llvm-project/llvm/lib/CodeGen/EarlyIfConversion.cpp

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//===-- EarlyIfConversion.cpp - If-conversion on SSA form machine code ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Early if-conversion is for out-of-order CPUs that don't have a lot of
// predicable instructions. The goal is to eliminate conditional branches that
// may mispredict.
//
// Instructions from both sides of the branch are executed specutatively, and a
// cmov instruction selects the result.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "early-ifcvt"
#include "llvm/Function.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SparseSet.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
// Absolute maximum number of instructions allowed per speculated block.
// This bypasses all other heuristics, so it should be set fairly high.
static cl::opt<unsigned>
BlockInstrLimit("early-ifcvt-limit", cl::init(30), cl::Hidden,
cl::desc("Maximum number of instructions per speculated block."));
// Stress testing mode - disable heuristics.
static cl::opt<bool> Stress("stress-early-ifcvt", cl::Hidden,
cl::desc("Turn all knobs to 11"));
typedef SmallSetVector<MachineBasicBlock*, 8> BlockSetVector;
//===----------------------------------------------------------------------===//
// SSAIfConv
//===----------------------------------------------------------------------===//
//
// The SSAIfConv class performs if-conversion on SSA form machine code after
// determining if it is possible. The class contains no heuristics, external
// code should be used to determine when if-conversion is a good idea.
//
// SSAIfConv con convert both triangles and diamonds:
//
// Triangle: Head Diamond: Head
// | \ / \
// | \ / \
// | [TF]BB FBB TBB
// | / \ /
// | / \ /
// Tail Tail
//
// Instructions in the conditional blocks TBB and/or FBB are spliced into the
// Head block, and phis in the Tail black are converted to select instruction.
//
namespace {
class SSAIfConv {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI;
/// The block containing the conditional branch.
MachineBasicBlock *Head;
/// The block containing phis after the if-then-else.
MachineBasicBlock *Tail;
/// The 'true' conditional block as determined by AnalyzeBranch.
MachineBasicBlock *TBB;
/// The 'false' conditional block as determined by AnalyzeBranch.
MachineBasicBlock *FBB;
/// isTriangle - When there is no 'else' block, either TBB or FBB will be
/// equal to Tail.
bool isTriangle() const { return TBB == Tail || FBB == Tail; }
/// The branch condition determined by AnalyzeBranch.
SmallVector<MachineOperand, 4> Cond;
/// Information about each phi in the Tail block.
struct PHIInfo {
MachineInstr *PHI;
unsigned TReg, FReg;
// Latencies from Cond+Branch, TReg, and FReg to DstReg.
int CondCycles, TCycles, FCycles;
PHIInfo(MachineInstr *phi)
: PHI(phi), TReg(0), FReg(0), CondCycles(0), TCycles(0), FCycles(0) {}
};
SmallVector<PHIInfo, 8> PHIs;
/// Instructions in Head that define values used by the conditional blocks.
/// The hoisted instructions must be inserted after these instructions.
SmallPtrSet<MachineInstr*, 8> InsertAfter;
/// Register units clobbered by the conditional blocks.
BitVector ClobberedRegUnits;
// Scratch pad for findInsertionPoint.
SparseSet<unsigned> LiveRegUnits;
/// Insertion point in Head for speculatively executed instructions form TBB
/// and FBB.
MachineBasicBlock::iterator InsertionPoint;
/// Return true if all non-terminator instructions in MBB can be safely
/// speculated.
bool canSpeculateInstrs(MachineBasicBlock *MBB);
/// Find a valid insertion point in Head.
bool findInsertionPoint();
public:
/// runOnMachineFunction - Initialize per-function data structures.
void runOnMachineFunction(MachineFunction &MF) {
TII = MF.getTarget().getInstrInfo();
TRI = MF.getTarget().getRegisterInfo();
MRI = &MF.getRegInfo();
LiveRegUnits.clear();
LiveRegUnits.setUniverse(TRI->getNumRegUnits());
ClobberedRegUnits.clear();
ClobberedRegUnits.resize(TRI->getNumRegUnits());
}
/// canConvertIf - If the sub-CFG headed by MBB can be if-converted,
/// initialize the internal state, and return true.
bool canConvertIf(MachineBasicBlock *MBB);
/// convertIf - If-convert the last block passed to canConvertIf(), assuming
/// it is possible. Remove any erased blocks from WorkList
void convertIf(BlockSetVector &WorkList);
};
} // end anonymous namespace
/// canSpeculateInstrs - Returns true if all the instructions in MBB can safely
/// be speculated. The terminators are not considered.
///
/// If instructions use any values that are defined in the head basic block,
/// the defining instructions are added to InsertAfter.
///
/// Any clobbered regunits are added to ClobberedRegUnits.
