstantIndexOp::create(b, loc, 0));
}
 
/// Return "true" if the conversion to async copy is supported by "async copy".
static bool resultsInSupportedAsyncCopy(MemRefType memrefType,
                                        VectorType vecType) {
  assert(vecType.getRank() == 1 && "expected 1-D vector");
  constexpr int64_t kSupportedCpAsyncAlignmentsInBytes[3] = {4, 8, 16};
 
  // Condition 1: the copy size must be supported.
  bool supportedCopySize = false;
  int64_t numElements = vecType.getNumElements();
  Type elementType = vecType.getElementType();
  for (int64_t alignmentInBytes : kSupportedCpAsyncAlignmentsInBytes) {
    if (alignmentInBytes * 8 ==
        numElements * elementType.getIntOrFloatBitWidth()) {
      supportedCopySize = true;
      break;
    }
  }
  if (!supportedCopySize)
    return false;
 
  // TODO: Condition 2: the alignments must be supported. For cp.async the
  // NVIDIA doc (section 6.4.1) says: "The address must be naturally aligned to
  // a multiple of the access size. If an address is not properly aligned, the
  // resulting behavior is undefined.".
  return true;
}
 
void nvgpu::createAsyncGroups(RewriterBase &rewriter, Operation *op,
                              bool bypassL1) {
  llvm::SmallSetVector<Operation *, 16> copyToSharedMem;
 
  // Look for all the copy that can be converted to async copy ops.
  op->walk([&](Operation *writeOp) {
    // Look for contiguous 1D vector store into shared memory.
    if (!isContiguousStore(writeOp))
      return;
    Value vectorVal = nvgpu::getValueStored(writeOp);
    if (cast<VectorType>(vectorVal.getType()).getRank() != 1)
      return;
    Value storeBase = nvgpu::getMemrefOperand(writeOp);
    if (!nvgpu::NVGPUDialect::hasSharedMemoryAddressSpace(
            cast<MemRefType>(storeBase.getType())))
      return;
 
    // The stored vector must originate from a contiguous 1D vector load.
    Operation *readOp = vectorVal.getDefiningOp();
    if (readOp == nullptr || !isContiguousRead(readOp))
      return;
    Value loadBase = nvgpu::getMemrefOperand(readOp);
    // Should be reading from global memory (not shared memory).
    if (nvgpu::NVGPUDialect::hasSharedMemoryAddressSpace(
            cast<MemRefType>(loadBase.getType())))
      return;
 
    // Look for compatible mask and padding.
    if (auto transferRead = dyn_cast<vector::TransferReadOp>(readOp)) {
      if (Value mask = transferRead.getMask()) {
        if (getConstantIntValue(transferRead.getPadding()) ==
            static_cast<int64_t>(0))
          return;
        if (failed(getMaskOp(readOp)))
          return;
      }
    }
 
    // Check whether both accesses are supported before we emit: this is
    // necessary to ensure the correctness of DeviceAsyncCopyOp.
    VectorType vecType = cast<VectorType>(vectorVal.getType());
 
    if (!resultsInSupportedAsyncCopy(cast<MemRefType>(loadBase.getType()),
                                     vecType) ||
        !resultsInSupportedAsyncCopy(cast<MemRefType>(storeBase.getType()),
                                     vecType))
      return;
 
    copyToSharedMem.insert(writeOp);
    return;
  });
 
  while (!copyToSharedMem.empty()) {
    // Start a group with the first write.
    SmallVector<Operation *> group;
    Operation *writeOp = *copyToSharedMem.begin();
    copyToSharedMem.remove(writeOp);
    group.push_back(writeOp);
    Operation *nextNode = writeOp;
 
    // Look in the next nodes for more copies to add to the same group.
    while ((nextNode = nextNode->getNextNode())) {
      // Ignore ops without side effects.
      auto memInterface = dyn_cast<MemoryEffectOpInterface>(nextNode);
      if (memInterface && memInterface.hasNoEffect() &&
          !nextNode->hasTrait<OpTrait::HasRecursiveMemoryEffects>())
        continue;
      // Ignore read from a different address space.
      if (isa<vector::TransferReadOp, vector::LoadOp>(nextNode)) {
        Operation *readOp = nextNode;
        Value memrefOperand = nvgpu::getMemrefOperand(readOp);
        if (!nvgpu::NVGPUDialect::hasSharedMemoryAddressSpace(
                cast<MemRefType>(memrefOperand.getType()))) {
          continue;
        }
      }
      if (copyToSharedMem.count(nextNode)) {
        // Found another copy, add it to the group.
        copyToSharedMem.remove(nextNode);
        group.push_back(nextNode);
        continue;
      }
      // If the op is something else stop the accumulating op in the group.
      break;
    }
 
    // Emit the group.
    SmallVector<Value> tokens;
    for (Operation *writeOp : group) {
      rewriter.setInsertionPoint(writeOp);
      Value vectorVal = nvgpu::getValueStored(writeOp);
      auto vectorType = cast<VectorType>(vectorVal.getType());
      int64_t numElements = vectorType.getNumElements();
      Operation *readOp = vectorVal.getDefiningOp();
      Value storeBase = nvgpu::getMemrefOperand(writeOp);
      Value loadBase = nvgpu::getMemrefOperand(readOp);
      Value numReadElements =
          buildNumReadElements(rewriter, writeOp->getLoc(), readOp);
      auto dstMemref = cast<MemRefType>(storeBase.getType());
      int64_t sizeInBytes =
          (dstMemref.getElementTypeBitWidth() * numElements) / 8;
      // bypass_l1 only possible with 16 byte transfer.
      Value token = nvgpu::DeviceAsyncCopyOp::create(
          rewriter, writeOp->getLoc(),
          nvgpu::DeviceAsyncTokenType::get(op->getContext()),
          /*dst=*/storeBase, /*dstIndices=*/nvgpu::getIndices(writeOp),
          /*src=*/loadBase,
          /*srcIndices=*/nvgpu::getIndices(readOp),
          /*dstElements=*/rewriter.getIndexAttr(numElements),
          /*srcElements=*/numReadElements,
          /*bypassL1=*/bypassL1 && sizeInBytes == 16 ? rewriter.getUnitAttr()
                                                     : UnitAttr());
      tokens.push_back(token);
    }
 
    // Create the group and wait for it right after.
    Value groupToken = nvgpu::DeviceAsyncCreateGroupOp::create(
        rewriter, op->getLoc(),
        nvgpu::DeviceAsyncTokenType::get(op->getContext()), tokens);
    nvgpu::DeviceAsyncWaitOp::create(rewriter, op->getLoc(), groupToken,
                                     nullptr);
    // Clean up old stores.
    for (Operation *writeOp : group)
      rewriter.eraseOp(writeOp);
  }
}