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llvm-mirror/lib/CodeGen/IfConversion.cpp
Krzysztof Parzyszek d1195ef11e Proper handling of diamond-like cases in if-conversion 
If converter was somewhat careless about "diamond" cases, where there
was no join block, or in other words, where the true/false blocks did
not have analyzable branches. In such cases, it was possible for it to
remove (needed) branches, resulting in a loss of entire basic blocks.

Differential Revision: http://reviews.llvm.org/D16156

llvm-svn: 258310
2016-01-20 13:14:52 +00:00

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66 KiB
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//===-- IfConversion.cpp - Machine code if conversion pass. ---------------===//
//
// 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 machine instruction level if-conversion pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/Passes.h"
#include "BranchFolding.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "ifcvt"
// Hidden options for help debugging.
static cl::opt<int> IfCvtFnStart("ifcvt-fn-start", cl::init(-1), cl::Hidden);
static cl::opt<int> IfCvtFnStop("ifcvt-fn-stop", cl::init(-1), cl::Hidden);
static cl::opt<int> IfCvtLimit("ifcvt-limit", cl::init(-1), cl::Hidden);
static cl::opt<bool> DisableSimple("disable-ifcvt-simple",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableSimpleF("disable-ifcvt-simple-false",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableTriangle("disable-ifcvt-triangle",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableTriangleR("disable-ifcvt-triangle-rev",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableTriangleF("disable-ifcvt-triangle-false",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableTriangleFR("disable-ifcvt-triangle-false-rev",
cl::init(false), cl::Hidden);
static cl::opt<bool> DisableDiamond("disable-ifcvt-diamond",
cl::init(false), cl::Hidden);
static cl::opt<bool> IfCvtBranchFold("ifcvt-branch-fold",
cl::init(true), cl::Hidden);
STATISTIC(NumSimple, "Number of simple if-conversions performed");
STATISTIC(NumSimpleFalse, "Number of simple (F) if-conversions performed");
STATISTIC(NumTriangle, "Number of triangle if-conversions performed");
STATISTIC(NumTriangleRev, "Number of triangle (R) if-conversions performed");
STATISTIC(NumTriangleFalse,"Number of triangle (F) if-conversions performed");
STATISTIC(NumTriangleFRev, "Number of triangle (F/R) if-conversions performed");
STATISTIC(NumDiamonds, "Number of diamond if-conversions performed");
STATISTIC(NumIfConvBBs, "Number of if-converted blocks");
STATISTIC(NumDupBBs, "Number of duplicated blocks");
STATISTIC(NumUnpred, "Number of true blocks of diamonds unpredicated");
namespace {
class IfConverter : public MachineFunctionPass {
enum IfcvtKind {
ICNotClassfied, // BB data valid, but not classified.
ICSimpleFalse, // Same as ICSimple, but on the false path.
ICSimple, // BB is entry of an one split, no rejoin sub-CFG.
ICTriangleFRev, // Same as ICTriangleFalse, but false path rev condition.
ICTriangleRev, // Same as ICTriangle, but true path rev condition.
ICTriangleFalse, // Same as ICTriangle, but on the false path.
ICTriangle, // BB is entry of a triangle sub-CFG.
ICDiamond // BB is entry of a diamond sub-CFG.
};
/// BBInfo - One per MachineBasicBlock, this is used to cache the result
/// if-conversion feasibility analysis. This includes results from
/// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
/// classification, and common tail block of its successors (if it's a
/// diamond shape), its size, whether it's predicable, and whether any
/// instruction can clobber the 'would-be' predicate.
///
/// IsDone - True if BB is not to be considered for ifcvt.
/// IsBeingAnalyzed - True if BB is currently being analyzed.
/// IsAnalyzed - True if BB has been analyzed (info is still valid).
/// IsEnqueued - True if BB has been enqueued to be ifcvt'ed.
/// IsBrAnalyzable - True if AnalyzeBranch() returns false.
/// HasFallThrough - True if BB may fallthrough to the following BB.
/// IsUnpredicable - True if BB is known to be unpredicable.
/// ClobbersPred - True if BB could modify predicates (e.g. has
/// cmp, call, etc.)
/// NonPredSize - Number of non-predicated instructions.
/// ExtraCost - Extra cost for multi-cycle instructions.
/// ExtraCost2 - Some instructions are slower when predicated
/// BB - Corresponding MachineBasicBlock.
/// TrueBB / FalseBB- See AnalyzeBranch().
/// BrCond - Conditions for end of block conditional branches.
/// Predicate - Predicate used in the BB.
struct BBInfo {
bool IsDone : 1;
bool IsBeingAnalyzed : 1;
bool IsAnalyzed : 1;
bool IsEnqueued : 1;
bool IsBrAnalyzable : 1;
bool HasFallThrough : 1;
bool IsUnpredicable : 1;
bool CannotBeCopied : 1;
bool ClobbersPred : 1;
unsigned NonPredSize;
unsigned ExtraCost;
unsigned ExtraCost2;
MachineBasicBlock *BB;
MachineBasicBlock *TrueBB;
MachineBasicBlock *FalseBB;
SmallVector<MachineOperand, 4> BrCond;
SmallVector<MachineOperand, 4> Predicate;
BBInfo() : IsDone(false), IsBeingAnalyzed(false),
IsAnalyzed(false), IsEnqueued(false), IsBrAnalyzable(false),
HasFallThrough(false), IsUnpredicable(false),
CannotBeCopied(false), ClobbersPred(false), NonPredSize(0),
ExtraCost(0), ExtraCost2(0), BB(nullptr), TrueBB(nullptr),
FalseBB(nullptr) {}
};
/// IfcvtToken - Record information about pending if-conversions to attempt:
/// BBI - Corresponding BBInfo.
/// Kind - Type of block. See IfcvtKind.
/// NeedSubsumption - True if the to-be-predicated BB has already been
/// predicated.
/// NumDups - Number of instructions that would be duplicated due
/// to this if-conversion. (For diamonds, the number of
/// identical instructions at the beginnings of both
/// paths).
/// NumDups2 - For diamonds, the number of identical instructions
/// at the ends of both paths.
struct IfcvtToken {
BBInfo &BBI;
IfcvtKind Kind;
bool NeedSubsumption;
unsigned NumDups;
unsigned NumDups2;
IfcvtToken(BBInfo &b, IfcvtKind k, bool s, unsigned d, unsigned d2 = 0)
: BBI(b), Kind(k), NeedSubsumption(s), NumDups(d), NumDups2(d2) {}
};
/// BBAnalysis - Results of if-conversion feasibility analysis indexed by
/// basic block number.
std::vector<BBInfo> BBAnalysis;
TargetSchedModel SchedModel;
const TargetLoweringBase *TLI;
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
const MachineBlockFrequencyInfo *MBFI;
const MachineBranchProbabilityInfo *MBPI;
MachineRegisterInfo *MRI;
LivePhysRegs Redefs;
LivePhysRegs DontKill;
bool PreRegAlloc;
bool MadeChange;
int FnNum;
std::function<bool(const Function &)> PredicateFtor;
public:
static char ID;
IfConverter(std::function<bool(const Function &)> Ftor = nullptr)
: MachineFunctionPass(ID), FnNum(-1), PredicateFtor(Ftor) {
initializeIfConverterPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineBranchProbabilityInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
bool ReverseBranchCondition(BBInfo &BBI);
bool ValidSimple(BBInfo &TrueBBI, unsigned &Dups,
BranchProbability Prediction) const;
bool ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
bool FalseBranch, unsigned &Dups,
BranchProbability Prediction) const;
bool ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
unsigned &Dups1, unsigned &Dups2) const;
void ScanInstructions(BBInfo &BBI);
void AnalyzeBlock(MachineBasicBlock *MBB, std::vector<IfcvtToken*> &Tokens);
bool FeasibilityAnalysis(BBInfo &BBI, SmallVectorImpl<MachineOperand> &Cond,
bool isTriangle = false, bool RevBranch = false);
void AnalyzeBlocks(MachineFunction &MF, std::vector<IfcvtToken*> &Tokens);
void InvalidatePreds(MachineBasicBlock *BB);
void RemoveExtraEdges(BBInfo &BBI);
bool IfConvertSimple(BBInfo &BBI, IfcvtKind Kind);
bool IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind);
bool IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
unsigned NumDups1, unsigned NumDups2);
void PredicateBlock(BBInfo &BBI,
MachineBasicBlock::iterator E,
SmallVectorImpl<MachineOperand> &Cond,
SmallSet<unsigned, 4> *LaterRedefs = nullptr);
void CopyAndPredicateBlock(BBInfo &ToBBI, BBInfo &FromBBI,
SmallVectorImpl<MachineOperand> &Cond,
bool IgnoreBr = false);
void MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges = true);
bool MeetIfcvtSizeLimit(MachineBasicBlock &BB,
unsigned Cycle, unsigned Extra,
BranchProbability Prediction) const {
return Cycle > 0 && TII->isProfitableToIfCvt(BB, Cycle, Extra,
Prediction);
}
bool MeetIfcvtSizeLimit(MachineBasicBlock &TBB,
unsigned TCycle, unsigned TExtra,
MachineBasicBlock &FBB,
unsigned FCycle, unsigned FExtra,
BranchProbability Prediction) const {
return TCycle > 0 && FCycle > 0 &&
TII->isProfitableToIfCvt(TBB, TCycle, TExtra, FBB, FCycle, FExtra,
Prediction);
}
// blockAlwaysFallThrough - Block ends without a terminator.
bool blockAlwaysFallThrough(BBInfo &BBI) const {
return BBI.IsBrAnalyzable && BBI.TrueBB == nullptr;
}
// IfcvtTokenCmp - Used to sort if-conversion candidates.
static bool IfcvtTokenCmp(IfcvtToken *C1, IfcvtToken *C2) {
int Incr1 = (C1->Kind == ICDiamond)
? -(int)(C1->NumDups + C1->NumDups2) : (int)C1->NumDups;
int Incr2 = (C2->Kind == ICDiamond)
? -(int)(C2->NumDups + C2->NumDups2) : (int)C2->NumDups;
if (Incr1 > Incr2)
return true;
else if (Incr1 == Incr2) {
// Favors subsumption.
if (!C1->NeedSubsumption && C2->NeedSubsumption)
return true;
else if (C1->NeedSubsumption == C2->NeedSubsumption) {
// Favors diamond over triangle, etc.
if ((unsigned)C1->Kind < (unsigned)C2->Kind)
return true;
else if (C1->Kind == C2->Kind)
return C1->BBI.BB->getNumber() < C2->BBI.BB->getNumber();
}
}
return false;
}
};
char IfConverter::ID = 0;
}
char &llvm::IfConverterID = IfConverter::ID;
INITIALIZE_PASS_BEGIN(IfConverter, "if-converter", "If Converter", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_END(IfConverter, "if-converter", "If Converter", false, false)
bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
if (PredicateFtor && !PredicateFtor(*MF.getFunction()))
return false;
const TargetSubtargetInfo &ST = MF.getSubtarget();
TLI = ST.getTargetLowering();
TII = ST.getInstrInfo();
TRI = ST.getRegisterInfo();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MRI = &MF.getRegInfo();
SchedModel.init(ST.getSchedModel(), &ST, TII);
if (!TII) return false;
PreRegAlloc = MRI->isSSA();
bool BFChange = false;
if (!PreRegAlloc) {
// Tail merge tend to expose more if-conversion opportunities.
BranchFolder BF(true, false, *MBFI, *MBPI);
BFChange = BF.OptimizeFunction(MF, TII, ST.getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>());
}
DEBUG(dbgs() << "\nIfcvt: function (" << ++FnNum << ") \'"
<< MF.getName() << "\'");
if (FnNum < IfCvtFnStart || (IfCvtFnStop != -1 && FnNum > IfCvtFnStop)) {
DEBUG(dbgs() << " skipped\n");
return false;
}
DEBUG(dbgs() << "\n");
MF.RenumberBlocks();
BBAnalysis.resize(MF.getNumBlockIDs());
std::vector<IfcvtToken*> Tokens;
MadeChange = false;
unsigned NumIfCvts = NumSimple + NumSimpleFalse + NumTriangle +
NumTriangleRev + NumTriangleFalse + NumTriangleFRev + NumDiamonds;
while (IfCvtLimit == -1 || (int)NumIfCvts < IfCvtLimit) {
// Do an initial analysis for each basic block and find all the potential
// candidates to perform if-conversion.
bool Change = false;
AnalyzeBlocks(MF, Tokens);
while (!Tokens.empty()) {
IfcvtToken *Token = Tokens.back();
Tokens.pop_back();
BBInfo &BBI = Token->BBI;
IfcvtKind Kind = Token->Kind;
unsigned NumDups = Token->NumDups;
unsigned NumDups2 = Token->NumDups2;
delete Token;
// If the block has been evicted out of the queue or it has already been
// marked dead (due to it being predicated), then skip it.
if (BBI.IsDone)
BBI.IsEnqueued = false;
if (!BBI.IsEnqueued)
continue;
BBI.IsEnqueued = false;
bool RetVal = false;
switch (Kind) {
default: llvm_unreachable("Unexpected!");
case ICSimple:
case ICSimpleFalse: {
bool isFalse = Kind == ICSimpleFalse;
if ((isFalse && DisableSimpleF) || (!isFalse && DisableSimple)) break;
DEBUG(dbgs() << "Ifcvt (Simple" << (Kind == ICSimpleFalse ?
" false" : "")
<< "): BB#" << BBI.BB->getNumber() << " ("
<< ((Kind == ICSimpleFalse)
? BBI.FalseBB->getNumber()
: BBI.TrueBB->getNumber()) << ") ");
RetVal = IfConvertSimple(BBI, Kind);
DEBUG(dbgs() << (RetVal ? "succeeded!" : "failed!") << "\n");
if (RetVal) {
if (isFalse) ++NumSimpleFalse;
else ++NumSimple;
}
break;
}
case ICTriangle:
case ICTriangleRev:
case ICTriangleFalse:
case ICTriangleFRev: {
bool isFalse = Kind == ICTriangleFalse;
bool isRev = (Kind == ICTriangleRev || Kind == ICTriangleFRev);
if (DisableTriangle && !isFalse && !isRev) break;
if (DisableTriangleR && !isFalse && isRev) break;
if (DisableTriangleF && isFalse && !isRev) break;
if (DisableTriangleFR && isFalse && isRev) break;
DEBUG(dbgs() << "Ifcvt (Triangle");
if (isFalse)
DEBUG(dbgs() << " false");
if (isRev)
DEBUG(dbgs() << " rev");
DEBUG(dbgs() << "): BB#" << BBI.BB->getNumber() << " (T:"
<< BBI.TrueBB->getNumber() << ",F:"
<< BBI.FalseBB->getNumber() << ") ");
RetVal = IfConvertTriangle(BBI, Kind);
DEBUG(dbgs() << (RetVal ? "succeeded!" : "failed!") << "\n");
if (RetVal) {
if (isFalse) {
if (isRev) ++NumTriangleFRev;
else ++NumTriangleFalse;
} else {
if (isRev) ++NumTriangleRev;
else ++NumTriangle;
}
}
break;
}
case ICDiamond: {
if (DisableDiamond) break;
DEBUG(dbgs() << "Ifcvt (Diamond): BB#" << BBI.BB->getNumber() << " (T:"
<< BBI.TrueBB->getNumber() << ",F:"
<< BBI.FalseBB->getNumber() << ") ");
RetVal = IfConvertDiamond(BBI, Kind, NumDups, NumDups2);
DEBUG(dbgs() << (RetVal ? "succeeded!" : "failed!") << "\n");
if (RetVal) ++NumDiamonds;
break;
}
}
Change |= RetVal;
NumIfCvts = NumSimple + NumSimpleFalse + NumTriangle + NumTriangleRev +
NumTriangleFalse + NumTriangleFRev + NumDiamonds;
if (IfCvtLimit != -1 && (int)NumIfCvts >= IfCvtLimit)
break;
}
if (!Change)
break;
MadeChange |= Change;
}
// Delete tokens in case of early exit.
while (!Tokens.empty()) {
IfcvtToken *Token = Tokens.back();
Tokens.pop_back();
delete Token;
}
Tokens.clear();
BBAnalysis.clear();
if (MadeChange && IfCvtBranchFold) {
BranchFolder BF(false, false, *MBFI, *MBPI);
BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>());
}
MadeChange |= BFChange;
return MadeChange;
}
/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
/// its 'true' successor.
static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
MachineBasicBlock *TrueBB) {
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
E = BB->succ_end(); SI != E; ++SI) {
MachineBasicBlock *SuccBB = *SI;
if (SuccBB != TrueBB)
return SuccBB;
}
return nullptr;
}
/// ReverseBranchCondition - Reverse the condition of the end of the block
/// branch. Swap block's 'true' and 'false' successors.
