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Reapply r185393. 

Original commit message:

Remove floating point computations from SpillPlacement.cpp.

Patch by Benjamin Kramer!

Use the BlockFrequency class instead of floats in the Hopfield network
computations. This rescales the node Bias field from a [-2;2] float
range to two block frequencies BiasN and BiasP pulling in opposite
directions. This construct has a more predictable behavior when block
frequencies saturate.

The per-node scaling factors are no longer necessary, assuming the block
frequencies around a bundle are consistent.

This patch can cause the register allocator to make different spilling
decisions. The differences should be small.

llvm-svn: 186434
This commit is contained in:
Jakob Stoklund Olesen 2013-07-16 18:26:15 +00:00
parent 6ab5468637
commit c5454ff046
2 changed files with 80 additions and 82 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

155
llvm/lib/CodeGen/SpillPlacement.cpp
View File

@ -59,6 +59,10 @@ void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
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,
@ -69,31 +73,25 @@ void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
/// 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.
@ -102,28 +100,24 @@ struct SpillPlacement::Node {
/// 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 = Threshold;
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)
@ -135,33 +129,48 @@ struct SpillPlacement::Node {
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 + Threshold)
Value = -1;
else if (Sum > Thres)
else if (SumP >= SumN + Threshold)
Value = 1;
else
Value = 0;
@ -178,23 +187,13 @@ bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
nodes = new Node[bundles->getNumBundles()];
// Compute total ingoing and outgoing block frequencies for all bundles.
BlockFrequency.resize(mf.getNumBlockIDs());
BlockFrequencies.resize(mf.getNumBlockIDs());
MachineBlockFrequencyInfo &MBFI = getAnalysis<MachineBlockFrequencyInfo>();
float EntryFreq = BlockFrequency::getEntryFrequency();
for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
float Freq = MBFI.getBlockFreq(I).getFrequency() / EntryFreq;
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;
}
@ -215,12 +214,15 @@ void SpillPlacement::activate(unsigned n) {
// 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 1/32 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].Bias = -0.0625f;
// 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);
}
}
@ -229,27 +231,20 @@ void SpillPlacement::activate(unsigned n) {
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
0, // PrefBoth
-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);
}
}
}
@ -258,15 +253,15 @@ void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) {
for (ArrayRef<unsigned>::iterator I = Blocks.begin(), E = Blocks.end();
I != E; ++I) {
float Freq = getBlockFrequency(*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, 1);
nodes[ob].addBias(-Freq, 0);
nodes[ib].addBias(Freq, PrefSpill);
nodes[ob].addBias(Freq, PrefSpill);
}
}
@ -286,9 +281,9 @@ void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
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);
}
}

7
llvm/lib/CodeGen/SpillPlacement.h
View File

@ -30,6 +30,7 @@
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Support/BlockFrequency.h"
namespace llvm {
@ -57,7 +58,7 @@ class SpillPlacement : public MachineFunctionPass {
SmallVector<unsigned, 8> RecentPositive;
// Block frequencies are computed once. Indexed by block number.
SmallVector<float, 4> BlockFrequency;
SmallVector<BlockFrequency, 4> BlockFrequencies;
public:
static char ID; // Pass identification, replacement for typeid.
@ -140,7 +141,9 @@ public:
/// getBlockFrequency - Return the estimated block execution frequency per
/// function invocation.
float getBlockFrequency(unsigned Number) const {
return BlockFrequency[Number];
// FIXME: Return the BlockFrequency directly.
const float Scale = 1.0f / BlockFrequency::getEntryFrequency();
return BlockFrequencies[Number].getFrequency() * Scale;
}
private: