llvm/llvm/lib/CodeGen/SpillPlacement.cpp

//===- SpillPlacement.cpp - Optimal Spill Code Placement ------------------===//
//
// 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 the spill code placement analysis.
//
// Each edge bundle corresponds to a node in a Hopfield network. Constraints on
// basic blocks are weighted by the block frequency and added to become the node
// bias.
//
// Transparent basic blocks have the variable live through, but don't care if it
// is spilled or in a register. These blocks become connections in the Hopfield
// network, again weighted by block frequency.
//
// The Hopfield network minimizes (possibly locally) its energy function:
//
//   E = -sum_n V_n * ( B_n + sum_{n, m linked by b} V_m * F_b )
//
// The energy function represents the expected spill code execution frequency,
// or the cost of spilling. This is a Lyapunov function which never increases
// when a node is updated. It is guaranteed to converge to a local minimum.
//
//===----------------------------------------------------------------------===//

#include "SpillPlacement.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/CodeGen/EdgeBundles.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>

usingnamespacellvm;

#define DEBUG_TYPE …

char SpillPlacement::ID = …;

char &llvm::SpillPlacementID = …;

INITIALIZE_PASS_BEGIN(SpillPlacement, DEBUG_TYPE,
                      "Spill Code Placement Analysis", true, true)
INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
INITIALIZE_PASS_END(SpillPlacement, DEBUG_TYPE,
                    "Spill Code Placement Analysis", true, true)

void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const { … }

/// Node - Each edge bundle corresponds to a Hopfield node.
///
/// The node contains precomputed frequency data that only depends on the CFG,
/// but Bias and Links are computed each time placeSpills is called.
///
/// The node Value is positive when the variable should be in a register. The
/// value can change when linked nodes change, but convergence is very fast
/// because all weights are positive.
struct SpillPlacement::Node { … };

bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) { … }

void SpillPlacement::releaseMemory() { … }

/// activate - mark node n as active if it wasn't already.
void SpillPlacement::activate(unsigned n) { … }

/// Set the threshold for a given entry frequency.
///
/// Set the threshold relative to \c Entry.  Since the threshold is used as a
/// bound on the open interval (-Threshold;Threshold), 1 is the minimum
/// threshold.
void SpillPlacement::setThreshold(BlockFrequency Entry) { … }

/// addConstraints - Compute node biases and weights from a set of constraints.
/// Set a bit in NodeMask for each active node.
void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) { … }

/// addPrefSpill - Same as addConstraints(PrefSpill)
void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) { … }

void SpillPlacement::addLinks(ArrayRef<unsigned> Links) { … }

bool SpillPlacement::scanActiveBundles() { … }

bool SpillPlacement::update(unsigned n) { … }

/// iterate - Repeatedly update the Hopfield nodes until stability or the
/// maximum number of iterations is reached.
void SpillPlacement::iterate() { … }

void SpillPlacement::prepare(BitVector &RegBundles) { … }

bool
SpillPlacement::finish() { … }

void SpillPlacement::BlockConstraint::print(raw_ostream &OS) const { … }

void SpillPlacement::BlockConstraint::dump() const { … }