bool IfConverter::ReverseBranchCondition(BBInfo &BBI) {
DebugLoc dl; // FIXME: this is nowhere
if (!TII->ReverseBranchCondition(BBI.BrCond)) {
TII->RemoveBranch(*BBI.BB);
TII->InsertBranch(*BBI.BB, BBI.FalseBB, BBI.TrueBB, BBI.BrCond, dl);
std::swap(BBI.TrueBB, BBI.FalseBB);
return true;
}
return false;
}
/// getNextBlock - Returns the next block in the function blocks ordering. If
/// it is the end, returns NULL.
static inline MachineBasicBlock *getNextBlock(MachineBasicBlock *BB) {
MachineFunction::iterator I = BB->getIterator();
MachineFunction::iterator E = BB->getParent()->end();
if (++I == E)
return nullptr;
return &*I;
}
/// ValidSimple - Returns true if the 'true' block (along with its
/// predecessor) forms a valid simple shape for ifcvt. It also returns the
/// number of instructions that the ifcvt would need to duplicate if performed
/// in Dups.
bool IfConverter::ValidSimple(BBInfo &TrueBBI, unsigned &Dups,
BranchProbability Prediction) const {
Dups = 0;
if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone)
return false;
if (TrueBBI.IsBrAnalyzable)
return false;
if (TrueBBI.BB->pred_size() > 1) {
if (TrueBBI.CannotBeCopied ||
!TII->isProfitableToDupForIfCvt(*TrueBBI.BB, TrueBBI.NonPredSize,
Prediction))
return false;
Dups = TrueBBI.NonPredSize;
}
return true;
}
/// ValidTriangle - Returns true if the 'true' and 'false' blocks (along
/// with their common predecessor) forms a valid triangle shape for ifcvt.
/// If 'FalseBranch' is true, it checks if 'true' block's false branch
/// branches to the 'false' block rather than the other way around. It also
/// returns the number of instructions that the ifcvt would need to duplicate
/// if performed in 'Dups'.
bool IfConverter::ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
bool FalseBranch, unsigned &Dups,
BranchProbability Prediction) const {
Dups = 0;
if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone)
return false;
if (TrueBBI.BB->pred_size() > 1) {
if (TrueBBI.CannotBeCopied)
return false;
unsigned Size = TrueBBI.NonPredSize;
if (TrueBBI.IsBrAnalyzable) {
if (TrueBBI.TrueBB && TrueBBI.BrCond.empty())
// Ends with an unconditional branch. It will be removed.
--Size;
else {
MachineBasicBlock *FExit = FalseBranch
? TrueBBI.TrueBB : TrueBBI.FalseBB;
if (FExit)
// Require a conditional branch
++Size;
}
}
if (!TII->isProfitableToDupForIfCvt(*TrueBBI.BB, Size, Prediction))
return false;
Dups = Size;
}
MachineBasicBlock *TExit = FalseBranch ? TrueBBI.FalseBB : TrueBBI.TrueBB;
if (!TExit && blockAlwaysFallThrough(TrueBBI)) {
MachineFunction::iterator I = TrueBBI.BB->getIterator();
if (++I == TrueBBI.BB->getParent()->end())
return false;
TExit = &*I;
}
return TExit && TExit == FalseBBI.BB;
}
/// ValidDiamond - Returns true if the 'true' and 'false' blocks (along
/// with their common predecessor) forms a valid diamond shape for ifcvt.
bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
unsigned &Dups1, unsigned &Dups2) const {
Dups1 = Dups2 = 0;
if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone ||
FalseBBI.IsBeingAnalyzed || FalseBBI.IsDone)
return false;
MachineBasicBlock *TT = TrueBBI.TrueBB;
MachineBasicBlock *FT = FalseBBI.TrueBB;
if (!TT && blockAlwaysFallThrough(TrueBBI))
TT = getNextBlock(TrueBBI.BB);
if (!FT && blockAlwaysFallThrough(FalseBBI))
FT = getNextBlock(FalseBBI.BB);
if (TT != FT)
return false;
if (!TT && (TrueBBI.IsBrAnalyzable || FalseBBI.IsBrAnalyzable))
return false;
if (TrueBBI.BB->pred_size() > 1 || FalseBBI.BB->pred_size() > 1)
return false;
// FIXME: Allow true block to have an early exit?
if (TrueBBI.FalseBB || FalseBBI.FalseBB ||
(TrueBBI.ClobbersPred && FalseBBI.ClobbersPred))
return false;
// Count duplicate instructions at the beginning of the true and false blocks.
MachineBasicBlock::iterator TIB = TrueBBI.BB->begin();
MachineBasicBlock::iterator FIB = FalseBBI.BB->begin();
MachineBasicBlock::iterator TIE = TrueBBI.BB->end();
MachineBasicBlock::iterator FIE = FalseBBI.BB->end();
while (TIB != TIE && FIB != FIE) {
// Skip dbg_value instructions. These do not count.
if (TIB->isDebugValue()) {
while (TIB != TIE && TIB->isDebugValue())
++TIB;
if (TIB == TIE)
break;
}
if (FIB->isDebugValue()) {
while (FIB != FIE && FIB->isDebugValue())
++FIB;
if (FIB == FIE)
break;
}
if (!TIB->isIdenticalTo(FIB))
break;
++Dups1;
++TIB;
++FIB;
}
// Now, in preparation for counting duplicate instructions at the ends of the
// blocks, move the end iterators up past any branch instructions.
// If both blocks are returning don't skip the branches, since they will
// likely be both identical return instructions. In such cases the return
// can be left unpredicated.
// Check for already containing all of the block.
if (TIB == TIE || FIB == FIE)
return true;
--TIE;
--FIE;
if (!TrueBBI.BB->succ_empty() || !FalseBBI.BB->succ_empty()) {
while (TIE != TIB && TIE->isBranch())
--TIE;
while (FIE != FIB && FIE->isBranch())
--FIE;
}
// If Dups1 includes all of a block, then don't count duplicate
// instructions at the end of the blocks.
if (TIB == TIE || FIB == FIE)
return true;
// Count duplicate instructions at the ends of the blocks.
while (TIE != TIB && FIE != FIB) {
// Skip dbg_value instructions. These do not count.
if (TIE->isDebugValue()) {
while (TIE != TIB && TIE->isDebugValue())
--TIE;
if (TIE == TIB)
break;
}
if (FIE->isDebugValue()) {
while (FIE != FIB && FIE->isDebugValue())
--FIE;
if (FIE == FIB)
break;
}
if (!TIE->isIdenticalTo(FIE))
break;
++Dups2;
--TIE;
--FIE;
}
return true;
}
/// ScanInstructions - Scan all the instructions in the block to determine if
/// the block is predicable. In most cases, that means all the instructions
/// in the block are isPredicable(). Also checks if the block contains any
/// instruction which can clobber a predicate (e.g. condition code register).
/// If so, the block is not predicable unless it's the last instruction.
void IfConverter::ScanInstructions(BBInfo &BBI) {
if (BBI.IsDone)
return;
bool AlreadyPredicated = !BBI.Predicate.empty();
// First analyze the end of BB branches.
BBI.TrueBB = BBI.FalseBB = nullptr;
BBI.BrCond.clear();
BBI.IsBrAnalyzable =
!TII->AnalyzeBranch(*BBI.BB, BBI.TrueBB, BBI.FalseBB, BBI.BrCond);
BBI.HasFallThrough = BBI.IsBrAnalyzable && BBI.FalseBB == nullptr;
if (BBI.BrCond.size()) {
// No false branch. This BB must end with a conditional branch and a
// fallthrough.
if (!BBI.FalseBB)
BBI.FalseBB = findFalseBlock(BBI.BB, BBI.TrueBB);
if (!BBI.FalseBB) {
// Malformed bcc? True and false blocks are the same?
BBI.IsUnpredicable = true;
return;
}
}
// Then scan all the instructions.
BBI.NonPredSize = 0;
BBI.ExtraCost = 0;
BBI.ExtraCost2 = 0;
BBI.ClobbersPred = false;
for (MachineBasicBlock::iterator I = BBI.BB->begin(), E = BBI.BB->end();
I != E; ++I) {
if (I->isDebugValue())
continue;
if (I->isNotDuplicable())
BBI.CannotBeCopied = true;
bool isPredicated = TII->isPredicated(I);
bool isCondBr = BBI.IsBrAnalyzable && I->isConditionalBranch();
// A conditional branch is not predicable, but it may be eliminated.
if (isCondBr)
continue;
if (!isPredicated) {
BBI.NonPredSize++;
unsigned ExtraPredCost = TII->getPredicationCost(&*I);
unsigned NumCycles = SchedModel.computeInstrLatency(&*I, false);
if (NumCycles > 1)
BBI.ExtraCost += NumCycles-1;
BBI.ExtraCost2 += ExtraPredCost;
} else if (!AlreadyPredicated) {
// FIXME: This instruction is already predicated before the
// if-conversion pass. It's probably something like a conditional move.
