mlir/examples/toy/Ch6/mlir/LowerToLLVM.cpp

//====- LowerToLLVM.cpp - Lowering from Toy+Affine+Std to LLVM ------------===//
//
// 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 full lowering of Toy operations to LLVM MLIR dialect.
// 'toy.print' is lowered to a loop nest that calls `printf` on each element of
// the input array. The file also sets up the ToyToLLVMLoweringPass. This pass
// lowers the combination of Affine + SCF + Standard dialects to the LLVM one:
//
//                         Affine --
//                                  |
//                                  v
//                                  Standard --> LLVM (Dialect)
//                                  ^
//                                  |
//     'toy.print' --> Loop (SCF) --
//
//===----------------------------------------------------------------------===//

#include "toy/Dialect.h"
#include "toy/Passes.h"

#include "mlir/Conversion/AffineToStandard/AffineToStandard.h"
#include "mlir/Conversion/SCFToStandard/SCFToStandard.h"
#include "mlir/Conversion/StandardToLLVM/ConvertStandardToLLVM.h"
#include "mlir/Conversion/StandardToLLVM/ConvertStandardToLLVMPass.h"
#include "mlir/Dialect/Affine/IR/AffineOps.h"
#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
#include "mlir/Dialect/SCF/SCF.h"
#include "mlir/Dialect/StandardOps/IR/Ops.h"
#include "mlir/Pass/Pass.h"
#include "mlir/Transforms/DialectConversion.h"
#include "llvm/ADT/Sequence.h"

using namespace mlir;

//===----------------------------------------------------------------------===//
// ToyToLLVM RewritePatterns
//===----------------------------------------------------------------------===//

namespace {
/// Lowers `toy.print` to a loop nest calling `printf` on each of the individual
/// elements of the array.
class PrintOpLowering : public ConversionPattern {
public:
  explicit PrintOpLowering(MLIRContext *context)
      : ConversionPattern(toy::PrintOp::getOperationName(), 1, context) {}

  LogicalResult
  matchAndRewrite(Operation *op, ArrayRef<Value> operands,
                  ConversionPatternRewriter &rewriter) const override {
    auto memRefType = (*op->operand_type_begin()).cast<MemRefType>();
    auto memRefShape = memRefType.getShape();
    auto loc = op->getLoc();

    ModuleOp parentModule = op->getParentOfType<ModuleOp>();

    // Get a symbol reference to the printf function, inserting it if necessary.
    auto printfRef = getOrInsertPrintf(rewriter, parentModule);
    Value formatSpecifierCst = getOrCreateGlobalString(
        loc, rewriter, "frmt_spec", StringRef("%f \0", 4), parentModule);
    Value newLineCst = getOrCreateGlobalString(
        loc, rewriter, "nl", StringRef("\n\0", 2), parentModule);

    // Create a loop for each of the dimensions within the shape.
    SmallVector<Value, 4> loopIvs;
    for (unsigned i = 0, e = memRefShape.size(); i != e; ++i) {
      auto lowerBound = rewriter.create<ConstantIndexOp>(loc, 0);
      auto upperBound = rewriter.create<ConstantIndexOp>(loc, memRefShape[i]);
      auto step = rewriter.create<ConstantIndexOp>(loc, 1);
      auto loop =
          rewriter.create<scf::ForOp>(loc, lowerBound, upperBound, step);
      for (Operation &nested : *loop.getBody())
        rewriter.eraseOp(&nested);
      loopIvs.push_back(loop.getInductionVar());

      // Terminate the loop body.
      rewriter.setInsertionPointToEnd(loop.getBody());

      // Insert a newline after each of the inner dimensions of the shape.
      if (i != e - 1)
        rewriter.create<CallOp>(loc, printfRef, rewriter.getIntegerType(32),
                                newLineCst);
      rewriter.create<scf::YieldOp>(loc);
      rewriter.setInsertionPointToStart(loop.getBody());
    }

    // Generate a call to printf for the current element of the loop.
    auto printOp = cast<toy::PrintOp>(op);
    auto elementLoad = rewriter.create<LoadOp>(loc, printOp.input(), loopIvs);
    rewriter.create<CallOp>(loc, printfRef, rewriter.getIntegerType(32),
                            ArrayRef<Value>({formatSpecifierCst, elementLoad}));

    // Notify the rewriter that this operation has been removed.
    rewriter.eraseOp(op);
    return success();
  }

private:
  /// Return a symbol reference to the printf function, inserting it into the
  /// module if necessary.
  static FlatSymbolRefAttr getOrInsertPrintf(PatternRewriter &rewriter,
                                             ModuleOp module) {
    auto *context = module.getContext();
    if (module.lookupSymbol<LLVM::LLVMFuncOp>("printf"))
      return SymbolRefAttr::get("printf", context);

