LoopGenerators.h

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//===- LoopGenerators.h - IR helper to create loops -------------*- C++ -*-===//
//
// 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 contains functions to create scalar and OpenMP parallel loops
// as LLVM-IR.
//
//===----------------------------------------------------------------------===//
#ifndef POLLY_LOOP_GENERATORS_H
#define POLLY_LOOP_GENERATORS_H
 
#include "polly/CodeGen/IRBuilder.h"
#include "polly/Support/ScopHelper.h"
#include "llvm/ADT/SetVector.h"
 
namespace polly {
using llvm::AllocaInst;
using llvm::BasicBlock;
using llvm::DataLayout;
using llvm::DominatorTree;
using llvm::Function;
using llvm::ICmpInst;
using llvm::LoopInfo;
using llvm::Module;
using llvm::SetVector;
using llvm::Type;
using llvm::Value;
 
/// General scheduling types of parallel OpenMP for loops.
/// Initialization values taken from OpenMP's enum in kmp.h: sched_type.
/// Currently, only 'static' scheduling may change from chunked to non-chunked.
enum class OMPGeneralSchedulingType {
  StaticChunked = 33,
  StaticNonChunked = 34,
  Dynamic = 35,
  Guided = 36,
  Runtime = 37
};
 
extern int PollyNumThreads;
extern OMPGeneralSchedulingType PollyScheduling;
extern int PollyChunkSize;
 
/// Create a scalar do/for-style loop.
///
/// @param LowerBound         The starting value of the induction variable.
/// @param UpperBound         The upper bound of the induction variable.
/// @param Stride             The value by which the induction variable
///                           is incremented.
///
/// @param Builder            The builder used to create the loop.
/// @param P                  A pointer to the pass that uses this function.
///                           It is used to update analysis information.
/// @param LI                 The loop info we need to update
/// @param DT                 The dominator tree we need to update
/// @param ExitBlock          The block the loop will exit to.
/// @param Predicate          The predicate used to generate the upper loop
///                           bound.
/// @param Annotator          This function can (optionally) take
///                           a ScopAnnotator which
///                           annotates loops and alias information in the SCoP.
/// @param Parallel           If this loop should be marked parallel in
///                           the Annotator.
/// @param UseGuard           Create a guard in front of the header to check if
///                           the loop is executed at least once, otherwise just
///                           assume it.
/// @param LoopVectDisabled   If the Loop vectorizer should be disabled for this
///                           loop.
///
/// @return Value*    The newly created induction variable for this loop.
Value *createLoop(Value *LowerBound, Value *UpperBound, Value *Stride,
                  PollyIRBuilder &Builder, LoopInfo &LI, DominatorTree &DT,
                  BasicBlock *&ExitBlock, ICmpInst::Predicate Predicate,
                  ScopAnnotator *Annotator = nullptr, bool Parallel = false,
                  bool UseGuard = true, bool LoopVectDisabled = false);
 
/// Create a DebugLoc representing generated instructions.
///
/// The IR verifier requires !dbg metadata to be set in some situations. For
/// instance, if an (inlinable) function has debug info, all its call site must
/// have debug info as well.
llvm::DebugLoc createDebugLocForGeneratedCode(Function *F);
 
/// The ParallelLoopGenerator allows to create parallelized loops
///
/// To parallelize a loop, we perform the following steps:
///   o  Generate a subfunction which will hold the loop body.
///   o  Create a struct to hold all outer values needed in the loop body.
///   o  Create calls to a runtime library to achieve the actual parallelism.
///      These calls will spawn and join threads, define how the work (here the
///      iterations) are distributed between them and make sure each has access
///      to the struct holding all needed values.
///
/// At the moment we support only one parallel runtime, OpenMP.
///
/// If we parallelize the outer loop of the following loop nest,
///
///   S0;
///   for (int i = 0; i < N; i++)
///     for (int j = 0; j < M; j++)
///       S1(i, j);
///   S2;
///
/// we will generate the following code (with different runtime function names):
///
///   S0;
///   auto *values = storeValuesIntoStruct();
///   // Execute subfunction with multiple threads
///   spawn_threads(subfunction, values);
///   join_threads();
///   S2;
///
///  // This function is executed in parallel by different threads
///   void subfunction(values) {
///     while (auto *WorkItem = getWorkItem()) {
///       int LB = WorkItem.begin();
///       int UB = WorkItem.end();
///       for (int i = LB; i < UB; i++)
///         for (int j = 0; j < M; j++)
///           S1(i, j);
///     }
///     cleanup_thread();
///   }
class ParallelLoopGenerator {
public:
  /// Create a parallel loop generator for the current function.
  ParallelLoopGenerator(PollyIRBuilder &Builder, const DataLayout &DL)
      : Builder(Builder), LongType(Type::getIntNTy(Builder.getContext(),
                                                   DL.getPointerSizeInBits())),
        M(Builder.GetInsertBlock()->getParent()->getParent()),
        DLGenerated(createDebugLocForGeneratedCode(
            Builder.GetInsertBlock()->getParent())) {}
 