// Mark this block unpredicable for now.
BBI.IsUnpredicable = true;
return;
}
if (BBI.ClobbersPred && !isPredicated) {
// Predicate modification instruction should end the block (except for
// already predicated instructions and end of block branches).
// Predicate may have been modified, the subsequent (currently)
// unpredicated instructions cannot be correctly predicated.
BBI.IsUnpredicable = true;
return;
}
// FIXME: Make use of PredDefs? e.g. ADDC, SUBC sets predicates but are
// still potentially predicable.
std::vector<MachineOperand> PredDefs;
if (TII->DefinesPredicate(I, PredDefs))
BBI.ClobbersPred = true;
if (!TII->isPredicable(I)) {
BBI.IsUnpredicable = true;
return;
}
}
}
/// FeasibilityAnalysis - Determine if the block is a suitable candidate to be
/// predicated by the specified predicate.
bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
SmallVectorImpl<MachineOperand> &Pred,
bool isTriangle, bool RevBranch) {
// If the block is dead or unpredicable, then it cannot be predicated.
if (BBI.IsDone || BBI.IsUnpredicable)
return false;
// If it is already predicated but we couldn't analyze its terminator, the
// latter might fallthrough, but we can't determine where to.
// Conservatively avoid if-converting again.
if (BBI.Predicate.size() && !BBI.IsBrAnalyzable)
return false;
// If it is already predicated, check if the new predicate subsumes
// its predicate.
if (BBI.Predicate.size() && !TII->SubsumesPredicate(Pred, BBI.Predicate))
return false;
if (BBI.BrCond.size()) {
if (!isTriangle)
return false;
// Test predicate subsumption.
SmallVector<MachineOperand, 4> RevPred(Pred.begin(), Pred.end());
SmallVector<MachineOperand, 4> Cond(BBI.BrCond.begin(), BBI.BrCond.end());
if (RevBranch) {
if (TII->ReverseBranchCondition(Cond))
return false;
}
if (TII->ReverseBranchCondition(RevPred) ||
!TII->SubsumesPredicate(Cond, RevPred))
return false;
}
return true;
}
/// AnalyzeBlock - Analyze the structure of the sub-CFG starting from
/// the specified block. Record its successors and whether it looks like an
/// if-conversion candidate.
void IfConverter::AnalyzeBlock(MachineBasicBlock *MBB,
std::vector<IfcvtToken*> &Tokens) {
struct BBState {
BBState(MachineBasicBlock *BB) : MBB(BB), SuccsAnalyzed(false) {}
MachineBasicBlock *MBB;
/// This flag is true if MBB's successors have been analyzed.
bool SuccsAnalyzed;
};
// Push MBB to the stack.
SmallVector<BBState, 16> BBStack(1, MBB);
while (!BBStack.empty()) {
BBState &State = BBStack.back();
MachineBasicBlock *BB = State.MBB;
BBInfo &BBI = BBAnalysis[BB->getNumber()];
if (!State.SuccsAnalyzed) {
if (BBI.IsAnalyzed || BBI.IsBeingAnalyzed) {
BBStack.pop_back();
continue;
}
BBI.BB = BB;
BBI.IsBeingAnalyzed = true;
ScanInstructions(BBI);
// Unanalyzable or ends with fallthrough or unconditional branch, or if is
// not considered for ifcvt anymore.
if (!BBI.IsBrAnalyzable || BBI.BrCond.empty() || BBI.IsDone) {
BBI.IsBeingAnalyzed = false;
BBI.IsAnalyzed = true;
BBStack.pop_back();
continue;
}
// Do not ifcvt if either path is a back edge to the entry block.
if (BBI.TrueBB == BB || BBI.FalseBB == BB) {
BBI.IsBeingAnalyzed = false;
BBI.IsAnalyzed = true;
BBStack.pop_back();
continue;
}
// Do not ifcvt if true and false fallthrough blocks are the same.
if (!BBI.FalseBB) {
BBI.IsBeingAnalyzed = false;
BBI.IsAnalyzed = true;
BBStack.pop_back();
continue;
}
// Push the False and True blocks to the stack.
State.SuccsAnalyzed = true;
BBStack.push_back(BBI.FalseBB);
BBStack.push_back(BBI.TrueBB);
continue;
}
BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
if (TrueBBI.IsDone && FalseBBI.IsDone) {
BBI.IsBeingAnalyzed = false;
BBI.IsAnalyzed = true;
BBStack.pop_back();
continue;
}
SmallVector<MachineOperand, 4>
RevCond(BBI.BrCond.begin(), BBI.BrCond.end());
bool CanRevCond = !TII->ReverseBranchCondition(RevCond);
unsigned Dups = 0;
unsigned Dups2 = 0;
bool TNeedSub = !TrueBBI.Predicate.empty();
bool FNeedSub = !FalseBBI.Predicate.empty();
bool Enqueued = false;
BranchProbability Prediction = MBPI->getEdgeProbability(BB, TrueBBI.BB);
if (CanRevCond && ValidDiamond(TrueBBI, FalseBBI, Dups, Dups2) &&
MeetIfcvtSizeLimit(*TrueBBI.BB, (TrueBBI.NonPredSize - (Dups + Dups2) +
TrueBBI.ExtraCost), TrueBBI.ExtraCost2,
*FalseBBI.BB, (FalseBBI.NonPredSize - (Dups + Dups2) +
FalseBBI.ExtraCost),FalseBBI.ExtraCost2,
Prediction) &&
FeasibilityAnalysis(TrueBBI, BBI.BrCond) &&
FeasibilityAnalysis(FalseBBI, RevCond)) {
// Diamond:
// EBB
// / \_
// | |
// TBB FBB
// \ /
// TailBB
// Note TailBB can be empty.
Tokens.push_back(new IfcvtToken(BBI, ICDiamond, TNeedSub|FNeedSub, Dups,
Dups2));
Enqueued = true;
}
if (ValidTriangle(TrueBBI, FalseBBI, false, Dups, Prediction) &&
MeetIfcvtSizeLimit(*TrueBBI.BB, TrueBBI.NonPredSize + TrueBBI.ExtraCost,
TrueBBI.ExtraCost2, Prediction) &&
FeasibilityAnalysis(TrueBBI, BBI.BrCond, true)) {
// Triangle:
// EBB
// | \_
// | |
// | TBB
// | /
// FBB
Tokens.push_back(new IfcvtToken(BBI, ICTriangle, TNeedSub, Dups));
Enqueued = true;
}
if (ValidTriangle(TrueBBI, FalseBBI, true, Dups, Prediction) &&
MeetIfcvtSizeLimit(*TrueBBI.BB, TrueBBI.NonPredSize + TrueBBI.ExtraCost,
TrueBBI.ExtraCost2, Prediction) &&
FeasibilityAnalysis(TrueBBI, BBI.BrCond, true, true)) {
Tokens.push_back(new IfcvtToken(BBI, ICTriangleRev, TNeedSub, Dups));
Enqueued = true;
}
if (ValidSimple(TrueBBI, Dups, Prediction) &&
MeetIfcvtSizeLimit(*TrueBBI.BB, TrueBBI.NonPredSize + TrueBBI.ExtraCost,
TrueBBI.ExtraCost2, Prediction) &&
FeasibilityAnalysis(TrueBBI, BBI.BrCond)) {
// Simple (split, no rejoin):
// EBB
// | \_
// | |
// | TBB---> exit
// |
// FBB
Tokens.push_back(new IfcvtToken(BBI, ICSimple, TNeedSub, Dups));
Enqueued = true;
}
if (CanRevCond) {
// Try the other path...