    // Create a function declaration for printf, the signature is:
    //   * `i32 (i8*, ...)`
    auto llvmI32Ty = IntegerType::get(context, 32);
    auto llvmI8PtrTy = LLVM::LLVMPointerType::get(IntegerType::get(context, 8));
    auto llvmFnType = LLVM::LLVMFunctionType::get(llvmI32Ty, llvmI8PtrTy,
                                                  /*isVarArg=*/true);

    // Insert the printf function into the body of the parent module.
    PatternRewriter::InsertionGuard insertGuard(rewriter);
    rewriter.setInsertionPointToStart(module.getBody());
    rewriter.create<LLVM::LLVMFuncOp>(module.getLoc(), "printf", llvmFnType);
    return SymbolRefAttr::get("printf", context);
  }

  /// Return a value representing an access into a global string with the given
  /// name, creating the string if necessary.
  static Value getOrCreateGlobalString(Location loc, OpBuilder &builder,
                                       StringRef name, StringRef value,
                                       ModuleOp module) {
    // Create the global at the entry of the module.
    LLVM::GlobalOp global;
    if (!(global = module.lookupSymbol<LLVM::GlobalOp>(name))) {
      OpBuilder::InsertionGuard insertGuard(builder);
      builder.setInsertionPointToStart(module.getBody());
      auto type = LLVM::LLVMArrayType::get(
          IntegerType::get(builder.getContext(), 8), value.size());
      global = builder.create<LLVM::GlobalOp>(loc, type, /*isConstant=*/true,
                                              LLVM::Linkage::Internal, name,
                                              builder.getStringAttr(value));
    }

    // Get the pointer to the first character in the global string.
    Value globalPtr = builder.create<LLVM::AddressOfOp>(loc, global);
    Value cst0 = builder.create<LLVM::ConstantOp>(
        loc, IntegerType::get(builder.getContext(), 64),
        builder.getIntegerAttr(builder.getIndexType(), 0));
    return builder.create<LLVM::GEPOp>(
        loc,
        LLVM::LLVMPointerType::get(IntegerType::get(builder.getContext(), 8)),
        globalPtr, ArrayRef<Value>({cst0, cst0}));
  }
};
} // end anonymous namespace

//===----------------------------------------------------------------------===//
// ToyToLLVMLoweringPass
//===----------------------------------------------------------------------===//

namespace {
struct ToyToLLVMLoweringPass
    : public PassWrapper<ToyToLLVMLoweringPass, OperationPass<ModuleOp>> {
  void getDependentDialects(DialectRegistry &registry) const override {
    registry.insert<LLVM::LLVMDialect, scf::SCFDialect>();
  }
  void runOnOperation() final;
};
} // end anonymous namespace

void ToyToLLVMLoweringPass::runOnOperation() {
  // The first thing to define is the conversion target. This will define the
  // final target for this lowering. For this lowering, we are only targeting
  // the LLVM dialect.
  LLVMConversionTarget target(getContext());
  target.addLegalOp<ModuleOp, ModuleTerminatorOp>();

  // During this lowering, we will also be lowering the MemRef types, that are
  // currently being operated on, to a representation in LLVM. To perform this
  // conversion we use a TypeConverter as part of the lowering. This converter
  // details how one type maps to another. This is necessary now that we will be
  // doing more complicated lowerings, involving loop region arguments.
  LLVMTypeConverter typeConverter(&getContext());

  // Now that the conversion target has been defined, we need to provide the
  // patterns used for lowering. At this point of the compilation process, we
  // have a combination of `toy`, `affine`, and `std` operations. Luckily, there
  // are already exists a set of patterns to transform `affine` and `std`
  // dialects. These patterns lowering in multiple stages, relying on transitive
  // lowerings. Transitive lowering, or A->B->C lowering, is when multiple
  // patterns must be applied to fully transform an illegal operation into a
  // set of legal ones.
  OwningRewritePatternList patterns;
  populateAffineToStdConversionPatterns(patterns, &getContext());
  populateLoopToStdConversionPatterns(patterns, &getContext());
  populateStdToLLVMConversionPatterns(typeConverter, patterns);

  // The only remaining operation to lower from the `toy` dialect, is the
  // PrintOp.
  patterns.insert<PrintOpLowering>(&getContext());

  // We want to completely lower to LLVM, so we use a `FullConversion`. This
  // ensures that only legal operations will remain after the conversion.
  auto module = getOperation();
  if (failed(applyFullConversion(module, target, std::move(patterns))))
    signalPassFailure();
}

/// Create a pass for lowering operations the remaining `Toy` operations, as
/// well as `Affine` and `Std`, to the LLVM dialect for codegen.
std::unique_ptr<mlir::Pass> mlir::toy::createLowerToLLVMPass() {
  return std::make_unique<ToyToLLVMLoweringPass>();
}