//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "spillplacement"
#include "SpillPlacement.h"
+#include "llvm/ADT/BitVector.h"
#include "llvm/CodeGen/EdgeBundles.h"
-#include "llvm/CodeGen/LiveIntervalAnalysis.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"
using namespace llvm;
+#define DEBUG_TYPE "spillplacement"
+
char SpillPlacement::ID = 0;
INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
"Spill Code Placement Analysis", true, true)
void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
+ AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequiredTransitive<EdgeBundles>();
AU.addRequiredTransitive<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
+namespace {
+static BlockFrequency Threshold;
+}
+
+/// Decision threshold. A node gets the output value 0 if the weighted sum of
+/// its inputs falls in the open interval (-Threshold;Threshold).
+static BlockFrequency getThreshold() { return Threshold; }
+
+/// \brief 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.
+static void setThreshold(const BlockFrequency &Entry) {
+ // Apparently 2 is a good threshold when Entry==2^14, but we need to scale
+ // it. Divide by 2^13, rounding as appropriate.
+ uint64_t Freq = Entry.getFrequency();
+ uint64_t Scaled = (Freq >> 13) + bool(Freq & (1 << 12));
+ Threshold = std::max(UINT64_C(1), Scaled);
+}
+
/// Node - Each edge bundle corresponds to a Hopfield node.
///
/// The node contains precomputed frequency data that only depends on the CFG,
/// because all weights are positive.
///
struct SpillPlacement::Node {
- /// Scale - Inverse 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 Scale[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;
+ /// 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 in the range [-1;1]. A positive number means the variable
- /// should go in a register through this bundle.
- float Value;
+ /// 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<float, unsigned>, 4> LinkVector;
+ typedef SmallVector<std::pair<BlockFrequency, 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.
+ /// 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.
/// 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() {
- Scale[0] = Scale[1] = 0;
+ // 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() {
- Bias = Value = 0;
+ BiasN = BiasP = Value = 0;
+ SumLinkWeights = getThreshold();
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 *= Scale[out];
+ 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)
Links.push_back(std::make_pair(w, b));
}
- /// addBias - Bias this node from an ingoing[0] or outgoing[1] link.
- /// Return the change to the total number of positive biases.
- void addBias(float w, bool out) {
- // Normalize w relative to all connected blocks from that direction.
- w *= Scale[out];
- Bias += w;
+ /// 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.
- float Sum = Bias;
- for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
- Sum += I->first * nodes[I->second].Value;
+ 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;
+ }
- // 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:
+ // 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.
- const float Thres = 1e-4f;
+ //
bool Before = preferReg();
- if (Sum < -Thres)
+ if (SumN >= SumP + getThreshold())
Value = -1;
- else if (Sum > Thres)
+ else if (SumP >= SumN + getThreshold())
Value = 1;
else
Value = 0;
nodes = new Node[bundles->getNumBundles()];
// Compute total ingoing and outgoing block frequencies for all bundles.
- BlockFrequency.resize(mf.getNumBlockIDs());
+ BlockFrequencies.resize(mf.getNumBlockIDs());
+ MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
+ setThreshold(MBFI->getEntryFreq());
for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
- float Freq = LiveIntervals::getSpillWeight(true, false,
- loops->getLoopDepth(I));
unsigned Num = I->getNumber();
- BlockFrequency[Num] = Freq;
- nodes[bundles->getBundle(Num, 1)].Scale[0] += Freq;
- nodes[bundles->getBundle(Num, 0)].Scale[1] += Freq;
+ BlockFrequencies[Num] = MBFI->getBlockFreq(I);
}
- // Scales are reciprocal frequencies.
- for (unsigned i = 0, e = bundles->getNumBundles(); i != e; ++i)
- for (unsigned d = 0; d != 2; ++d)
- if (nodes[i].Scale[d] > 0)
- nodes[i].Scale[d] = 1 / nodes[i].Scale[d];
-
// We never change the function.
return false;
}
void SpillPlacement::releaseMemory() {
delete[] nodes;
- nodes = 0;
+ nodes = nullptr;
}
/// activate - mark node n as active if it wasn't already.
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 = (MBFI->getEntryFreq() / 16);
+ }
}
void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
E = LiveBlocks.end(); I != E; ++I) {
- float Freq = getBlockFrequency(I->Number);
- const float Bias[] = {
- 0, // DontCare,
- 1, // PrefReg,
- -1, // PrefSpill
- -HUGE_VALF // MustSpill
- };
+ 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 * Bias[I->Entry], 1);
+ 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 * Bias[I->Exit], 0);
+ 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) {
Linked.push_back(ib);
if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
Linked.push_back(ob);
- float Freq = getBlockFrequency(Number);
- nodes[ib].addLink(ob, Freq, 1);
- nodes[ob].addLink(ib, Freq, 0);
+ BlockFrequency Freq = BlockFrequencies[Number];
+ nodes[ib].addLink(ob, Freq);
+ nodes[ob].addLink(ib, Freq);
}
}
// 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.
+ // Scan backwards, skipping the last node when iteration is not zero. When
+ // iteration is not zero, the last node was just updated.
bool Changed = false;
for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
- llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
+ iteration == 0 ? Linked.rbegin() : std::next(Linked.rbegin()),
+ E = Linked.rend(); I != E; ++I) {
unsigned n = *I;
if (nodes[n].update(nodes)) {
Changed = true;
// 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) {
+ std::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
unsigned n = *I;
if (nodes[n].update(nodes)) {
Changed = true;
ActiveNodes->reset(n);
Perfect = false;
}
- ActiveNodes = 0;
+ ActiveNodes = nullptr;
return Perfect;
}
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