if (ValidTriangle(FalseBBI, TrueBBI, false, Dups,
Prediction.getCompl()) &&
MeetIfcvtSizeLimit(*FalseBBI.BB,
FalseBBI.NonPredSize + FalseBBI.ExtraCost,
FalseBBI.ExtraCost2, Prediction.getCompl()) &&
FeasibilityAnalysis(FalseBBI, RevCond, true)) {
Tokens.push_back(new IfcvtToken(BBI, ICTriangleFalse, FNeedSub, Dups));
Enqueued = true;
}
if (ValidTriangle(FalseBBI, TrueBBI, true, Dups,
Prediction.getCompl()) &&
MeetIfcvtSizeLimit(*FalseBBI.BB,
FalseBBI.NonPredSize + FalseBBI.ExtraCost,
FalseBBI.ExtraCost2, Prediction.getCompl()) &&
FeasibilityAnalysis(FalseBBI, RevCond, true, true)) {
Tokens.push_back(new IfcvtToken(BBI, ICTriangleFRev, FNeedSub, Dups));
Enqueued = true;
}
if (ValidSimple(FalseBBI, Dups, Prediction.getCompl()) &&
MeetIfcvtSizeLimit(*FalseBBI.BB,
FalseBBI.NonPredSize + FalseBBI.ExtraCost,
FalseBBI.ExtraCost2, Prediction.getCompl()) &&
FeasibilityAnalysis(FalseBBI, RevCond)) {
Tokens.push_back(new IfcvtToken(BBI, ICSimpleFalse, FNeedSub, Dups));
Enqueued = true;
}
}
BBI.IsEnqueued = Enqueued;
BBI.IsBeingAnalyzed = false;
BBI.IsAnalyzed = true;
BBStack.pop_back();
}
}
/// AnalyzeBlocks - Analyze all blocks and find entries for all if-conversion
/// candidates.
void IfConverter::AnalyzeBlocks(MachineFunction &MF,
std::vector<IfcvtToken*> &Tokens) {
for (auto &BB : MF)
AnalyzeBlock(&BB, Tokens);
// Sort to favor more complex ifcvt scheme.
std::stable_sort(Tokens.begin(), Tokens.end(), IfcvtTokenCmp);
}
/// canFallThroughTo - Returns true either if ToBB is the next block after BB or
/// that all the intervening blocks are empty (given BB can fall through to its
/// next block).
static bool canFallThroughTo(MachineBasicBlock *BB, MachineBasicBlock *ToBB) {
MachineFunction::iterator PI = BB->getIterator();
MachineFunction::iterator I = std::next(PI);
MachineFunction::iterator TI = ToBB->getIterator();
MachineFunction::iterator E = BB->getParent()->end();
while (I != TI) {
// Check isSuccessor to avoid case where the next block is empty, but
// it's not a successor.
if (I == E || !I->empty() || !PI->isSuccessor(&*I))
return false;
PI = I++;
}
return true;
}
/// InvalidatePreds - Invalidate predecessor BB info so it would be re-analyzed
/// to determine if it can be if-converted. If predecessor is already enqueued,
/// dequeue it!
void IfConverter::InvalidatePreds(MachineBasicBlock *BB) {
for (const auto &Predecessor : BB->predecessors()) {
BBInfo &PBBI = BBAnalysis[Predecessor->getNumber()];
if (PBBI.IsDone || PBBI.BB == BB)
continue;
PBBI.IsAnalyzed = false;
PBBI.IsEnqueued = false;
}
}
/// InsertUncondBranch - Inserts an unconditional branch from BB to ToBB.
///
static void InsertUncondBranch(MachineBasicBlock *BB, MachineBasicBlock *ToBB,
const TargetInstrInfo *TII) {
DebugLoc dl; // FIXME: this is nowhere
SmallVector<MachineOperand, 0> NoCond;
TII->InsertBranch(*BB, ToBB, nullptr, NoCond, dl);
}
/// RemoveExtraEdges - Remove true / false edges if either / both are no longer
/// successors.
void IfConverter::RemoveExtraEdges(BBInfo &BBI) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(*BBI.BB, TBB, FBB, Cond))
BBI.BB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
}
/// Behaves like LiveRegUnits::StepForward() but also adds implicit uses to all
/// values defined in MI which are not live/used by MI.
static void UpdatePredRedefs(MachineInstr *MI, LivePhysRegs &Redefs) {
SmallVector<std::pair<unsigned, const MachineOperand*>, 4> Clobbers;
Redefs.stepForward(*MI, Clobbers);
// Now add the implicit uses for each of the clobbered values.
for (auto Reg : Clobbers) {
// FIXME: Const cast here is nasty, but better than making StepForward
// take a mutable instruction instead of const.
MachineOperand &Op = const_cast<MachineOperand&>(*Reg.second);
MachineInstr *OpMI = Op.getParent();
MachineInstrBuilder MIB(*OpMI->getParent()->getParent(), OpMI);
if (Op.isRegMask()) {
// First handle regmasks. They clobber any entries in the mask which
// means that we need a def for those registers.
MIB.addReg(Reg.first, RegState::Implicit | RegState::Undef);
// We also need to add an implicit def of this register for the later
// use to read from.
// For the register allocator to have allocated a register clobbered
// by the call which is used later, it must be the case that
// the call doesn't return.
MIB.addReg(Reg.first, RegState::Implicit | RegState::Define);
continue;
}
assert(Op.isReg() && "Register operand required");
if (Op.isDead()) {
// If we found a dead def, but it needs to be live, then remove the dead
// flag.
if (Redefs.contains(Op.getReg()))
Op.setIsDead(false);
}
MIB.addReg(Reg.first, RegState::Implicit | RegState::Undef);
}
}
/**
* Remove kill flags from operands with a registers in the @p DontKill set.
*/
static void RemoveKills(MachineInstr &MI, const LivePhysRegs &DontKill) {
for (MIBundleOperands O(&MI); O.isValid(); ++O) {
if (!O->isReg() || !O->isKill())
continue;
if (DontKill.contains(O->getReg()))
O->setIsKill(false);
}
}
/**
* Walks a range of machine instructions and removes kill flags for registers
* in the @p DontKill set.
*/
static void RemoveKills(MachineBasicBlock::iterator I,
MachineBasicBlock::iterator E,
const LivePhysRegs &DontKill,
const MCRegisterInfo &MCRI) {
for ( ; I != E; ++I)
RemoveKills(*I, DontKill);
}
/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
///
bool IfConverter::IfConvertSimple(BBInfo &BBI, IfcvtKind Kind) {
BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
BBInfo *CvtBBI = &TrueBBI;
BBInfo *NextBBI = &FalseBBI;
SmallVector<MachineOperand, 4> Cond(BBI.BrCond.begin(), BBI.BrCond.end());
if (Kind == ICSimpleFalse)
std::swap(CvtBBI, NextBBI);
if (CvtBBI->IsDone ||
(CvtBBI->CannotBeCopied && CvtBBI->BB->pred_size() > 1)) {
// Something has changed. It's no longer safe to predicate this block.
BBI.IsAnalyzed = false;
CvtBBI->IsAnalyzed = false;
return false;
}
if (CvtBBI->BB->hasAddressTaken())
// Conservatively abort if-conversion if BB's address is taken.
return false;
if (Kind == ICSimpleFalse)
if (TII->ReverseBranchCondition(Cond))
llvm_unreachable("Unable to reverse branch condition!");
// Initialize liveins to the first BB. These are potentiall redefined by
// predicated instructions.
Redefs.init(TRI);
Redefs.addLiveIns(CvtBBI->BB);
Redefs.addLiveIns(NextBBI->BB);
// Compute a set of registers which must not be killed by instructions in
// BB1: This is everything live-in to BB2.
DontKill.init(TRI);
DontKill.addLiveIns(NextBBI->BB);
if (CvtBBI->BB->pred_size() > 1) {
BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
// Copy instructions in the true block, predicate them, and add them to
// the entry block.
CopyAndPredicateBlock(BBI, *CvtBBI, Cond);
// RemoveExtraEdges won't work if the block has an unanalyzable branch, so
// explicitly remove CvtBBI as a successor.
BBI.BB->removeSuccessor(CvtBBI->BB, true);
} else {
RemoveKills(CvtBBI->BB->begin(), CvtBBI->BB->end(), DontKill, *TRI);
PredicateBlock(*CvtBBI, CvtBBI->BB->end(), Cond);
// Merge converted block into entry block.
BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
MergeBlocks(BBI, *CvtBBI);
}
bool IterIfcvt = true;
if (!canFallThroughTo(BBI.BB, NextBBI->BB)) {
InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
BBI.HasFallThrough = false;
// Now ifcvt'd block will look like this:
// BB:
// ...
// t, f = cmp
// if t op
// b BBf
//
// We cannot further ifcvt this block because the unconditional branch
// will have to be predicated on the new condition, that will not be
// available if cmp executes.
