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+//===-- SpillPlacement.cpp - Optimal Spill Code Placement -----------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "spillplacement"
+#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/MachineLoopInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/Format.h"
+
+using namespace llvm;
+
+char SpillPlacement::ID = 0;
+INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
+ "Spill Code Placement Analysis", true, true)
+INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
+INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
+INITIALIZE_PASS_END(SpillPlacement, "spill-code-placement",
+ "Spill Code Placement Analysis", true, true)
+
+char &llvm::SpillPlacementID = SpillPlacement::ID;
+
+void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.setPreservesAll();
+ AU.addRequired<MachineBlockFrequencyInfo>();
+ AU.addRequiredTransitive<EdgeBundles>();
+ AU.addRequiredTransitive<MachineLoopInfo>();
+ MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+/// Decision threshold. A node gets the output value 0 if the weighted sum of
+/// its inputs falls in the open interval (-Threshold;Threshold).
+static const BlockFrequency Threshold = 2;
+
+/// 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 {
+ /// BiasN - Sum of blocks that prefer a spill.
+ BlockFrequency BiasN;
+ /// BiasP - Sum of blocks that prefer a register.
+ BlockFrequency BiasP;
+
+ /// Value - Output value of this node computed from the Bias and links.
+ /// This is always on of the values {-1, 0, 1}. A positive number means the
+ /// variable should go in a register through this bundle.
+ int Value;
+
+ typedef SmallVector<std::pair<BlockFrequency, unsigned>, 4> LinkVector;
+
+ /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
+ /// bundles. The weights are all positive block frequencies.
+ LinkVector Links;
+
+ /// SumLinkWeights - Cached sum of the weights of all links + ThresHold.
+ BlockFrequency SumLinkWeights;
+
+ /// preferReg - Return true when this node prefers to be in a register.
+ bool preferReg() const {
+ // Undecided nodes (Value==0) go on the stack.
+ return Value > 0;
+ }
+
+ /// mustSpill - Return True if this node is so biased that it must spill.
+ bool mustSpill() const {
+ // We must spill if Bias < -sum(weights) or the MustSpill flag was set.
+ // BiasN is saturated when MustSpill is set, make sure this still returns
+ // true when the RHS saturates. Note that SumLinkWeights includes Threshold.
+ return BiasN >= BiasP + SumLinkWeights;
+ }
+
+ /// clear - Reset per-query data, but preserve frequencies that only depend on
+ // the CFG.
+ void clear() {
+ BiasN = BiasP = Value = 0;
+ SumLinkWeights = Threshold;
+ Links.clear();
+ }
+
+ /// addLink - Add a link to bundle b with weight w.
+ void addLink(unsigned b, BlockFrequency w) {
+ // Update cached sum.
+ SumLinkWeights += w;
+
+ // There can be multiple links to the same bundle, add them up.
+ for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
+ if (I->second == b) {
+ I->first += w;
+ return;
+ }
+ // This must be the first link to b.
+ Links.push_back(std::make_pair(w, b));
+ }
+
+ /// addBias - Bias this node.
+ void addBias(BlockFrequency freq, BorderConstraint direction) {
+ switch (direction) {
+ default:
+ break;
+ case PrefReg:
+ BiasP += freq;
+ break;
+ case PrefSpill:
+ BiasN += freq;
+ break;
+ case MustSpill:
+ BiasN = BlockFrequency::getMaxFrequency();
+ break;
+ }
+ }
+
+ /// update - Recompute Value from Bias and Links. Return true when node
+ /// preference changes.
+ bool update(const Node nodes[]) {
+ // Compute the weighted sum of inputs.
+ BlockFrequency SumN = BiasN;
+ BlockFrequency SumP = BiasP;
+ for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I) {
+ if (nodes[I->second].Value == -1)
+ SumN += I->first;
+ else if (nodes[I->second].Value == 1)
+ SumP += I->first;
+ }
+
+ // Each weighted sum is going to be less than the total frequency of the
+ // bundle. Ideally, we should simply set Value = sign(SumP - SumN), but we
+ // will add a dead zone around 0 for two reasons:
+ //
+ // 1. It avoids arbitrary bias when all links are 0 as is possible during
+ // initial iterations.
+ // 2. It helps tame rounding errors when the links nominally sum to 0.
+ //
+ bool Before = preferReg();
+ if (SumN >= SumP + Threshold)
+ Value = -1;
+ else if (SumP >= SumN + Threshold)
+ Value = 1;
+ else
+ Value = 0;
+ return Before != preferReg();
+ }
+};
+
+bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
+ MF = &mf;
+ bundles = &getAnalysis<EdgeBundles>();
+ loops = &getAnalysis<MachineLoopInfo>();
+
+ assert(!nodes && "Leaking node array");
+ nodes = new Node[bundles->getNumBundles()];
+
+ // Compute total ingoing and outgoing block frequencies for all bundles.
+ BlockFrequencies.resize(mf.getNumBlockIDs());
+ MachineBlockFrequencyInfo &MBFI = getAnalysis<MachineBlockFrequencyInfo>();
+ for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
+ unsigned Num = I->getNumber();
+ BlockFrequencies[Num] = MBFI.getBlockFreq(I);
+ }
+
+ // We never change the function.
+ return false;
+}
+
+void SpillPlacement::releaseMemory() {
+ delete[] nodes;
+ nodes = 0;
+}
+
+/// activate - mark node n as active if it wasn't already.
+void SpillPlacement::activate(unsigned n) {
+ if (ActiveNodes->test(n))
+ return;
+ ActiveNodes->set(n);
+ nodes[n].clear();
+
+ // Very large bundles usually come from big switches, indirect branches,
+ // landing pads, or loops with many 'continue' statements. It is difficult to
+ // allocate registers when so many different blocks are involved.