///
bool SSAIfConv::canSpeculateInstrs(MachineBasicBlock *MBB) {
// Reject any live-in physregs. It's probably CPSR/EFLAGS, and very hard to
// get right.
if (!MBB->livein_empty()) {
DEBUG(dbgs() << "BB#" << MBB->getNumber() << " has live-ins.\n");
return false;
}
unsigned InstrCount = 0;
for (MachineBasicBlock::iterator I = MBB->begin(),
E = MBB->getFirstTerminator(); I != E; ++I) {
if (I->isDebugValue())
continue;
if (++InstrCount > BlockInstrLimit && !Stress) {
DEBUG(dbgs() << "BB#" << MBB->getNumber() << " has more than "
<< BlockInstrLimit << " instructions.\n");
return false;
}
// There shouldn't normally be any phis in a single-predecessor block.
if (I->isPHI()) {
DEBUG(dbgs() << "Can't hoist: " << *I);
return false;
}
// Don't speculate loads. Note that it may be possible and desirable to
// speculate GOT or constant pool loads that are guaranteed not to trap,
// but we don't support that for now.
if (I->mayLoad()) {
DEBUG(dbgs() << "Won't speculate load: " << *I);
return false;
}
// We never speculate stores, so an AA pointer isn't necessary.
bool DontMoveAcrossStore = true;
if (!I->isSafeToMove(TII, 0, DontMoveAcrossStore)) {
DEBUG(dbgs() << "Can't speculate: " << *I);
return false;
}
// Check for any dependencies on Head instructions.
for (MIOperands MO(I); MO.isValid(); ++MO) {
if (MO->isRegMask()) {
DEBUG(dbgs() << "Won't speculate regmask: " << *I);
return false;
}
if (!MO->isReg())
continue;
unsigned Reg = MO->getReg();
// Remember clobbered regunits.
if (MO->isDef() && TargetRegisterInfo::isPhysicalRegister(Reg))
for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units)
ClobberedRegUnits.set(*Units);
if (!MO->readsReg() || !TargetRegisterInfo::isVirtualRegister(Reg))
continue;
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (!DefMI || DefMI->getParent() != Head)
continue;
if (InsertAfter.insert(DefMI))
DEBUG(dbgs() << "BB#" << MBB->getNumber() << " depends on " << *DefMI);
if (DefMI->isTerminator()) {
DEBUG(dbgs() << "Can't insert instructions below terminator.\n");
return false;
}
}
}
return true;
}
/// Find an insertion point in Head for the speculated instructions. The
/// insertion point must be:
///
/// 1. Before any terminators.
/// 2. After any instructions in InsertAfter.
/// 3. Not have any clobbered regunits live.
///
/// This function sets InsertionPoint and returns true when successful, it
/// returns false if no valid insertion point could be found.
///
bool SSAIfConv::findInsertionPoint() {
// Keep track of live regunits before the current position.
// Only track RegUnits that are also in ClobberedRegUnits.
LiveRegUnits.clear();
SmallVector<unsigned, 8> Reads;
MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
MachineBasicBlock::iterator I = Head->end();
MachineBasicBlock::iterator B = Head->begin();
while (I != B) {
--I;
// Some of the conditional code depends in I.
if (InsertAfter.count(I)) {
DEBUG(dbgs() << "Can't insert code after " << *I);
return false;
}
// Update live regunits.
for (MIOperands MO(I); MO.isValid(); ++MO) {
// We're ignoring regmask operands. That is conservatively correct.
if (!MO->isReg())
continue;
unsigned Reg = MO->getReg();
if (!TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
// I clobbers Reg, so it isn't live before I.
if (MO->isDef())
for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units)
LiveRegUnits.erase(*Units);
// Unless I reads Reg.
if (MO->readsReg())
Reads.push_back(Reg);
}
// Anything read by I is live before I.
while (!Reads.empty())
for (MCRegUnitIterator Units(Reads.pop_back_val(), TRI); Units.isValid();
++Units)
if (ClobberedRegUnits.test(*Units))
LiveRegUnits.insert(*Units);
// We can't insert before a terminator.
if (I != FirstTerm && I->isTerminator())
continue;
// Some of the clobbered registers are live before I, not a valid insertion
// point.
if (!LiveRegUnits.empty()) {
DEBUG({
dbgs() << "Would clobber";
for (SparseSet<unsigned>::const_iterator
i = LiveRegUnits.begin(), e = LiveRegUnits.end(); i != e; ++i)
dbgs() << ' ' << PrintRegUnit(*i, TRI);
dbgs() << " live before " << *I;
});
continue;
}
// This is a valid insertion point.
InsertionPoint = I;
DEBUG(dbgs() << "Can insert before " << *I);
return true;
}
DEBUG(dbgs() << "No legal insertion point found.\n");
return false;
}
/// canConvertIf - analyze the sub-cfg rooted in MBB, and return true if it is
/// a potential candidate for if-conversion. Fill out the internal state.
