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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/CodeGen/EdgeBundles.h"
+#include "llvm/CodeGen/LiveIntervalAnalysis.h"
+#include "llvm/CodeGen/MachineBasicBlock.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.addRequiredTransitive<EdgeBundles>();
+ AU.addRequiredTransitive<MachineLoopInfo>();
+ MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+/// 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 {
+ /// Frequency - Total block frequency feeding into[0] or out of[1] the bundle.
+ /// Ideally, these two numbers should be identical, but inaccuracies in the
+ /// block frequency estimates means that we need to normalize ingoing and
+ /// outgoing frequencies separately so they are commensurate.
+ float Frequency[2];
+
+ /// Bias - Normalized contributions from non-transparent blocks.
+ /// A bundle connected to a MustSpill block has a huge negative bias,
+ /// otherwise it is a number in the range [-2;2].
+ float Bias;
+
+ /// Value - Output value of this node computed from the Bias and links.
+ /// This is always in the range [-1;1]. A positive number means the variable
+ /// should go in a register through this bundle.
+ float Value;
+
+ typedef SmallVector<std::pair<float, unsigned>, 4> LinkVector;
+
+ /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
+ /// bundles. The weights are all positive and add up to at most 2, weights
+ /// from ingoing and outgoing nodes separately add up to a most 1. The weight
+ /// sum can be less than 2 when the variable is not live into / out of some
+ /// connected basic blocks.
+ LinkVector Links;
+
+ /// 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 {
+ // Actually, we must spill if Bias < sum(weights).
+ // It may be worth it to compute the weight sum here?
+ return Bias < -2.0f;
+ }
+
+ /// Node - Create a blank Node.
+ Node() {
+ Frequency[0] = Frequency[1] = 0;
+ }
+
+ /// clear - Reset per-query data, but preserve frequencies that only depend on
+ // the CFG.
+ void clear() {
+ Bias = Value = 0;
+ Links.clear();
+ }
+
+ /// addLink - Add a link to bundle b with weight w.
+ /// out=0 for an ingoing link, and 1 for an outgoing link.
+ void addLink(unsigned b, float w, bool out) {
+ // Normalize w relative to all connected blocks from that direction.
+ w /= Frequency[out];
+
+ // 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 from an ingoing[0] or outgoing[1] link.
+ void addBias(float w, bool out) {
+ // Normalize w relative to all connected blocks from that direction.
+ w /= Frequency[out];
+ Bias += w;
+ }
+
+ /// update - Recompute Value from Bias and Links. Return true when node
+ /// preference changes.
+ bool update(const Node nodes[]) {
+ // Compute the weighted sum of inputs.
+ float Sum = Bias;
+ for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
+ Sum += I->first * nodes[I->second].Value;
+
+ // The weighted sum is going to be in the range [-2;2]. Ideally, we should
+ // simply set Value = sign(Sum), 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.
+ const float Thres = 1e-4f;
+ bool Before = preferReg();
+ if (Sum < -Thres)
+ Value = -1;
+ else if (Sum > Thres)
+ 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.
+ for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
+ float Freq = getBlockFrequency(I);
+ unsigned Num = I->getNumber();
+ nodes[bundles->getBundle(Num, 1)].Frequency[0] += Freq;
+ nodes[bundles->getBundle(Num, 0)].Frequency[1] += Freq;
+ }
+
+ // 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();
+}
+
+
+/// prepareNodes - Compute node biases and weights from a set of constraints.
+/// Set a bit in NodeMask for each active node.
+void SpillPlacement::
+prepareNodes(const SmallVectorImpl<BlockConstraint> &LiveBlocks) {
+ for (SmallVectorImpl<BlockConstraint>::const_iterator I = LiveBlocks.begin(),
+ E = LiveBlocks.end(); I != E; ++I) {
+ MachineBasicBlock *MBB = MF->getBlockNumbered(I->Number);
+ float Freq = getBlockFrequency(MBB);
+
+ // Is this a transparent block? Link ingoing and outgoing bundles.
+ if (I->Entry == DontCare && I->Exit == DontCare) {
+ unsigned ib = bundles->getBundle(I->Number, 0);
+ unsigned ob = bundles->getBundle(I->Number, 1);
+
+ // Ignore self-loops.
+ if (ib == ob)
+ continue;
+ activate(ib);
+ activate(ob);
+ nodes[ib].addLink(ob, Freq, 1);
+ nodes[ob].addLink(ib, Freq, 0);
+ continue;
+ }
+
+ // This block is not transparent, but it can still add bias.
+ const float Bias[] = {
+ 0, // DontCare,
+ 1, // PrefReg,
+ -1, // PrefSpill
+ -HUGE_VALF // MustSpill
+ };
+
+ // Live-in to block?
+ if (I->Entry != DontCare) {
+ unsigned ib = bundles->getBundle(I->Number, 0);
+ activate(ib);
+ nodes[ib].addBias(Freq * Bias[I->Entry], 1);
+ }
+
+ // Live-out from block?
+ if (I->Exit != DontCare) {
+ unsigned ob = bundles->getBundle(I->Number, 1);
+ activate(ob);
+ nodes[ob].addBias(Freq * Bias[I->Exit], 0);
+ }
+ }
+}
+
+/// 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(const SmallVectorImpl<unsigned> &Linked) {
+ 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;
+ bool C = nodes[n].update(nodes);
+ Changed |= C;
+ }
+ if (!Changed)
+ 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;
+ bool C = nodes[n].update(nodes);
+ Changed |= C;
+ }
+ if (!Changed)
+ return;
+ }
+}
+
+bool
+SpillPlacement::placeSpills(const SmallVectorImpl<BlockConstraint> &LiveBlocks,
+ BitVector &RegBundles) {
+ // Reuse RegBundles as our ActiveNodes vector.
+ ActiveNodes = &RegBundles;
+ ActiveNodes->clear();
+ ActiveNodes->resize(bundles->getNumBundles());
+
+ // Compute active nodes, links and biases.
+ prepareNodes(LiveBlocks);
+
+ // Update all active nodes, and find the ones that are actually linked to
+ // something so their value may change when iterating.
+ SmallVector<unsigned, 8> Linked;
+ for (int n = RegBundles.find_first(); n>=0; n = RegBundles.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].Links.empty() && !nodes[n].mustSpill())
+ Linked.push_back(n);
+ }
+
+ // Iterate the network to convergence.
+ iterate(Linked);
+
+ // Write preferences back to RegBundles.
+ bool Perfect = true;
+ for (int n = RegBundles.find_first(); n>=0; n = RegBundles.find_next(n))
+ if (!nodes[n].preferReg()) {
+ RegBundles.reset(n);
+ Perfect = false;
+ }
+ return Perfect;
+}
+
+/// getBlockFrequency - Return our best estimate of the block frequency which is
+/// the expected number of block executions per function invocation.
+float SpillPlacement::getBlockFrequency(const MachineBasicBlock *MBB) {
+ // Use the unnormalized spill weight for real block frequencies.
+ return LiveIntervals::getSpillWeight(true, false, loops->getLoopDepth(MBB));
+}
+
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