+ //
+ // Give a small negative bias to large bundles such that a substantial
+ // fraction of the connected blocks need to be interested before we consider
+ // expanding the region through the bundle. This helps compile time by
+ // limiting the number of blocks visited and the number of links in the
+ // Hopfield network.
+ if (bundles->getBlocks(n).size() > 100) {
+ nodes[n].BiasP = 0;
+ nodes[n].BiasN = (BlockFrequency::getEntryFrequency() / 16);
+ }
+}
+
+
+/// 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) {
+ for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
+ E = LiveBlocks.end(); I != E; ++I) {
+ BlockFrequency Freq = BlockFrequencies[I->Number];
+
+ // Live-in to block?
+ if (I->Entry != DontCare) {
+ unsigned ib = bundles->getBundle(I->Number, 0);
+ activate(ib);
+ nodes[ib].addBias(Freq, I->Entry);
+ }
+
+ // Live-out from block?
+ if (I->Exit != DontCare) {
+ unsigned ob = bundles->getBundle(I->Number, 1);
+ activate(ob);
+ nodes[ob].addBias(Freq, I->Exit);
+ }
+ }
+}
+
+/// addPrefSpill - Same as addConstraints(PrefSpill)
+void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) {
+ for (ArrayRef<unsigned>::iterator I = Blocks.begin(), E = Blocks.end();
+ I != E; ++I) {
+ BlockFrequency Freq = BlockFrequencies[*I];
+ if (Strong)
+ Freq += Freq;
+ unsigned ib = bundles->getBundle(*I, 0);
+ unsigned ob = bundles->getBundle(*I, 1);
+ activate(ib);
+ activate(ob);
+ nodes[ib].addBias(Freq, PrefSpill);
+ nodes[ob].addBias(Freq, PrefSpill);
+ }
+}
+
+void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
+ for (ArrayRef<unsigned>::iterator I = Links.begin(), E = Links.end(); I != E;
+ ++I) {
+ unsigned Number = *I;
+ unsigned ib = bundles->getBundle(Number, 0);
+ unsigned ob = bundles->getBundle(Number, 1);
+
+ // Ignore self-loops.
+ if (ib == ob)
+ continue;
+ activate(ib);
+ activate(ob);
+ if (nodes[ib].Links.empty() && !nodes[ib].mustSpill())
+ Linked.push_back(ib);
+ if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
+ Linked.push_back(ob);
+ BlockFrequency Freq = BlockFrequencies[Number];
+ nodes[ib].addLink(ob, Freq);
+ nodes[ob].addLink(ib, Freq);
+ }
+}
+
+bool SpillPlacement::scanActiveBundles() {
+ Linked.clear();
+ RecentPositive.clear();
+ for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
+ nodes[n].update(nodes);
+ // A node that must spill, or a node without any links is not going to
+ // change its value ever again, so exclude it from iterations.
+ if (nodes[n].mustSpill())
+ continue;
+ if (!nodes[n].Links.empty())
+ Linked.push_back(n);
+ if (nodes[n].preferReg())
+ RecentPositive.push_back(n);
+ }
+ return !RecentPositive.empty();
+}
+
+/// iterate - Repeatedly update the Hopfield nodes until stability or the
+/// maximum number of iterations is reached.
+/// @param Linked - Numbers of linked nodes that need updating.
+void SpillPlacement::iterate() {
+ // First update the recently positive nodes. They have likely received new
+ // negative bias that will turn them off.
+ while (!RecentPositive.empty())
+ nodes[RecentPositive.pop_back_val()].update(nodes);
+
+ if (Linked.empty())
+ return;
+
+ // Run up to 10 iterations. The edge bundle numbering is closely related to
+ // basic block numbering, so there is a strong tendency towards chains of
+ // linked nodes with sequential numbers. By scanning the linked nodes
+ // backwards and forwards, we make it very likely that a single node can
+ // affect the entire network in a single iteration. That means very fast
+ // convergence, usually in a single iteration.
+ for (unsigned iteration = 0; iteration != 10; ++iteration) {
+ // Scan backwards, skipping the last node which was just updated.
+ bool Changed = false;
+ for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
+ llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
+ unsigned n = *I;
+ if (nodes[n].update(nodes)) {
+ Changed = true;
+ if (nodes[n].preferReg())
+ RecentPositive.push_back(n);
+ }
+ }
+ if (!Changed || !RecentPositive.empty())
+ return;
+
+ // Scan forwards, skipping the first node which was just updated.
+ Changed = false;
+ for (SmallVectorImpl<unsigned>::const_iterator I =
+ llvm::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
+ unsigned n = *I;
+ if (nodes[n].update(nodes)) {
+ Changed = true;
+ if (nodes[n].preferReg())
+ RecentPositive.push_back(n);
+ }
+ }
+ if (!Changed || !RecentPositive.empty())
+ return;
+ }
+}
+
+void SpillPlacement::prepare(BitVector &RegBundles) {
+ Linked.clear();
+ RecentPositive.clear();
+ // Reuse RegBundles as our ActiveNodes vector.
+ ActiveNodes = &RegBundles;
+ ActiveNodes->clear();
+ ActiveNodes->resize(bundles->getNumBundles());
+}
+
+bool
+SpillPlacement::finish() {
+ assert(ActiveNodes && "Call prepare() first");
+
+ // Write preferences back to ActiveNodes.
+ bool Perfect = true;
+ for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n))
+ if (!nodes[n].preferReg()) {
+ ActiveNodes->reset(n);
+ Perfect = false;
+ }
+ ActiveNodes = 0;
+ return Perfect;
+}
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