///
bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) {
Head = MBB;
TBB = FBB = Tail = 0;
if (Head->succ_size() != 2)
return false;
MachineBasicBlock *Succ0 = Head->succ_begin()[0];
MachineBasicBlock *Succ1 = Head->succ_begin()[1];
// Canonicalize so Succ0 has MBB as its single predecessor.
if (Succ0->pred_size() != 1)
std::swap(Succ0, Succ1);
if (Succ0->pred_size() != 1 || Succ0->succ_size() != 1)
return false;
// We could support additional Tail predecessors by updating phis instead of
// eliminating them. Let's see an example where it matters first.
Tail = Succ0->succ_begin()[0];
if (Tail->pred_size() != 2)
return false;
// This is not a triangle.
if (Tail != Succ1) {
// Check for a diamond. We won't deal with any critical edges.
if (Succ1->pred_size() != 1 || Succ1->succ_size() != 1 ||
Succ1->succ_begin()[0] != Tail)
return false;
DEBUG(dbgs() << "\nDiamond: BB#" << Head->getNumber()
<< " -> BB#" << Succ0->getNumber()
<< "/BB#" << Succ1->getNumber()
<< " -> BB#" << Tail->getNumber() << '\n');
// Live-in physregs are tricky to get right when speculating code.
if (!Tail->livein_empty()) {
DEBUG(dbgs() << "Tail has live-ins.\n");
return false;
}
} else {
DEBUG(dbgs() << "\nTriangle: BB#" << Head->getNumber()
<< " -> BB#" << Succ0->getNumber()
<< " -> BB#" << Tail->getNumber() << '\n');
}
// This is a triangle or a diamond.
// If Tail doesn't have any phis, there must be side effects.
if (Tail->empty() || !Tail->front().isPHI()) {
DEBUG(dbgs() << "No phis in tail.\n");
return false;
}
// The branch we're looking to eliminate must be analyzable.
Cond.clear();
if (TII->AnalyzeBranch(*Head, TBB, FBB, Cond)) {
DEBUG(dbgs() << "Branch not analyzable.\n");
return false;
}
// This is weird, probably some sort of degenerate CFG.
if (!TBB) {
DEBUG(dbgs() << "AnalyzeBranch didn't find conditional branch.\n");
return false;
}
// AnalyzeBranch doesn't set FBB on a fall-through branch.
// Make sure it is always set.
FBB = TBB == Succ0 ? Succ1 : Succ0;
// Any phis in the tail block must be convertible to selects.
PHIs.clear();
MachineBasicBlock *TPred = TBB == Tail ? Head : TBB;
MachineBasicBlock *FPred = FBB == Tail ? Head : FBB;
for (MachineBasicBlock::iterator I = Tail->begin(), E = Tail->end();
I != E && I->isPHI(); ++I) {
PHIs.push_back(&*I);
PHIInfo &PI = PHIs.back();
// Find PHI operands corresponding to TPred and FPred.
for (unsigned i = 1; i != PI.PHI->getNumOperands(); i += 2) {
if (PI.PHI->getOperand(i+1).getMBB() == TPred)
PI.TReg = PI.PHI->getOperand(i).getReg();
if (PI.PHI->getOperand(i+1).getMBB() == FPred)
PI.FReg = PI.PHI->getOperand(i).getReg();
}
assert(TargetRegisterInfo::isVirtualRegister(PI.TReg) && "Bad PHI");
assert(TargetRegisterInfo::isVirtualRegister(PI.FReg) && "Bad PHI");
// Get target information.
if (!TII->canInsertSelect(*Head, Cond, PI.TReg, PI.FReg,
PI.CondCycles, PI.TCycles, PI.FCycles)) {
DEBUG(dbgs() << "Can't convert: " << *PI.PHI);
return false;
}
}
// Check that the conditional instructions can be speculated.
InsertAfter.clear();
ClobberedRegUnits.reset();
if (TBB != Tail && !canSpeculateInstrs(TBB))
return false;
if (FBB != Tail && !canSpeculateInstrs(FBB))
return false;
// Try to find a valid insertion point for the speculated instructions in the
// head basic block.
if (!findInsertionPoint())
return false;
return true;
}
static void eraseBlock(BlockSetVector &WorkList, MachineBasicBlock *MBB) {
WorkList.remove(MBB);
MBB->eraseFromParent();
}
/// convertIf - Execute the if conversion after canConvertIf has determined the
/// feasibility.
///
/// Any basic blocks erased will also be removed from WorkList.
