OptimizeSharedMemory.cpp

Go to the documentation of this file.

//===- OptimizeSharedMemory.cpp - MLIR NVGPU pass implementation ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements transforms to optimize accesses to shared memory.
//
//===----------------------------------------------------------------------===//
 
#include "mlir/Dialect/NVGPU/Transforms/Passes.h"
 
#include "mlir/Dialect/Arith/IR/Arith.h"
#include "mlir/Dialect/GPU/IR/GPUDialect.h"
#include "mlir/Dialect/MemRef/IR/MemRef.h"
#include "mlir/Dialect/NVGPU/IR/NVGPUDialect.h"
#include "mlir/Dialect/NVGPU/Transforms/Transforms.h"
#include "mlir/Dialect/NVGPU/Transforms/Utils.h"
#include "mlir/Dialect/Vector/IR/VectorOps.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/MathExtras.h"
 
namespace mlir {
namespace nvgpu {
#define GEN_PASS_DEF_OPTIMIZESHAREDMEMORY
#include "mlir/Dialect/NVGPU/Transforms/Passes.h.inc"
} // namespace nvgpu
} // namespace mlir
 
using namespace mlir;
using namespace mlir::nvgpu;
 
/// The size of a shared memory line according to NV documentation.
constexpr int64_t kSharedMemoryLineSizeBytes = 128;
/// We optimize for 128bit accesses, but this can be made an argument in the
/// future.
constexpr int64_t kDefaultVectorSizeBits = 128;
 
/// Uses `srcIndexValue` to permute `tgtIndexValue` via
/// `result = xor(floordiv(srcIdxVal,permuteEveryN),
///               floordiv(tgtIdxVal,vectorSize)))
///            + tgtIdxVal % vectorSize`
/// This is done using an optimized sequence of `arith` operations.
static Value permuteVectorOffset(OpBuilder &b, Location loc,
                                 ArrayRef<Value> indices, MemRefType memrefTy,
                                 int64_t srcDim, int64_t tgtDim) {
  // Adjust the src index to change how often the permutation changes
  // if necessary.
  Value src = indices[srcDim];
 
  // We only want to permute every N iterations of the target dim where N is
  // ceil(sharedMemoryLineSizeBytes / dimSizeBytes(tgtDim)).
  const int64_t permuteEveryN = std::max<int64_t>(
      1, kSharedMemoryLineSizeBytes / ((memrefTy.getDimSize(tgtDim) *
                                        memrefTy.getElementTypeBitWidth()) /
                                       8));
 
  // clang-format off
  // Index bit representation (b0 = least significant bit) for dim(1)
  // of a `memref<?x?xDT>` is as follows:
  // N := log2(128/elementSizeBits)
  // M := log2(dimSize(1))
  // then
  // bits[0:N] = sub-vector element offset
  // bits[N:M] = vector index
  // clang-format on
  int64_t n =
      llvm::Log2_64(kDefaultVectorSizeBits / memrefTy.getElementTypeBitWidth());
  int64_t m = llvm::Log2_64(memrefTy.getDimSize(tgtDim));
 
  // Capture bits[0:(M-N)] of src by first creating a (M-N) mask.
  int64_t mask = (1LL << (m - n)) - 1;
  if (permuteEveryN > 1)
    mask = mask << llvm::Log2_64(permuteEveryN);
  Value srcBits = b.create<arith::ConstantIndexOp>(loc, mask);
  srcBits = b.create<arith::AndIOp>(loc, src, srcBits);
 
  // Use the src bits to permute the target bits b[N:M] containing the
  // vector offset.
  if (permuteEveryN > 1) {
    int64_t shlBits = n - llvm::Log2_64(permuteEveryN);
    if (shlBits > 0) {
      Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, shlBits);
      srcBits = b.createOrFold<arith::ShLIOp>(loc, srcBits, finalShiftVal);
    } else if (shlBits < 0) {
      Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, -1 * shlBits);
      srcBits = b.createOrFold<arith::ShRUIOp>(loc, srcBits, finalShiftVal);
    }
  } else {
    Value finalShiftVal = b.create<arith::ConstantIndexOp>(loc, n);
    srcBits = b.createOrFold<arith::ShLIOp>(loc, srcBits, finalShiftVal);
  }
 
  Value permutedVectorIdx =
      b.create<arith::XOrIOp>(loc, indices[tgtDim], srcBits);
  return permutedVectorIdx;
}
 
static void transformIndices(OpBuilder &builder, Location loc,
                             SmallVector<Value, 4> &indices,
                             MemRefType memrefTy, int64_t srcDim,
                             int64_t tgtDim) {
  indices[tgtDim] =
      permuteVectorOffset(builder, loc, indices, memrefTy, srcDim, tgtDim);
}
 
