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//===- SpillPlacement.h - Optimal Spill Code Placement ---------*- 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 analysis computes the optimal spill code placement between basic blocks.
//
// The runOnMachineFunction() method only precomputes some profiling information
// about the CFG. The real work is done by prepare(), addConstraints(), and
// finish() which are called by the register allocator.
//
// Given a variable that is live across multiple basic blocks, and given
// constraints on the basic blocks where the variable is live, determine which
// edge bundles should have the variable in a register and which edge bundles
// should have the variable in a stack slot.
//
// The returned bit vector can be used to place optimal spill code at basic
// block entries and exits. Spill code placement inside a basic block is not
// considered.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
#define LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SparseSet.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Support/BlockFrequency.h"
namespace llvm {
class BitVector;
class EdgeBundles;
class MachineBlockFrequencyInfo;
class MachineFunction;
class MachineLoopInfo;
class SpillPlacement : public MachineFunctionPass {
struct Node;
const MachineFunction *MF;
const EdgeBundles *bundles;
const MachineLoopInfo *loops;
const MachineBlockFrequencyInfo *MBFI;
Node *nodes = nullptr;
// Nodes that are active in the current computation. Owned by the prepare()
// caller.
BitVector *ActiveNodes;
// Nodes with active links. Populated by scanActiveBundles.
SmallVector<unsigned, 8> Linked;
// Nodes that went positive during the last call to scanActiveBundles or
// iterate.
SmallVector<unsigned, 8> RecentPositive;
// Block frequencies are computed once. Indexed by block number.
SmallVector<BlockFrequency, 8> BlockFrequencies;
/// Decision threshold. A node gets the output value 0 if the weighted sum of
/// its inputs falls in the open interval (-Threshold;Threshold).
BlockFrequency Threshold;
/// List of nodes that need to be updated in ::iterate.
SparseSet<unsigned> TodoList;
public:
static char ID; // Pass identification, replacement for typeid.
SpillPlacement() : MachineFunctionPass(ID) {}
~SpillPlacement() override { releaseMemory(); }
/// BorderConstraint - A basic block has separate constraints for entry and
/// exit.
enum BorderConstraint {
DontCare, ///< Block doesn't care / variable not live.
PrefReg, ///< Block entry/exit prefers a register.
PrefSpill, ///< Block entry/exit prefers a stack slot.
PrefBoth, ///< Block entry prefers both register and stack.
MustSpill ///< A register is impossible, variable must be spilled.
};
/// BlockConstraint - Entry and exit constraints for a basic block.
struct BlockConstraint {
unsigned Number; ///< Basic block number (from MBB::getNumber()).
BorderConstraint Entry : 8; ///< Constraint on block entry.
BorderConstraint Exit : 8; ///< Constraint on block exit.
/// True when this block changes the value of the live range. This means
/// the block has a non-PHI def. When this is false, a live-in value on
/// the stack can be live-out on the stack without inserting a spill.
bool ChangesValue;
void print(raw_ostream &OS) const;
void dump() const;
};
/// prepare - Reset state and prepare for a new spill placement computation.
/// @param RegBundles Bit vector to receive the edge bundles where the
/// variable should be kept in a register. Each bit
/// corresponds to an edge bundle, a set bit means the
/// variable should be kept in a register through the
/// bundle. A clear bit means the variable should be
/// spilled. This vector is retained.
void prepare(BitVector &RegBundles);
/// addConstraints - Add constraints and biases. This method may be called
/// more than once to accumulate constraints.
/// @param LiveBlocks Constraints for blocks that have the variable live in or
/// live out.
void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
/// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is
/// equivalent to calling addConstraint with identical BlockConstraints with
/// Entry = Exit = PrefSpill, and ChangesValue = false.
///
/// @param Blocks Array of block numbers that prefer to spill in and out.
/// @param Strong When true, double the negative bias for these blocks.
void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
/// addLinks - Add transparent blocks with the given numbers.
void addLinks(ArrayRef<unsigned> Links);
/// scanActiveBundles - Perform an initial scan of all bundles activated by
/// addConstraints and addLinks, updating their state. Add all the bundles
/// that now prefer a register to RecentPositive.
/// Prepare internal data structures for iterate.
/// Return true is there are any positive nodes.
bool scanActiveBundles();
/// iterate - Update the network iteratively until convergence, or new bundles
/// are found.
void iterate();
/// getRecentPositive - Return an array of bundles that became positive during
/// the previous call to scanActiveBundles or iterate.
ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
/// finish - Compute the optimal spill code placement given the
/// constraints. No MustSpill constraints will be violated, and the smallest
/// possible number of PrefX constraints will be violated, weighted by
/// expected execution frequencies.
/// The selected bundles are returned in the bitvector passed to prepare().
/// @return True if a perfect solution was found, allowing the variable to be
/// in a register through all relevant bundles.
bool finish();
/// getBlockFrequency - Return the estimated block execution frequency per
/// function invocation.
BlockFrequency getBlockFrequency(unsigned Number) const {
return BlockFrequencies[Number];
}
private:
bool runOnMachineFunction(MachineFunction &mf) override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
void releaseMemory() override;
void activate(unsigned n);
void setThreshold(const BlockFrequency &Entry);
bool update(unsigned n);
};
} // end namespace llvm
#endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
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