IterIfcvt = false;
}
RemoveExtraEdges(BBI);
// Update block info. BB can be iteratively if-converted.
if (!IterIfcvt)
BBI.IsDone = true;
InvalidatePreds(BBI.BB);
CvtBBI->IsDone = true;
// FIXME: Must maintain LiveIns.
return true;
}
/// IfConvertTriangle - If convert a triangle sub-CFG.
///
bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
BBInfo *CvtBBI = &TrueBBI;
BBInfo *NextBBI = &FalseBBI;
DebugLoc dl; // FIXME: this is nowhere
SmallVector<MachineOperand, 4> Cond(BBI.BrCond.begin(), BBI.BrCond.end());
if (Kind == ICTriangleFalse || Kind == ICTriangleFRev)
std::swap(CvtBBI, NextBBI);
if (CvtBBI->IsDone ||
(CvtBBI->CannotBeCopied && CvtBBI->BB->pred_size() > 1)) {
// Something has changed. It's no longer safe to predicate this block.
BBI.IsAnalyzed = false;
CvtBBI->IsAnalyzed = false;
return false;
}
if (CvtBBI->BB->hasAddressTaken())
// Conservatively abort if-conversion if BB's address is taken.
return false;
if (Kind == ICTriangleFalse || Kind == ICTriangleFRev)
if (TII->ReverseBranchCondition(Cond))
llvm_unreachable("Unable to reverse branch condition!");
if (Kind == ICTriangleRev || Kind == ICTriangleFRev) {
if (ReverseBranchCondition(*CvtBBI)) {
// BB has been changed, modify its predecessors (except for this
// one) so they don't get ifcvt'ed based on bad intel.
for (MachineBasicBlock::pred_iterator PI = CvtBBI->BB->pred_begin(),
E = CvtBBI->BB->pred_end(); PI != E; ++PI) {
MachineBasicBlock *PBB = *PI;
if (PBB == BBI.BB)
continue;
BBInfo &PBBI = BBAnalysis[PBB->getNumber()];
if (PBBI.IsEnqueued) {
PBBI.IsAnalyzed = false;
PBBI.IsEnqueued = false;
}
}
}
}
// Initialize liveins to the first BB. These are potentially redefined by
// predicated instructions.
Redefs.init(TRI);
Redefs.addLiveIns(CvtBBI->BB);
Redefs.addLiveIns(NextBBI->BB);
DontKill.clear();
bool HasEarlyExit = CvtBBI->FalseBB != nullptr;
BranchProbability CvtNext, CvtFalse, BBNext, BBCvt;
if (HasEarlyExit) {
// Get probabilities before modifying CvtBBI->BB and BBI.BB.
CvtNext = MBPI->getEdgeProbability(CvtBBI->BB, NextBBI->BB);
CvtFalse = MBPI->getEdgeProbability(CvtBBI->BB, CvtBBI->FalseBB);
BBNext = MBPI->getEdgeProbability(BBI.BB, NextBBI->BB);
BBCvt = MBPI->getEdgeProbability(BBI.BB, CvtBBI->BB);
}
if (CvtBBI->BB->pred_size() > 1) {
BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
// Copy instructions in the true block, predicate them, and add them to
// the entry block.
CopyAndPredicateBlock(BBI, *CvtBBI, Cond, true);
// RemoveExtraEdges won't work if the block has an unanalyzable branch, so
// explicitly remove CvtBBI as a successor.
BBI.BB->removeSuccessor(CvtBBI->BB, true);
} else {
// Predicate the 'true' block after removing its branch.
CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
PredicateBlock(*CvtBBI, CvtBBI->BB->end(), Cond);
// Now merge the entry of the triangle with the true block.
BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
MergeBlocks(BBI, *CvtBBI, false);
}
// If 'true' block has a 'false' successor, add an exit branch to it.
if (HasEarlyExit) {
SmallVector<MachineOperand, 4> RevCond(CvtBBI->BrCond.begin(),
CvtBBI->BrCond.end());
if (TII->ReverseBranchCondition(RevCond))
llvm_unreachable("Unable to reverse branch condition!");
// Update the edge probability for both CvtBBI->FalseBB and NextBBI.
// NewNext = New_Prob(BBI.BB, NextBBI->BB) =
// Prob(BBI.BB, NextBBI->BB) +
// Prob(BBI.BB, CvtBBI->BB) * Prob(CvtBBI->BB, NextBBI->BB)
// NewFalse = New_Prob(BBI.BB, CvtBBI->FalseBB) =
// Prob(BBI.BB, CvtBBI->BB) * Prob(CvtBBI->BB, CvtBBI->FalseBB)
auto NewTrueBB = getNextBlock(BBI.BB);
auto NewNext = BBNext + BBCvt * CvtNext;
auto NewTrueBBIter =
std::find(BBI.BB->succ_begin(), BBI.BB->succ_end(), NewTrueBB);
if (NewTrueBBIter != BBI.BB->succ_end())
BBI.BB->setSuccProbability(NewTrueBBIter, NewNext);
auto NewFalse = BBCvt * CvtFalse;
TII->InsertBranch(*BBI.BB, CvtBBI->FalseBB, nullptr, RevCond, dl);
BBI.BB->addSuccessor(CvtBBI->FalseBB, NewFalse);
}
// Merge in the 'false' block if the 'false' block has no other
// predecessors. Otherwise, add an unconditional branch to 'false'.
bool FalseBBDead = false;
bool IterIfcvt = true;
bool isFallThrough = canFallThroughTo(BBI.BB, NextBBI->BB);
if (!isFallThrough) {
// Only merge them if the true block does not fallthrough to the false
// block. By not merging them, we make it possible to iteratively
// ifcvt the blocks.
if (!HasEarlyExit &&
NextBBI->BB->pred_size() == 1 && !NextBBI->HasFallThrough &&
!NextBBI->BB->hasAddressTaken()) {
MergeBlocks(BBI, *NextBBI);
FalseBBDead = true;
} else {
InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
BBI.HasFallThrough = false;
}
// Mixed predicated and unpredicated code. This cannot be iteratively
// predicated.
IterIfcvt = false;
}
RemoveExtraEdges(BBI);
// Update block info. BB can be iteratively if-converted.
if (!IterIfcvt)
BBI.IsDone = true;
InvalidatePreds(BBI.BB);
CvtBBI->IsDone = true;
if (FalseBBDead)
NextBBI->IsDone = true;
// FIXME: Must maintain LiveIns.
return true;
}
/// IfConvertDiamond - If convert a diamond sub-CFG.
///
bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
unsigned NumDups1, unsigned NumDups2) {
BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
MachineBasicBlock *TailBB = TrueBBI.TrueBB;
// True block must fall through or end with an unanalyzable terminator.
if (!TailBB) {
if (blockAlwaysFallThrough(TrueBBI))
TailBB = FalseBBI.TrueBB;
assert((TailBB || !TrueBBI.IsBrAnalyzable) && "Unexpected!");
}
if (TrueBBI.IsDone || FalseBBI.IsDone ||
TrueBBI.BB->pred_size() > 1 ||
FalseBBI.BB->pred_size() > 1) {
// Something has changed. It's no longer safe to predicate these blocks.
BBI.IsAnalyzed = false;
TrueBBI.IsAnalyzed = false;
FalseBBI.IsAnalyzed = false;
return false;
}
if (TrueBBI.BB->hasAddressTaken() || FalseBBI.BB->hasAddressTaken())
// Conservatively abort if-conversion if either BB has its address taken.
return false;
// Put the predicated instructions from the 'true' block before the
// instructions from the 'false' block, unless the true block would clobber
// the predicate, in which case, do the opposite.
BBInfo *BBI1 = &TrueBBI;
BBInfo *BBI2 = &FalseBBI;
SmallVector<MachineOperand, 4> RevCond(BBI.BrCond.begin(), BBI.BrCond.end());
if (TII->ReverseBranchCondition(RevCond))
llvm_unreachable("Unable to reverse branch condition!");
SmallVector<MachineOperand, 4> *Cond1 = &BBI.BrCond;
SmallVector<MachineOperand, 4> *Cond2 = &RevCond;
// Figure out the more profitable ordering.
bool DoSwap = false;
if (TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred)
DoSwap = true;
else if (TrueBBI.ClobbersPred == FalseBBI.ClobbersPred) {
if (TrueBBI.NonPredSize > FalseBBI.NonPredSize)
DoSwap = true;
}
if (DoSwap) {
std::swap(BBI1, BBI2);
std::swap(Cond1, Cond2);
}
// Remove the conditional branch from entry to the blocks.
BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
// Initialize liveins to the first BB. These are potentially redefined by
// predicated instructions.
Redefs.init(TRI);
Redefs.addLiveIns(BBI1->BB);
// Remove the duplicated instructions at the beginnings of both paths.
// Skip dbg_value instructions
MachineBasicBlock::iterator DI1 = BBI1->BB->getFirstNonDebugInstr();
MachineBasicBlock::iterator DI2 = BBI2->BB->getFirstNonDebugInstr();
BBI1->NonPredSize -= NumDups1;
BBI2->NonPredSize -= NumDups1;
// Skip past the dups on each side separately since there may be
// differing dbg_value entries.
for (unsigned i = 0; i < NumDups1; ++DI1) {
if (!DI1->isDebugValue())
++i;
}
while (NumDups1 != 0) {
++DI2;
if (!DI2->isDebugValue())
--NumDups1;
}
// Compute a set of registers which must not be killed by instructions in BB1:
// This is everything used+live in BB2 after the duplicated instructions. We
// can compute this set by simulating liveness backwards from the end of BB2.
DontKill.init(TRI);
for (MachineBasicBlock::reverse_iterator I = BBI2->BB->rbegin(),
E = MachineBasicBlock::reverse_iterator(DI2); I != E; ++I) {
DontKill.stepBackward(*I);
}
for (MachineBasicBlock::const_iterator I = BBI1->BB->begin(), E = DI1; I != E;
++I) {
SmallVector<std::pair<unsigned, const MachineOperand*>, 4> IgnoredClobbers;
Redefs.stepForward(*I, IgnoredClobbers);
}
BBI.BB->splice(BBI.BB->end(), BBI1->BB, BBI1->BB->begin(), DI1);
BBI2->BB->erase(BBI2->BB->begin(), DI2);
// Remove branch from the 'true' block, unless it was not analyzable.
// Non-analyzable branches need to be preserved, since in such cases,
// the CFG structure is not an actual diamond (the join block may not
// be present).
if (BBI1->IsBrAnalyzable)
BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
// Remove duplicated instructions.
DI1 = BBI1->BB->end();
for (unsigned i = 0; i != NumDups2; ) {
// NumDups2 only counted non-dbg_value instructions, so this won't
// run off the head of the list.
assert (DI1 != BBI1->BB->begin());
--DI1;
// skip dbg_value instructions
if (!DI1->isDebugValue())
++i;
}
BBI1->BB->erase(DI1, BBI1->BB->end());
// Kill flags in the true block for registers living into the false block
// must be removed.
RemoveKills(BBI1->BB->begin(), BBI1->BB->end(), DontKill, *TRI);
// Remove 'false' block branch (unless it was not analyzable), and find
// the last instruction to predicate.
if (BBI2->IsBrAnalyzable)
BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
DI2 = BBI2->BB->end();
while (NumDups2 != 0) {
// NumDups2 only counted non-dbg_value instructions, so this won't
// run off the head of the list.
assert (DI2 != BBI2->BB->begin());
--DI2;
// skip dbg_value instructions
if (!DI2->isDebugValue())
--NumDups2;
}
// Remember which registers would later be defined by the false block.
// This allows us not to predicate instructions in the true block that would
// later be re-defined. That is, rather than
// subeq r0, r1, #1
// addne r0, r1, #1
// generate:
// sub r0, r1, #1
// addne r0, r1, #1
SmallSet<unsigned, 4> RedefsByFalse;
SmallSet<unsigned, 4> ExtUses;
if (TII->isProfitableToUnpredicate(*BBI1->BB, *BBI2->BB)) {
for (MachineBasicBlock::iterator FI = BBI2->BB->begin(); FI != DI2; ++FI) {
if (FI->isDebugValue())
continue;
SmallVector<unsigned, 4> Defs;
for (unsigned i = 0, e = FI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = FI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isDef()) {
Defs.push_back(Reg);
} else if (!RedefsByFalse.count(Reg)) {
// These are defined before ctrl flow reach the 'false' instructions.
// They cannot be modified by the 'true' instructions.
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
ExtUses.insert(*SubRegs);
}
}
for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
unsigned Reg = Defs[i];
if (!ExtUses.count(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
RedefsByFalse.insert(*SubRegs);
}
}
}
}
// Predicate the 'true' block.
PredicateBlock(*BBI1, BBI1->BB->end(), *Cond1, &RedefsByFalse);
// After predicating BBI1, if there is a predicated terminator in BBI1 and
// a non-predicated in BBI2, then we don't want to predicate the one from
// BBI2. The reason is that if we merged these blocks, we would end up with
// two predicated terminators in the same block.
if (!BBI2->BB->empty() && (DI2 == BBI2->BB->end())) {
MachineBasicBlock::iterator BBI1T = BBI1->BB->getFirstTerminator();
MachineBasicBlock::iterator BBI2T = BBI2->BB->getFirstTerminator();
if ((BBI1T != BBI1->BB->end()) && TII->isPredicated(BBI1T) &&
((BBI2T != BBI2->BB->end()) && !TII->isPredicated(BBI2T)))
--DI2;
}
// Predicate the 'false' block.
PredicateBlock(*BBI2, DI2, *Cond2);
// Merge the true block into the entry of the diamond.
MergeBlocks(BBI, *BBI1, TailBB == nullptr);
MergeBlocks(BBI, *BBI2, TailBB == nullptr);
// If the if-converted block falls through or unconditionally branches into
// the tail block, and the tail block does not have other predecessors, then
// fold the tail block in as well. Otherwise, unless it falls through to the
// tail, add a unconditional branch to it.
if (TailBB) {
BBInfo &TailBBI = BBAnalysis[TailBB->getNumber()];
bool CanMergeTail = !TailBBI.HasFallThrough &&
!TailBBI.BB->hasAddressTaken();
// The if-converted block can still have a predicated terminator
// (e.g. a predicated return). If that is the case, we cannot merge
// it with the tail block.
MachineBasicBlock::const_iterator TI = BBI.BB->getFirstTerminator();
if (TI != BBI.BB->end() && TII->isPredicated(TI))
CanMergeTail = false;
// There may still be a fall-through edge from BBI1 or BBI2 to TailBB;
// check if there are any other predecessors besides those.
unsigned NumPreds = TailBB->pred_size();
if (NumPreds > 1)
CanMergeTail = false;
else if (NumPreds == 1 && CanMergeTail) {
MachineBasicBlock::pred_iterator PI = TailBB->pred_begin();
if (*PI != BBI1->BB && *PI != BBI2->BB)
CanMergeTail = false;
}
if (CanMergeTail) {
MergeBlocks(BBI, TailBBI);
TailBBI.IsDone = true;
} else {
BBI.BB->addSuccessor(TailBB, BranchProbability::getOne());
InsertUncondBranch(BBI.BB, TailBB, TII);
BBI.HasFallThrough = false;
}
}
// RemoveExtraEdges won't work if the block has an unanalyzable branch,
// which can happen here if TailBB is unanalyzable and is merged, so
// explicitly remove BBI1 and BBI2 as successors.
BBI.BB->removeSuccessor(BBI1->BB);
BBI.BB->removeSuccessor(BBI2->BB, true);
RemoveExtraEdges(BBI);
// Update block info.
BBI.IsDone = TrueBBI.IsDone = FalseBBI.IsDone = true;
InvalidatePreds(BBI.BB);
// FIXME: Must maintain LiveIns.
return true;
}
static bool MaySpeculate(const MachineInstr *MI,
SmallSet<unsigned, 4> &LaterRedefs) {
bool SawStore = true;
if (!MI->isSafeToMove(nullptr, SawStore))
return false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isDef() && !LaterRedefs.count(Reg))
return false;
}
return true;
}
/// PredicateBlock - Predicate instructions from the start of the block to the
/// specified end with the specified condition.
void IfConverter::PredicateBlock(BBInfo &BBI,
MachineBasicBlock::iterator E,
SmallVectorImpl<MachineOperand> &Cond,
SmallSet<unsigned, 4> *LaterRedefs) {
bool AnyUnpred = false;
bool MaySpec = LaterRedefs != nullptr;
for (MachineBasicBlock::iterator I = BBI.BB->begin(); I != E; ++I) {
if (I->isDebugValue() || TII->isPredicated(I))
continue;
// It may be possible not to predicate an instruction if it's the 'true'
// side of a diamond and the 'false' side may re-define the instruction's
// defs.
if (MaySpec && MaySpeculate(I, *LaterRedefs)) {
AnyUnpred = true;
continue;
}
// If any instruction is predicated, then every instruction after it must
// be predicated.
