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llvm-mirror/lib/CodeGen/SwitchLoweringUtils.cpp
Amara Emerson d06a69ea1d Factor out SelectionDAG's switch analysis and lowering into a separate component. 
In order for GlobalISel to re-use the significant amount of analysis and
optimization code in SDAG's switch lowering, we first have to extract it and
create an interface to be used by both frameworks.

No test changes as it's NFC.

Differential Revision: https://reviews.llvm.org/D62745

llvm-svn: 362857
2019-06-08 00:05:17 +00:00

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//===- SwitchLoweringUtils.cpp - Switch Lowering --------------------------===//
//
// 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 contains switch inst lowering optimizations and utilities for
// codegen, so that it can be used for both SelectionDAG and GlobalISel.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/SwitchLoweringUtils.h"
using namespace llvm;
using namespace SwitchCG;
uint64_t SwitchCG::getJumpTableRange(const CaseClusterVector &Clusters,
unsigned First, unsigned Last) {
assert(Last >= First);
const APInt &LowCase = Clusters[First].Low->getValue();
const APInt &HighCase = Clusters[Last].High->getValue();
assert(LowCase.getBitWidth() == HighCase.getBitWidth());
// FIXME: A range of consecutive cases has 100% density, but only requires one
// comparison to lower. We should discriminate against such consecutive ranges
// in jump tables.
return (HighCase - LowCase).getLimitedValue((UINT64_MAX - 1) / 100) + 1;
}
uint64_t
SwitchCG::getJumpTableNumCases(const SmallVectorImpl<unsigned> &TotalCases,
unsigned First, unsigned Last) {
assert(Last >= First);
assert(TotalCases[Last] >= TotalCases[First]);
uint64_t NumCases =
TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
return NumCases;
}
void SwitchCG::SwitchLowering::findJumpTables(CaseClusterVector &Clusters,
const SwitchInst *SI,
MachineBasicBlock *DefaultMBB) {
#ifndef NDEBUG
// Clusters must be non-empty, sorted, and only contain Range clusters.
assert(!Clusters.empty());
for (CaseCluster &C : Clusters)
assert(C.Kind == CC_Range);
for (unsigned i = 1, e = Clusters.size(); i < e; ++i)
assert(Clusters[i - 1].High->getValue().slt(Clusters[i].Low->getValue()));
#endif
if (!TLI->areJTsAllowed(SI->getParent()->getParent()))
return;
const int64_t N = Clusters.size();
const unsigned MinJumpTableEntries = TLI->getMinimumJumpTableEntries();
const unsigned SmallNumberOfEntries = MinJumpTableEntries / 2;
if (N < 2 || N < MinJumpTableEntries)
return;
// TotalCases[i]: Total nbr of cases in Clusters[0..i].
SmallVector<unsigned, 8> TotalCases(N);
for (unsigned i = 0; i < N; ++i) {
const APInt &Hi = Clusters[i].High->getValue();
const APInt &Lo = Clusters[i].Low->getValue();
TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
if (i != 0)
TotalCases[i] += TotalCases[i - 1];
}
// Cheap case: the whole range may be suitable for jump table.
uint64_t Range = getJumpTableRange(Clusters,0, N - 1);
uint64_t NumCases = getJumpTableNumCases(TotalCases, 0, N - 1);
assert(NumCases < UINT64_MAX / 100);
assert(Range >= NumCases);
if (TLI->isSuitableForJumpTable(SI, NumCases, Range)) {
CaseCluster JTCluster;
if (buildJumpTable(Clusters, 0, N - 1, SI, DefaultMBB, JTCluster)) {
Clusters[0] = JTCluster;
Clusters.resize(1);
return;
}
}
// The algorithm below is not suitable for -O0.
if (TM->getOptLevel() == CodeGenOpt::None)
return;
// Split Clusters into minimum number of dense partitions. The algorithm uses
// the same idea as Kannan & Proebsting "Correction to 'Producing Good Code
// for the Case Statement'" (1994), but builds the MinPartitions array in
// reverse order to make it easier to reconstruct the partitions in ascending
// order. In the choice between two optimal partitionings, it picks the one
// which yields more jump tables.
// MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
SmallVector<unsigned, 8> MinPartitions(N);
// LastElement[i] is the last element of the partition starting at i.
SmallVector<unsigned, 8> LastElement(N);
// PartitionsScore[i] is used to break ties when choosing between two
// partitionings resulting in the same number of partitions.
SmallVector<unsigned, 8> PartitionsScore(N);
// For PartitionsScore, a small number of comparisons is considered as good as
// a jump table and a single comparison is considered better than a jump
// table.
enum PartitionScores : unsigned {
NoTable = 0,
Table = 1,
FewCases = 1,
SingleCase = 2
};
// Base case: There is only one way to partition Clusters[N-1].
MinPartitions[N - 1] = 1;
LastElement[N - 1] = N - 1;
PartitionsScore[N - 1] = PartitionScores::SingleCase;
// Note: loop indexes are signed to avoid underflow.
for (int64_t i = N - 2; i >= 0; i--) {
// Find optimal partitioning of Clusters[i..N-1].
// Baseline: Put Clusters[i] into a partition on its own.
MinPartitions[i] = MinPartitions[i + 1] + 1;
LastElement[i] = i;
PartitionsScore[i] = PartitionsScore[i + 1] + PartitionScores::SingleCase;
// Search for a solution that results in fewer partitions.
for (int64_t j = N - 1; j > i; j--) {
// Try building a partition from Clusters[i..j].
uint64_t Range = getJumpTableRange(Clusters, i, j);
uint64_t NumCases = getJumpTableNumCases(TotalCases, i, j);
assert(NumCases < UINT64_MAX / 100);
assert(Range >= NumCases);
if (TLI->isSuitableForJumpTable(SI, NumCases, Range)) {
unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
unsigned Score = j == N - 1 ? 0 : PartitionsScore[j + 1];
int64_t NumEntries = j - i + 1;
if (NumEntries == 1)
Score += PartitionScores::SingleCase;
else if (NumEntries <= SmallNumberOfEntries)
Score += PartitionScores::FewCases;
else if (NumEntries >= MinJumpTableEntries)
Score += PartitionScores::Table;
// If this leads to fewer partitions, or to the same number of
// partitions with better score, it is a better partitioning.
if (NumPartitions < MinPartitions[i] ||
(NumPartitions == MinPartitions[i] && Score > PartitionsScore[i])) {
MinPartitions[i] = NumPartitions;
LastElement[i] = j;
PartitionsScore[i] = Score;
}
}
}
}
// Iterate over the partitions, replacing some with jump tables in-place.
unsigned DstIndex = 0;
for (unsigned First = 0, Last; First < N; First = Last + 1) {
Last = LastElement[First];
assert(Last >= First);
assert(DstIndex <= First);
unsigned NumClusters = Last - First + 1;
CaseCluster JTCluster;
if (NumClusters >= MinJumpTableEntries &&
buildJumpTable(Clusters, First, Last, SI, DefaultMBB, JTCluster)) {
Clusters[DstIndex++] = JTCluster;
} else {
for (unsigned I = First; I <= Last; ++I)
std::memmove(&Clusters[DstIndex++], &Clusters[I], sizeof(Clusters[I]));
}
}
Clusters.resize(DstIndex);
}
bool SwitchCG::SwitchLowering::buildJumpTable(const CaseClusterVector &Clusters,
unsigned First, unsigned Last,
const SwitchInst *SI,
MachineBasicBlock *DefaultMBB,
CaseCluster &JTCluster) {
assert(First <= Last);
auto Prob = BranchProbability::getZero();
unsigned NumCmps = 0;
std::vector<MachineBasicBlock*> Table;
DenseMap<MachineBasicBlock*, BranchProbability> JTProbs;
// Initialize probabilities in JTProbs.