///
void SSAIfConv::convertIf(BlockSetVector &WorkList) {
assert(Head && Tail && TBB && FBB && "Call canConvertIf first.");
// Move all instructions into Head, except for the terminators.
if (TBB != Tail)
Head->splice(InsertionPoint, TBB, TBB->begin(), TBB->getFirstTerminator());
if (FBB != Tail)
Head->splice(InsertionPoint, FBB, FBB->begin(), FBB->getFirstTerminator());
MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
assert(FirstTerm != Head->end() && "No terminators");
DebugLoc HeadDL = FirstTerm->getDebugLoc();
// Convert all PHIs to select instructions inserted before FirstTerm.
for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
PHIInfo &PI = PHIs[i];
DEBUG(dbgs() << "If-converting " << *PI.PHI);
assert(PI.PHI->getNumOperands() == 5 && "Unexpected PHI operands.");
unsigned DstReg = PI.PHI->getOperand(0).getReg();
TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg);
DEBUG(dbgs() << " --> " << *llvm::prior(FirstTerm));
PI.PHI->eraseFromParent();
PI.PHI = 0;
}
// Fix up the CFG, temporarily leave Head without any successors.
Head->removeSuccessor(TBB);
Head->removeSuccessor(FBB);
if (TBB != Tail)
TBB->removeSuccessor(Tail);
if (FBB != Tail)
FBB->removeSuccessor(Tail);
// Fix up Head's terminators.
// It should become a single branch or a fallthrough.
TII->RemoveBranch(*Head);
// Erase the now empty conditional blocks. It is likely that Head can fall
// through to Tail, and we can join the two blocks.
if (TBB != Tail)
eraseBlock(WorkList, TBB);
if (FBB != Tail)
eraseBlock(WorkList, FBB);
assert(Head->succ_empty() && "Additional head successors?");
if (Head->isLayoutSuccessor(Tail)) {
// Splice Tail onto the end of Head.
DEBUG(dbgs() << "Joining tail BB#" << Tail->getNumber()
<< " into head BB#" << Head->getNumber() << '\n');
Head->splice(Head->end(), Tail,
Tail->begin(), Tail->end());
Head->transferSuccessorsAndUpdatePHIs(Tail);
eraseBlock(WorkList, Tail);
} else {
// We need a branch to Tail, let code placement work it out later.
DEBUG(dbgs() << "Converting to unconditional branch.\n");
SmallVector<MachineOperand, 0> EmptyCond;
TII->InsertBranch(*Head, Tail, 0, EmptyCond, HeadDL);
Head->addSuccessor(Tail);
}
DEBUG(dbgs() << *Head);
}
//===----------------------------------------------------------------------===//
// EarlyIfConverter Pass
//===----------------------------------------------------------------------===//
namespace {
class EarlyIfConverter : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI;
SSAIfConv IfConv;
// Worklist of head blocks to try for if-conversion.
BlockSetVector WorkList;
public:
static char ID;
EarlyIfConverter() : MachineFunctionPass(ID) {}
void getAnalysisUsage(AnalysisUsage &AU) const;
bool runOnMachineFunction(MachineFunction &MF);
private:
bool tryConvertIf(MachineBasicBlock*);
};
} // end anonymous namespace
char EarlyIfConverter::ID = 0;
char &llvm::EarlyIfConverterID = EarlyIfConverter::ID;
INITIALIZE_PASS_BEGIN(EarlyIfConverter,
"early-ifcvt", "Early If Converter", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_END(EarlyIfConverter,
"early-ifcvt", "Early If Converter", false, false)
void EarlyIfConverter::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineBranchProbabilityInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
/// Attempt repeated if-conversion on MBB, return true if successful.
/// Update WorkList with new opportunities.
///
bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) {
if (!IfConv.canConvertIf(MBB))
return false;
// Repeatedly if-convert MBB, joining Head and Tail may expose more
// opportunities.
do IfConv.convertIf(WorkList);
while (IfConv.canConvertIf(MBB));
// It is possible that MBB is now itself a conditional block that can be
// if-converted.
if (MBB->pred_size() == 1 && MBB->succ_size() == 1)
WorkList.insert(MBB->pred_begin()[0]);
WorkList.remove(MBB);
return true;
}
bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) {
DEBUG(dbgs() << "********** EARLY IF-CONVERSION **********\n"
<< "********** Function: "
<< ((Value*)MF.getFunction())->getName() << '\n');
TII = MF.getTarget().getInstrInfo();
TRI = MF.getTarget().getRegisterInfo();
MRI = &MF.getRegInfo();
bool Changed = false;
IfConv.runOnMachineFunction(MF);
for (MachineFunction::iterator MFI = MF.begin(), MFE = MF.end(); MFI != MFE;
++MFI)
if (tryConvertIf(MFI))
Changed = true;
DEBUG(dbgs() << "Revisiting " << WorkList.size() << " blocks.\n");
while (!WorkList.empty())
tryConvertIf(WorkList.pop_back_val());
MF.verify(this, "After early if-conversion");
return Changed;
}