llvm/llvm/include/llvm/Transforms/Scalar/ConstantHoisting.h

//==- ConstantHoisting.h - Prepare code for expensive constants --*- 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 pass identifies expensive constants to hoist and coalesces them to
// better prepare it for SelectionDAG-based code generation. This works around
// the limitations of the basic-block-at-a-time approach.
//
// First it scans all instructions for integer constants and calculates its
// cost. If the constant can be folded into the instruction (the cost is
// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
// consider it expensive and leave it alone. This is the default behavior and
// the default implementation of getIntImmCostInst will always return TCC_Free.
//
// If the cost is more than TCC_BASIC, then the integer constant can't be folded
// into the instruction and it might be beneficial to hoist the constant.
// Similar constants are coalesced to reduce register pressure and
// materialization code.
//
// When a constant is hoisted, it is also hidden behind a bitcast to force it to
// be live-out of the basic block. Otherwise the constant would be just
// duplicated and each basic block would have its own copy in the SelectionDAG.
// The SelectionDAG recognizes such constants as opaque and doesn't perform
// certain transformations on them, which would create a new expensive constant.
//
// This optimization is only applied to integer constants in instructions and
// simple (this means not nested) constant cast expressions. For example:
// %0 = load i64* inttoptr (i64 big_constant to i64*)
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
#define LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/PassManager.h"
#include <algorithm>
#include <vector>

namespace llvm {

class BasicBlock;
class BlockFrequencyInfo;
class Constant;
class ConstantInt;
class ConstantExpr;
class DominatorTree;
class Function;
class GlobalVariable;
class Instruction;
class ProfileSummaryInfo;
class TargetTransformInfo;
class TargetTransformInfo;

/// A private "module" namespace for types and utilities used by
/// ConstantHoisting. These are implementation details and should not be used by
/// clients.
namespace consthoist {

/// Keeps track of the user of a constant and the operand index where the
/// constant is used.
struct ConstantUser { … };

ConstantUseListType;

/// Keeps track of a constant candidate and its uses.
struct ConstantCandidate { … };

/// This represents a constant that has been rebased with respect to a
/// base constant. The difference to the base constant is recorded in Offset.
struct RebasedConstantInfo { … };

RebasedConstantListType;

/// A base constant and all its rebased constants.
struct ConstantInfo { … };

} // end namespace consthoist

class ConstantHoistingPass : public PassInfoMixin<ConstantHoistingPass> { … };

} // end namespace llvm

#endif // LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H