for (unsigned I = First; I <= Last; ++I)
JTProbs[Clusters[I].MBB] = BranchProbability::getZero();
for (unsigned I = First; I <= Last; ++I) {
assert(Clusters[I].Kind == CC_Range);
Prob += Clusters[I].Prob;
const APInt &Low = Clusters[I].Low->getValue();
const APInt &High = Clusters[I].High->getValue();
NumCmps += (Low == High) ? 1 : 2;
if (I != First) {
// Fill the gap between this and the previous cluster.
const APInt &PreviousHigh = Clusters[I - 1].High->getValue();
assert(PreviousHigh.slt(Low));
uint64_t Gap = (Low - PreviousHigh).getLimitedValue() - 1;
for (uint64_t J = 0; J < Gap; J++)
Table.push_back(DefaultMBB);
}
uint64_t ClusterSize = (High - Low).getLimitedValue() + 1;
for (uint64_t J = 0; J < ClusterSize; ++J)
Table.push_back(Clusters[I].MBB);
JTProbs[Clusters[I].MBB] += Clusters[I].Prob;
}
unsigned NumDests = JTProbs.size();
if (TLI->isSuitableForBitTests(NumDests, NumCmps,
Clusters[First].Low->getValue(),
Clusters[Last].High->getValue(), *DL)) {
// Clusters[First..Last] should be lowered as bit tests instead.
return false;
}
// Create the MBB that will load from and jump through the table.
// Note: We create it here, but it's not inserted into the function yet.
MachineFunction *CurMF = FuncInfo.MF;
MachineBasicBlock *JumpTableMBB =
CurMF->CreateMachineBasicBlock(SI->getParent());
// Add successors. Note: use table order for determinism.
SmallPtrSet<MachineBasicBlock *, 8> Done;
for (MachineBasicBlock *Succ : Table) {
if (Done.count(Succ))
continue;
addSuccessorWithProb(JumpTableMBB, Succ, JTProbs[Succ]);
Done.insert(Succ);
}
JumpTableMBB->normalizeSuccProbs();
unsigned JTI = CurMF->getOrCreateJumpTableInfo(TLI->getJumpTableEncoding())
->createJumpTableIndex(Table);
// Set up the jump table info.
JumpTable JT(-1U, JTI, JumpTableMBB, nullptr);
JumpTableHeader JTH(Clusters[First].Low->getValue(),
Clusters[Last].High->getValue(), SI->getCondition(),
nullptr, false);
JTCases.emplace_back(std::move(JTH), std::move(JT));
JTCluster = CaseCluster::jumpTable(Clusters[First].Low, Clusters[Last].High,
JTCases.size() - 1, Prob);
return true;
}
void SwitchCG::SwitchLowering::findBitTestClusters(CaseClusterVector &Clusters,
const SwitchInst *SI) {
// Partition Clusters into as few subsets as possible, where each subset has a
// range that fits in a machine word and has <= 3 unique destinations.
#ifndef NDEBUG
// Clusters must be sorted and contain Range or JumpTable clusters.
assert(!Clusters.empty());
assert(Clusters[0].Kind == CC_Range || Clusters[0].Kind == CC_JumpTable);
for (const CaseCluster &C : Clusters)
assert(C.Kind == CC_Range || C.Kind == CC_JumpTable);
for (unsigned i = 1; i < Clusters.size(); ++i)
assert(Clusters[i-1].High->getValue().slt(Clusters[i].Low->getValue()));
#endif
// The algorithm below is not suitable for -O0.
if (TM->getOptLevel() == CodeGenOpt::None)
return;
// If target does not have legal shift left, do not emit bit tests at all.
EVT PTy = TLI->getPointerTy(*DL);
if (!TLI->isOperationLegal(ISD::SHL, PTy))
return;
int BitWidth = PTy.getSizeInBits();
const int64_t N = Clusters.size();
// MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
SmallVector<unsigned, 8> MinPartitions(N);
// LastElement[i] is the last element of the partition starting at i.
