IslExprBuilder.h

Go to the documentation of this file.

//===-IslExprBuilder.h - Helper to generate code for isl AST expressions --===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
//===----------------------------------------------------------------------===//
 
#ifndef POLLY_ISL_EXPR_BUILDER_H
#define POLLY_ISL_EXPR_BUILDER_H
 
#include "polly/CodeGen/IRBuilder.h"
#include "polly/Support/ScopHelper.h"
#include "isl/isl-noexceptions.h"
 
namespace llvm {
// Provide PointerLikeTypeTraits for isl_id.
template <> struct PointerLikeTypeTraits<isl_id *> {
 
public:
  static inline const void *getAsVoidPointer(isl_id *P) { return (void *)P; }
  static inline const Region *getFromVoidPointer(void *P) {
    return (Region *)P;
  }
  static constexpr int NumLowBitsAvailable = 0;
};
} // namespace llvm
 
namespace polly {
class ScopArrayInfo;
 
/// LLVM-IR generator for isl_ast_expr[essions]
///
/// This generator generates LLVM-IR that performs the computation described by
/// an isl_ast_expr[ession].
///
/// Example:
///
///   An isl_ast_expr[ession] can look like this:
///
///     (N + M) + 10
///
///   The IslExprBuilder could create the following LLVM-IR:
///
///     %tmp1 = add nsw i64 %N
///     %tmp2 = add nsw i64 %tmp1, %M
///     %tmp3 = add nsw i64 %tmp2, 10
///
/// The implementation of this class is mostly a mapping from isl_ast_expr
/// constructs to the corresponding LLVM-IR constructs.
///
/// The following decisions may need some explanation:
///
/// 1) Which data-type to choose
///
/// isl_ast_expr[essions] are untyped expressions that assume arbitrary
/// precision integer computations. LLVM-IR instead has fixed size integers.
/// When lowering to LLVM-IR we need to chose both the size of the data type and
/// the sign of the operations we use.
///
/// At the moment, we hardcode i64 bit signed computations. Our experience has
/// shown that 64 bit are generally large enough for the loop bounds that appear
/// in the wild. Signed computations are needed, as loop bounds may become
/// negative.
///
/// It is possible to track overflows that occurred in the generated IR. See the
/// description of @see OverflowState for more information.
///
/// FIXME: Hardcoding sizes can cause issues:
///
///   -  On embedded systems and especially for high-level-synthesis 64 bit
///      computations are very costly.
///
///   The right approach is to compute the minimal necessary bitwidth and
///   signedness for each subexpression during in the isl AST generation and
///   to use this information in our IslAstGenerator. Preliminary patches are
///   available, but have not been committed yet.
///
class IslExprBuilder final {
public:
  /// A map from isl_ids to llvm::Values.
  typedef llvm::MapVector<isl_id *, llvm::AssertingVH<llvm::Value>> IDToValueTy;
 
  typedef llvm::MapVector<isl_id *, const ScopArrayInfo *> IDToScopArrayInfoTy;
 
  /// A map from isl_ids to ScopArrayInfo objects.
  ///
  /// This map is used to obtain ScopArrayInfo objects for isl_ids which do not
  /// carry a ScopArrayInfo object in their user pointer. This is useful if the
  /// construction of ScopArrayInfo objects happens only after references (e.g.
  /// in an AST) to an isl_id are generated and the user pointer of the isl_id
  /// can not be changed any more.
  ///
  /// This is useful for external users who just use the IslExprBuilder for
  /// code generation.
  IDToScopArrayInfoTy *IDToSAI = nullptr;
 
  /// Set the isl_id to ScopArrayInfo map.
  ///
  /// @param NewIDToSAI The new isl_id to ScopArrayInfo map to use.
  void setIDToSAI(IDToScopArrayInfoTy *NewIDToSAI) { IDToSAI = NewIDToSAI; }
 
  /// Construct an IslExprBuilder.
  ///
  /// @param Builder     The IRBuilder used to construct the
  ///                    isl_ast_expr[ession]. The insert location of this
  ///                    IRBuilder defines WHERE the  corresponding LLVM-IR
  ///                    is generated.
  /// @param IDToValue   The isl_ast_expr[ession] may reference parameters or
  ///                    variables (identified by an isl_id). The IDTOValue map
  ///                    specifies the LLVM-IR Values that correspond to these
  ///                    parameters and variables.
  /// @param GlobalMap   A mapping from llvm::Values used in the original scop
  ///                    region to a new set of llvm::Values.
  /// @param DL          DataLayout for the current Module.
  /// @param SE          ScalarEvolution analysis for the current function.
  /// @param DT          DominatorTree analysis for the current function.
  /// @param LI          LoopInfo analysis for the current function.
  /// @param StartBlock The first basic block after the RTC.
  IslExprBuilder(Scop &S, PollyIRBuilder &Builder, IDToValueTy &IDToValue,
                 ValueMapT &GlobalMap, const llvm::DataLayout &DL,
                 llvm::ScalarEvolution &SE, llvm::DominatorTree &DT,
                 llvm::LoopInfo &LI, llvm::BasicBlock *StartBlock);
 
  /// Create LLVM-IR for an isl_ast_expr[ession].
  ///
  /// @param Expr The ast expression for which we generate LLVM-IR.
  ///
  /// @return The llvm::Value* containing the result of the computation.
  llvm::Value *create(__isl_take isl_ast_expr *Expr);
 
  /// Return the largest of two types.
  ///
  /// @param T1 The first type.
  /// @param T2 The second type.
  ///
  /// @return The largest of the two types.
  llvm::Type *getWidestType(llvm::Type *T1, llvm::Type *T2);
 
  /// Return the type with which this expression should be computed.
  ///
  /// The type needs to be large enough to hold all possible input and all
  /// possible output values.
  ///
  /// @param Expr The expression for which to find the type.
  /// @return The type with which the expression should be computed.
  llvm::IntegerType *getType(__isl_keep isl_ast_expr *Expr);
 
  /// Change if runtime overflows are tracked or not.
  ///
  /// @param Enable Flag to enable/disable the tracking.
  ///
  /// Note that this will reset the tracking state and that tracking is only
  /// allowed if the last tracked expression dominates the current insert point.
  void setTrackOverflow(bool Enable);
 
  /// Return the current overflow status or nullptr if it is not tracked.
  ///
  /// @return A nullptr if tracking is disabled or otherwise an i1 that has the
  ///         value of "0" if and only if no overflow happened since tracking
  ///         was enabled.
  llvm::Value *getOverflowState() const;
 
  /// Create LLVM-IR that computes the memory location of an access expression.
  ///
  /// For a given isl_ast_expr[ession] of type isl_ast_op_access this function
  /// creates IR that computes the address the access expression refers to.
  ///
  /// @param Expr The ast expression of type isl_ast_op_access
  ///             for which we generate LLVM-IR.
  ///
  /// @return A pair of the llvm::Value* containing the result of the
  ///         computation and the llvm::Type* it points to.
  std::pair<llvm::Value *, llvm::Type *>
  createAccessAddress(__isl_take isl_ast_expr *Expr);
 
  /// Check if an @p Expr contains integer constants larger than 64 bit.
  ///
  /// @param Expr The expression to check.
  ///
  /// @return True if the ast expression is larger than 64 bit.
  bool hasLargeInts(isl::ast_expr Expr);
 
private:
  Scop &S;
 
  /// Flag that will be set if an overflow occurred at runtime.
  ///
  /// Note that this flag is by default a nullptr and if it is a nullptr
  /// we will not record overflows but simply perform the computations.
  /// The intended usage is as follows:
  ///   - If overflows in [an] expression[s] should be tracked, call
  ///     the setTrackOverflow(true) function.
  ///   - Use create(...) for all expressions that should be checked.
  ///   - Call getOverflowState() to get the value representing the current
  ///     state of the overflow flag.
  ///   - To stop tracking call setTrackOverflow(false).
  llvm::Value *OverflowState;
 
  PollyIRBuilder &Builder;
  IDToValueTy &IDToValue;
  ValueMapT &GlobalMap;
 
  const llvm::DataLayout &DL;
  llvm::ScalarEvolution &SE;
  llvm::DominatorTree &DT;
  llvm::LoopInfo &LI;
  llvm::BasicBlock *StartBlock;
 
  llvm::Value *createOp(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpUnary(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpAccess(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpBin(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpNAry(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpSelect(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpICmp(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpBoolean(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpBooleanConditional(__isl_take isl_ast_expr *Expr);
  llvm::Value *createId(__isl_take isl_ast_expr *Expr);
  llvm::Value *createInt(__isl_take isl_ast_expr *Expr);
  llvm::Value *createOpAddressOf(__isl_take isl_ast_expr *Expr);
 
  /// Create a binary operation @p Opc and track overflows if requested.
  ///
  /// @param OpC  The binary operation that should be performed [Add/Sub/Mul].
  /// @param LHS  The left operand.
  /// @param RHS  The right operand.
  /// @param Name The (base) name of the new IR operations.
  ///
  /// @return A value that represents the result of the binary operation.
  llvm::Value *createBinOp(llvm::BinaryOperator::BinaryOps Opc,
                           llvm::Value *LHS, llvm::Value *RHS,
                           const llvm::Twine &Name);
 
  /// Create an addition and track overflows if requested.
  ///
  /// @param LHS  The left operand.
  /// @param RHS  The right operand.
  /// @param Name The (base) name of the new IR operations.
  ///
  /// @return A value that represents the result of the addition.
  llvm::Value *createAdd(llvm::Value *LHS, llvm::Value *RHS,
                         const llvm::Twine &Name = "");
 
  /// Create a subtraction and track overflows if requested.
  ///
  /// @param LHS  The left operand.
  /// @param RHS  The right operand.
  /// @param Name The (base) name of the new IR operations.
  ///
  /// @return A value that represents the result of the subtraction.
  llvm::Value *createSub(llvm::Value *LHS, llvm::Value *RHS,
                         const llvm::Twine &Name = "");
 
  /// Create a multiplication and track overflows if requested.
  ///
  /// @param LHS  The left operand.
  /// @param RHS  The right operand.
  /// @param Name The (base) name of the new IR operations.
  ///
  /// @return A value that represents the result of the multiplication.
  llvm::Value *createMul(llvm::Value *LHS, llvm::Value *RHS,
                         const llvm::Twine &Name = "");
};
} // namespace polly
 
#endif