/// Return all operations within `parentOp` that read from or write to
/// `shmMemRef`.
static LogicalResult
getShmReadAndWriteOps(Operation *parentOp, Value shmMemRef,
                      SmallVector<Operation *, 16> &readOps,
                      SmallVector<Operation *, 16> &writeOps) {
  parentOp->walk([&](Operation *op) {
    MemoryEffectOpInterface iface = dyn_cast<MemoryEffectOpInterface>(op);
    if (!iface)
      return;
    std::optional<MemoryEffects::EffectInstance> effect =
        iface.getEffectOnValue<MemoryEffects::Read>(shmMemRef);
    if (effect) {
      readOps.push_back(op);
      return;
    }
    effect = iface.getEffectOnValue<MemoryEffects::Write>(shmMemRef);
    if (effect)
      writeOps.push_back(op);
  });
 
  // Restrict to a supported set of ops. We also require at least 2D access,
  // although this could be relaxed.
  if (llvm::any_of(readOps, [](Operation *op) {
        return !isa<memref::LoadOp, vector::LoadOp, nvgpu::LdMatrixOp>(op) ||
               getIndices(op).size() < 2;
      }))
    return failure();
  if (llvm::any_of(writeOps, [](Operation *op) {
        return !isa<memref::StoreOp, vector::StoreOp, nvgpu::DeviceAsyncCopyOp>(
                   op) ||
               getIndices(op).size() < 2;
      }))
    return failure();
 
  return success();
}
 
llvm::LogicalResult
mlir::nvgpu::optimizeSharedMemoryReadsAndWrites(Operation *parentOp,
                                                Value memrefValue) {
  auto memRefType = dyn_cast<MemRefType>(memrefValue.getType());
  if (!memRefType || !NVGPUDialect::hasSharedMemoryAddressSpace(memRefType))
    return failure();
 
  // Abort if the given value has any sub-views; we do not do any alias
  // analysis.
  bool hasSubView = false;
  parentOp->walk([&](memref::SubViewOp subView) { hasSubView = true; });
  if (hasSubView)
    return failure();
 
  // Check if this is necessary given the assumption of 128b accesses:
  // If dim[rank-1] is small enough to fit 8 rows in a 128B line.
  const int64_t rowSize = memRefType.getDimSize(memRefType.getRank() - 1);
  const int64_t rowsPerLine =
      (8 * kSharedMemoryLineSizeBytes / memRefType.getElementTypeBitWidth()) /
      rowSize;
  const int64_t threadGroupSize =
      1LL << (7 - llvm::Log2_64(kDefaultVectorSizeBits / 8));
  if (rowsPerLine >= threadGroupSize)
    return failure();
 
  // Get sets of operations within the function that read/write to shared
  // memory.
  SmallVector<Operation *, 16> shmReadOps;
  SmallVector<Operation *, 16> shmWriteOps;
  if (failed(getShmReadAndWriteOps(parentOp, memrefValue, shmReadOps,
                                   shmWriteOps)))
    return failure();
 
  if (shmReadOps.empty() || shmWriteOps.empty())
    return failure();
 
  OpBuilder builder(parentOp->getContext());
 
  int64_t tgtDim = memRefType.getRank() - 1;
  int64_t srcDim = memRefType.getRank() - 2;
 
  // Transform indices for the ops writing to shared memory.
  while (!shmWriteOps.empty()) {
    Operation *shmWriteOp = shmWriteOps.back();
    shmWriteOps.pop_back();
    builder.setInsertionPoint(shmWriteOp);
 
    auto indices = getIndices(shmWriteOp);
    SmallVector<Value, 4> transformedIndices(indices.begin(), indices.end());
    transformIndices(builder, shmWriteOp->getLoc(), transformedIndices,
                     memRefType, srcDim, tgtDim);
    setIndices(shmWriteOp, transformedIndices);
  }
 
  // Transform indices for the ops reading from shared memory.
  while (!shmReadOps.empty()) {
    Operation *shmReadOp = shmReadOps.back();
    shmReadOps.pop_back();
    builder.setInsertionPoint(shmReadOp);
 
    auto indices = getIndices(shmReadOp);
    SmallVector<Value, 4> transformedIndices(indices.begin(), indices.end());
    transformIndices(builder, shmReadOp->getLoc(), transformedIndices,
                     memRefType, srcDim, tgtDim);
    setIndices(shmReadOp, transformedIndices);
  }
 
  return success();
}
 
namespace {
class OptimizeSharedMemoryPass
    : public nvgpu::impl::OptimizeSharedMemoryBase<OptimizeSharedMemoryPass> {
public:
  OptimizeSharedMemoryPass() = default;
 
  void runOnOperation() override {
    Operation *op = getOperation();
    SmallVector<memref::AllocOp> shmAllocOps;
    op->walk([&](memref::AllocOp allocOp) {
      if (!NVGPUDialect::hasSharedMemoryAddressSpace(allocOp.getType()))
        return;
      shmAllocOps.push_back(allocOp);
    });
    for (auto allocOp : shmAllocOps) {
      if (failed(optimizeSharedMemoryReadsAndWrites(getOperation(),
                                                    allocOp.getMemref())))
        return;
    }
  }
};
} // namespace
 
std::unique_ptr<Pass> mlir::nvgpu::createOptimizeSharedMemoryPass() {
  return std::make_unique<OptimizeSharedMemoryPass>();
}