SmallVector<unsigned, 8> LastElement(N);
// FIXME: This might not be the best algorithm for finding bit test clusters.
// Base case: There is only one way to partition Clusters[N-1].
MinPartitions[N - 1] = 1;
LastElement[N - 1] = N - 1;
// Note: loop indexes are signed to avoid underflow.
for (int64_t i = N - 2; i >= 0; --i) {
// Find optimal partitioning of Clusters[i..N-1].
// Baseline: Put Clusters[i] into a partition on its own.
MinPartitions[i] = MinPartitions[i + 1] + 1;
LastElement[i] = i;
// Search for a solution that results in fewer partitions.
// Note: the search is limited by BitWidth, reducing time complexity.
for (int64_t j = std::min(N - 1, i + BitWidth - 1); j > i; --j) {
// Try building a partition from Clusters[i..j].
// Check the range.
if (!TLI->rangeFitsInWord(Clusters[i].Low->getValue(),
Clusters[j].High->getValue(), *DL))
continue;
// Check nbr of destinations and cluster types.
// FIXME: This works, but doesn't seem very efficient.
bool RangesOnly = true;
BitVector Dests(FuncInfo.MF->getNumBlockIDs());
for (int64_t k = i; k <= j; k++) {
if (Clusters[k].Kind != CC_Range) {
RangesOnly = false;
break;
}
Dests.set(Clusters[k].MBB->getNumber());
}
if (!RangesOnly || Dests.count() > 3)
break;
// Check if it's a better partition.
unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
if (NumPartitions < MinPartitions[i]) {
// Found a better partition.
MinPartitions[i] = NumPartitions;
LastElement[i] = j;
}
}
}
// Iterate over the partitions, replacing with bit-test clusters in-place.
unsigned DstIndex = 0;
for (unsigned First = 0, Last; First < N; First = Last + 1) {
Last = LastElement[First];
assert(First <= Last);
assert(DstIndex <= First);
CaseCluster BitTestCluster;
if (buildBitTests(Clusters, First, Last, SI, BitTestCluster)) {
Clusters[DstIndex++] = BitTestCluster;
} else {
size_t NumClusters = Last - First + 1;
std::memmove(&Clusters[DstIndex], &Clusters[First],
sizeof(Clusters[0]) * NumClusters);
DstIndex += NumClusters;
}
}
Clusters.resize(DstIndex);
}
bool SwitchCG::SwitchLowering::buildBitTests(CaseClusterVector &Clusters,
unsigned First, unsigned Last,
const SwitchInst *SI,
CaseCluster &BTCluster) {
assert(First <= Last);
if (First == Last)
return false;
BitVector Dests(FuncInfo.MF->getNumBlockIDs());
unsigned NumCmps = 0;
for (int64_t I = First; I <= Last; ++I) {
assert(Clusters[I].Kind == CC_Range);
Dests.set(Clusters[I].MBB->getNumber());
NumCmps += (Clusters[I].Low == Clusters[I].High) ? 1 : 2;
}
unsigned NumDests = Dests.count();
APInt Low = Clusters[First].Low->getValue();
APInt High = Clusters[Last].High->getValue();
assert(Low.slt(High));
if (!TLI->isSuitableForBitTests(NumDests, NumCmps, Low, High, *DL))
return false;
APInt LowBound;
APInt CmpRange;
const int BitWidth = TLI->getPointerTy(*DL).getSizeInBits();
assert(TLI->rangeFitsInWord(Low, High, *DL) &&
"Case range must fit in bit mask!");
// Check if the clusters cover a contiguous range such that no value in the
// range will jump to the default statement.
bool ContiguousRange = true;
for (int64_t I = First + 1; I <= Last; ++I) {
if (Clusters[I].Low->getValue() != Clusters[I - 1].High->getValue() + 1) {
ContiguousRange = false;
break;
}
}
if (Low.isStrictlyPositive() && High.slt(BitWidth)) {
// Optimize the case where all the case values fit in a word without having
// to subtract minValue. In this case, we can optimize away the subtraction.
LowBound = APInt::getNullValue(Low.getBitWidth());
CmpRange = High;
ContiguousRange = false;
} else {
LowBound = Low;
CmpRange = High - Low;
}
CaseBitsVector CBV;
auto TotalProb = BranchProbability::getZero();
for (unsigned i = First; i <= Last; ++i) {
// Find the CaseBits for this destination.
unsigned j;
for (j = 0; j < CBV.size(); ++j)
if (CBV[j].BB == Clusters[i].MBB)
break;
if (j == CBV.size())
CBV.push_back(
CaseBits(0, Clusters[i].MBB, 0, BranchProbability::getZero()));
CaseBits *CB = &CBV[j];
// Update Mask, Bits and ExtraProb.
uint64_t Lo = (Clusters[i].Low->getValue() - LowBound).getZExtValue();
uint64_t Hi = (Clusters[i].High->getValue() - LowBound).getZExtValue();
assert(Hi >= Lo && Hi < 64 && "Invalid bit case!");
CB->Mask |= (-1ULL >> (63 - (Hi - Lo))) << Lo;
CB->Bits += Hi - Lo + 1;
CB->ExtraProb += Clusters[i].Prob;
TotalProb += Clusters[i].Prob;
}
BitTestInfo BTI;
llvm::sort(CBV, [](const CaseBits &a, const CaseBits &b) {
// Sort by probability first, number of bits second, bit mask third.
if (a.ExtraProb != b.ExtraProb)
return a.ExtraProb > b.ExtraProb;
if (a.Bits != b.Bits)
return a.Bits > b.Bits;
return a.Mask < b.Mask;
});
for (auto &CB : CBV) {
MachineBasicBlock *BitTestBB =
FuncInfo.MF->CreateMachineBasicBlock(SI->getParent());
BTI.push_back(BitTestCase(CB.Mask, BitTestBB, CB.BB, CB.ExtraProb));
}
BitTestCases.emplace_back(std::move(LowBound), std::move(CmpRange),
SI->getCondition(), -1U, MVT::Other, false,
ContiguousRange, nullptr, nullptr, std::move(BTI),
TotalProb);
BTCluster = CaseCluster::bitTests(Clusters[First].Low, Clusters[Last].High,
BitTestCases.size() - 1, TotalProb);
return true;
}
void SwitchCG::sortAndRangeify(CaseClusterVector &Clusters) {
#ifndef NDEBUG
for (const CaseCluster &CC : Clusters)
assert(CC.Low == CC.High && "Input clusters must be single-case");
#endif
llvm::sort(Clusters, [](const CaseCluster &a, const CaseCluster &b) {
return a.Low->getValue().slt(b.Low->getValue());
});
// Merge adjacent clusters with the same destination.
const unsigned N = Clusters.size();
unsigned DstIndex = 0;
for (unsigned SrcIndex = 0; SrcIndex < N; ++SrcIndex) {
CaseCluster &CC = Clusters[SrcIndex];
const ConstantInt *CaseVal = CC.Low;
MachineBasicBlock *Succ = CC.MBB;
if (DstIndex != 0 && Clusters[DstIndex - 1].MBB == Succ &&
(CaseVal->getValue() - Clusters[DstIndex - 1].High->getValue()) == 1) {
// If this case has the same successor and is a neighbour, merge it into
// the previous cluster.
Clusters[DstIndex - 1].High = CaseVal;
Clusters[DstIndex - 1].Prob += CC.Prob;
} else {
std::memmove(&Clusters[DstIndex++], &Clusters[SrcIndex],
sizeof(Clusters[SrcIndex]));
}
}
Clusters.resize(DstIndex);
}
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