  virtual ~ParallelLoopGenerator() {}
 
  /// Create a parallel loop.
  ///
  /// This function is the main function to automatically generate a parallel
  /// loop with all its components.
  ///
  /// @param LB        The lower bound for the loop we parallelize.
  /// @param UB        The upper bound for the loop we parallelize.
  /// @param Stride    The stride of the loop we parallelize.
  /// @param Values    A set of LLVM-IR Values that should be available in
  ///                  the new loop body.
  /// @param VMap      A map to allow outside access to the new versions of
  ///                  the values in @p Values.
  /// @param LoopBody  A pointer to an iterator that is set to point to the
  ///                  body of the created loop. It should be used to insert
  ///                  instructions that form the actual loop body.
  ///
  /// @return The newly created induction variable for this loop.
  Value *createParallelLoop(Value *LB, Value *UB, Value *Stride,
                            SetVector<Value *> &Values, ValueMapT &VMap,
                            BasicBlock::iterator *LoopBody);
 
protected:
  /// The IR builder we use to create instructions.
  PollyIRBuilder &Builder;
 
  /// The loop info for the generated subfunction.
  std::unique_ptr<LoopInfo> SubFnLI;
 
  /// The dominance tree for the generated subfunction.
  std::unique_ptr<DominatorTree> SubFnDT;
 
  /// The type of a "long" on this hardware used for backend calls.
  Type *LongType;
 
  /// The current module
  Module *M;
 
  /// Debug location for generated code without direct link to any specific
  /// line.
  ///
  /// We only set the DebugLoc where the IR Verifier requires us to. Otherwise,
  /// absent debug location for optimized code should be fine.
  llvm::DebugLoc DLGenerated;
 
public:
  /// Returns the DominatorTree for the generated subfunction.
  DominatorTree *getCalleeDominatorTree() const { return SubFnDT.get(); }
 
  /// Returns the LoopInfo for the generated subfunction.
  LoopInfo *getCalleeLoopInfo() const { return SubFnLI.get(); }
 
  /// Create a struct for all @p Values and store them in there.
  ///
  /// @param Values The values which should be stored in the struct.
  ///
  /// @return The created struct.
  AllocaInst *storeValuesIntoStruct(SetVector<Value *> &Values);
 
  /// Extract all values from the @p Struct and construct the mapping.
  ///
  /// @param Values The values which were stored in the struct.
  /// @param Struct The struct holding all the values in @p Values.
  /// @param VMap   A map to associate every element of @p Values with the
  ///               new llvm value loaded from the @p Struct.
  void extractValuesFromStruct(SetVector<Value *> Values, Type *Ty,
                               Value *Struct, ValueMapT &VMap);
 
  /// Create the definition of the parallel subfunction.
  ///
  /// @return A pointer to the subfunction.
  Function *createSubFnDefinition();
 
  /// Create the runtime library calls for spawn and join of the worker threads.
  /// Additionally, places a call to the specified subfunction.
  ///
  /// @param SubFn      The subfunction which holds the loop body.
  /// @param SubFnParam The parameter for the subfunction (basically the struct
  ///                   filled with the outside values).
  /// @param LB         The lower bound for the loop we parallelize.
  /// @param UB         The upper bound for the loop we parallelize.
  /// @param Stride     The stride of the loop we parallelize.
  virtual void deployParallelExecution(Function *SubFn, Value *SubFnParam,
                                       Value *LB, Value *UB, Value *Stride) = 0;
 
  /// Prepare the definition of the parallel subfunction.
  /// Creates the argument list and names them (as well as the subfunction).
  ///
  /// @param F A pointer to the (parallel) subfunction's parent function.
  ///
  /// @return The pointer to the (parallel) subfunction.
  virtual Function *prepareSubFnDefinition(Function *F) const = 0;
 
  /// Create the parallel subfunction.
  ///
  /// @param Stride The induction variable increment.
  /// @param Struct A struct holding all values in @p Values.
  /// @param Values A set of LLVM-IR Values that should be available in
  ///               the new loop body.
  /// @param VMap   A map to allow outside access to the new versions of
  ///               the values in @p Values.
  /// @param SubFn  The newly created subfunction is returned here.
  ///
  /// @return The newly created induction variable.
  virtual std::tuple<Value *, Function *>
  createSubFn(Value *Stride, AllocaInst *Struct, SetVector<Value *> UsedValues,
              ValueMapT &VMap) = 0;
};
} // end namespace polly
#endif