MaySpec = false;
if (!TII->PredicateInstruction(I, Cond)) {
#ifndef NDEBUG
dbgs() << "Unable to predicate " << *I << "!\n";
#endif
llvm_unreachable(nullptr);
}
// If the predicated instruction now redefines a register as the result of
// if-conversion, add an implicit kill.
UpdatePredRedefs(I, Redefs);
}
BBI.Predicate.append(Cond.begin(), Cond.end());
BBI.IsAnalyzed = false;
BBI.NonPredSize = 0;
++NumIfConvBBs;
if (AnyUnpred)
++NumUnpred;
}
/// CopyAndPredicateBlock - Copy and predicate instructions from source BB to
/// the destination block. Skip end of block branches if IgnoreBr is true.
void IfConverter::CopyAndPredicateBlock(BBInfo &ToBBI, BBInfo &FromBBI,
SmallVectorImpl<MachineOperand> &Cond,
bool IgnoreBr) {
MachineFunction &MF = *ToBBI.BB->getParent();
for (MachineBasicBlock::iterator I = FromBBI.BB->begin(),
E = FromBBI.BB->end(); I != E; ++I) {
// Do not copy the end of the block branches.
if (IgnoreBr && I->isBranch())
break;
MachineInstr *MI = MF.CloneMachineInstr(I);
ToBBI.BB->insert(ToBBI.BB->end(), MI);
ToBBI.NonPredSize++;
unsigned ExtraPredCost = TII->getPredicationCost(&*I);
unsigned NumCycles = SchedModel.computeInstrLatency(&*I, false);
if (NumCycles > 1)
ToBBI.ExtraCost += NumCycles-1;
ToBBI.ExtraCost2 += ExtraPredCost;
if (!TII->isPredicated(I) && !MI->isDebugValue()) {
if (!TII->PredicateInstruction(MI, Cond)) {
#ifndef NDEBUG
dbgs() << "Unable to predicate " << *I << "!\n";
#endif
llvm_unreachable(nullptr);
}
}
// If the predicated instruction now redefines a register as the result of
// if-conversion, add an implicit kill.
UpdatePredRedefs(MI, Redefs);
// Some kill flags may not be correct anymore.
if (!DontKill.empty())
RemoveKills(*MI, DontKill);
}
if (!IgnoreBr) {
std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
FromBBI.BB->succ_end());
MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : nullptr;
for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
MachineBasicBlock *Succ = Succs[i];
// Fallthrough edge can't be transferred.
if (Succ == FallThrough)
continue;
ToBBI.BB->addSuccessor(Succ);
}
}
ToBBI.Predicate.append(FromBBI.Predicate.begin(), FromBBI.Predicate.end());
ToBBI.Predicate.append(Cond.begin(), Cond.end());
ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
ToBBI.IsAnalyzed = false;
++NumDupBBs;
}
/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
/// This will leave FromBB as an empty block, so remove all of its
/// successor edges except for the fall-through edge. If AddEdges is true,
/// i.e., when FromBBI's branch is being moved, add those successor edges to
/// ToBBI.
void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
assert(!FromBBI.BB->hasAddressTaken() &&
"Removing a BB whose address is taken!");
// In case FromBBI.BB contains terminators (e.g. return instruction),
// first move the non-terminator instructions, then the terminators.
MachineBasicBlock::iterator FromTI = FromBBI.BB->getFirstTerminator();
MachineBasicBlock::iterator ToTI = ToBBI.BB->getFirstTerminator();
ToBBI.BB->splice(ToTI, FromBBI.BB, FromBBI.BB->begin(), FromTI);
// If FromBB has non-predicated terminator we should copy it at the end.
if ((FromTI != FromBBI.BB->end()) && !TII->isPredicated(FromTI))
ToTI = ToBBI.BB->end();
ToBBI.BB->splice(ToTI, FromBBI.BB, FromTI, FromBBI.BB->end());
// Force normalizing the successors' probabilities of ToBBI.BB to convert all
// unknown probabilities into known ones.
// FIXME: This usage is too tricky and in the future we would like to
// eliminate all unknown probabilities in MBB.
ToBBI.BB->normalizeSuccProbs();
SmallVector<MachineBasicBlock *, 4> FromSuccs(FromBBI.BB->succ_begin(),
FromBBI.BB->succ_end());
MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : nullptr;
// The edge probability from ToBBI.BB to FromBBI.BB, which is only needed when
// AddEdges is true and FromBBI.BB is a successor of ToBBI.BB.
auto To2FromProb = BranchProbability::getZero();
if (AddEdges && ToBBI.BB->isSuccessor(FromBBI.BB)) {
To2FromProb = MBPI->getEdgeProbability(ToBBI.BB, FromBBI.BB);
// Set the edge probability from ToBBI.BB to FromBBI.BB to zero to avoid the
// edge probability being merged to other edges when this edge is removed
// later.
ToBBI.BB->setSuccProbability(
std::find(ToBBI.BB->succ_begin(), ToBBI.BB->succ_end(), FromBBI.BB),
BranchProbability::getZero());
}
for (unsigned i = 0, e = FromSuccs.size(); i != e; ++i) {
MachineBasicBlock *Succ = FromSuccs[i];
// Fallthrough edge can't be transferred.
if (Succ == FallThrough)
continue;
auto NewProb = BranchProbability::getZero();
if (AddEdges) {
// Calculate the edge probability for the edge from ToBBI.BB to Succ,
// which is a portion of the edge probability from FromBBI.BB to Succ. The
// portion ratio is the edge probability from ToBBI.BB to FromBBI.BB (if
// FromBBI is a successor of ToBBI.BB. See comment below for excepion).
NewProb = MBPI->getEdgeProbability(FromBBI.BB, Succ);
// To2FromProb is 0 when FromBBI.BB is not a successor of ToBBI.BB. This
// only happens when if-converting a diamond CFG and FromBBI.BB is the
// tail BB. In this case FromBBI.BB post-dominates ToBBI.BB and hence we
// could just use the probabilities on FromBBI.BB's out-edges when adding
// new successors.
if (!To2FromProb.isZero())
NewProb *= To2FromProb;
}
FromBBI.BB->removeSuccessor(Succ);
if (AddEdges) {
// If the edge from ToBBI.BB to Succ already exists, update the
// probability of this edge by adding NewProb to it. An example is shown
// below, in which A is ToBBI.BB and B is FromBBI.BB. In this case we
// don't have to set C as A's successor as it already is. We only need to
// update the edge probability on A->C. Note that B will not be
// immediately removed from A's successors. It is possible that B->D is
// not removed either if D is a fallthrough of B. Later the edge A->D
// (generated here) and B->D will be combined into one edge. To maintain
// correct edge probability of this combined edge, we need to set the edge
// probability of A->B to zero, which is already done above. The edge
// probability on A->D is calculated by scaling the original probability
// on A->B by the probability of B->D.
//
// Before ifcvt: After ifcvt (assume B->D is kept):
//
// A A
// /| /|\
// / B / B|
// | /| | ||
// |/ | | |/
// C D C D
//
if (ToBBI.BB->isSuccessor(Succ))
ToBBI.BB->setSuccProbability(
std::find(ToBBI.BB->succ_begin(), ToBBI.BB->succ_end(), Succ),
MBPI->getEdgeProbability(ToBBI.BB, Succ) + NewProb);
else
ToBBI.BB->addSuccessor(Succ, NewProb);
}
}
// Now FromBBI always falls through to the next block!
if (NBB && !FromBBI.BB->isSuccessor(NBB))
FromBBI.BB->addSuccessor(NBB);
// Normalize the probabilities of ToBBI.BB's successors with all adjustment
// we've done above.
ToBBI.BB->normalizeSuccProbs();
ToBBI.Predicate.append(FromBBI.Predicate.begin(), FromBBI.Predicate.end());
FromBBI.Predicate.clear();
ToBBI.NonPredSize += FromBBI.NonPredSize;
ToBBI.ExtraCost += FromBBI.ExtraCost;
ToBBI.ExtraCost2 += FromBBI.ExtraCost2;
FromBBI.NonPredSize = 0;
FromBBI.ExtraCost = 0;
FromBBI.ExtraCost2 = 0;
ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
ToBBI.HasFallThrough = FromBBI.HasFallThrough;
ToBBI.IsAnalyzed = false;
FromBBI.IsAnalyzed = false;
}
FunctionPass *
llvm::createIfConverter(std::function<bool(const Function &)> Ftor) {
return new IfConverter(Ftor);
}
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