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f2b87eabed
llvm-mirror/lib/CodeGen/TailDuplicator.cpp
Hongtao Yu f2b87eabed [CSSPGO] Unblocking optimizations by dangling pseudo probes. 
This change fixes a couple places where the pseudo probe intrinsic blocks optimizations because they are not naturally removable. To unblock those optimizations, the blocking pseudo probes are moved out of the original blocks and tagged dangling, instead of allowing pseudo probes to be literally removed. The reason is that when the original block is removed, we won't be able to sample it. Instead of assigning it a zero weight, moving all its pseudo probes into another block and marking them dangling should allow the counts inference a chance to assign them a more reasonable weight. We have not seen counts quality degradation from our experiments.

The optimizations being unblocked are:

	1. Removing conditional probes for if-converted branches. Conditional probes are tagged dangling when their homing branch arms are folded so that they will not be over-counted.
	2. Unblocking jump threading from removing empty blocks. Pseudo probe prevents jump threading from removing logically empty blocks that only has one unconditional jump instructions.
	3. Unblocking SimplifyCFG and MIR tail duplicate to thread empty blocks and blocks with redundant branch checks.

Since dangling probes are logically deleted, they should not consume any samples in LTO postLink. This can be achieved by setting their distribution factors to zero when dangled.

Reviewed By: wmi

Differential Revision: https://reviews.llvm.org/D97481
2021-03-03 22:44:42 -08:00

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//===- TailDuplicator.cpp - Duplicate blocks into predecessors' tails -----===//
//
// 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 utility class duplicates basic blocks ending in unconditional branches
// into the tails of their predecessors.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/TailDuplicator.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSizeOpts.h"
#include "llvm/CodeGen/MachineSSAUpdater.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.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/TargetMachine.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "tailduplication"
STATISTIC(NumTails, "Number of tails duplicated");
STATISTIC(NumTailDups, "Number of tail duplicated blocks");
STATISTIC(NumTailDupAdded,
"Number of instructions added due to tail duplication");
STATISTIC(NumTailDupRemoved,
"Number of instructions removed due to tail duplication");
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumAddedPHIs, "Number of phis added");
// Heuristic for tail duplication.
static cl::opt<unsigned> TailDuplicateSize(
"tail-dup-size",
cl::desc("Maximum instructions to consider tail duplicating"), cl::init(2),
cl::Hidden);
static cl::opt<unsigned> TailDupIndirectBranchSize(
"tail-dup-indirect-size",
cl::desc("Maximum instructions to consider tail duplicating blocks that "
"end with indirect branches."), cl::init(20),
cl::Hidden);
static cl::opt<bool>
TailDupVerify("tail-dup-verify",
cl::desc("Verify sanity of PHI instructions during taildup"),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> TailDupLimit("tail-dup-limit", cl::init(~0U),
cl::Hidden);
void TailDuplicator::initMF(MachineFunction &MFin, bool PreRegAlloc,
const MachineBranchProbabilityInfo *MBPIin,
MBFIWrapper *MBFIin,
ProfileSummaryInfo *PSIin,
bool LayoutModeIn, unsigned TailDupSizeIn) {
MF = &MFin;
TII = MF->getSubtarget().getInstrInfo();
TRI = MF->getSubtarget().getRegisterInfo();
MRI = &MF->getRegInfo();
MMI = &MF->getMMI();
MBPI = MBPIin;
MBFI = MBFIin;
PSI = PSIin;
TailDupSize = TailDupSizeIn;
assert(MBPI != nullptr && "Machine Branch Probability Info required");
LayoutMode = LayoutModeIn;
this->PreRegAlloc = PreRegAlloc;
}
static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
MachineBasicBlock *MBB = &*I;
SmallSetVector<MachineBasicBlock *, 8> Preds(MBB->pred_begin(),
MBB->pred_end());
MachineBasicBlock::iterator MI = MBB->begin();
while (MI != MBB->end()) {
if (!MI->isPHI())
break;
for (MachineBasicBlock *PredBB : Preds) {
bool Found = false;
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
if (PHIBB == PredBB) {
Found = true;
break;
}
}
if (!Found) {
dbgs() << "Malformed PHI in " << printMBBReference(*MBB) << ": "
<< *MI;
dbgs() << " missing input from predecessor "
<< printMBBReference(*PredBB) << '\n';
llvm_unreachable(nullptr);
}
}
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
if (CheckExtra && !Preds.count(PHIBB)) {
dbgs() << "Warning: malformed PHI in " << printMBBReference(*MBB)
<< ": " << *MI;
dbgs() << " extra input from predecessor "
<< printMBBReference(*PHIBB) << '\n';
llvm_unreachable(nullptr);
}
if (PHIBB->getNumber() < 0) {
dbgs() << "Malformed PHI in " << printMBBReference(*MBB) << ": "
<< *MI;
dbgs() << " non-existing " << printMBBReference(*PHIBB) << '\n';
llvm_unreachable(nullptr);
}
}
++MI;
}
}
}
/// Tail duplicate the block and cleanup.
/// \p IsSimple - return value of isSimpleBB
/// \p MBB - block to be duplicated
/// \p ForcedLayoutPred - If non-null, treat this block as the layout
/// predecessor, instead of using the ordering in MF
/// \p DuplicatedPreds - if non-null, \p DuplicatedPreds will contain a list of
/// all Preds that received a copy of \p MBB.
/// \p RemovalCallback - if non-null, called just before MBB is deleted.
bool TailDuplicator::tailDuplicateAndUpdate(
bool IsSimple, MachineBasicBlock *MBB,
MachineBasicBlock *ForcedLayoutPred,
SmallVectorImpl<MachineBasicBlock*> *DuplicatedPreds,
function_ref<void(MachineBasicBlock *)> *RemovalCallback,
SmallVectorImpl<MachineBasicBlock *> *CandidatePtr) {
// Save the successors list.
SmallSetVector<MachineBasicBlock *, 8> Succs(MBB->succ_begin(),
MBB->succ_end());
SmallVector<MachineBasicBlock *, 8> TDBBs;
SmallVector<MachineInstr *, 16> Copies;
if (!tailDuplicate(IsSimple, MBB, ForcedLayoutPred,
TDBBs, Copies, CandidatePtr))
return false;
++NumTails;
SmallVector<MachineInstr *, 8> NewPHIs;
MachineSSAUpdater SSAUpdate(*MF, &NewPHIs);
// TailBB's immediate successors are now successors of those predecessors
// which duplicated TailBB. Add the predecessors as sources to the PHI
// instructions.
bool isDead = MBB->pred_empty() && !MBB->hasAddressTaken();
if (PreRegAlloc)
updateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
// If it is dead, remove it.
if (isDead) {
NumTailDupRemoved += MBB->size();
removeDeadBlock(MBB, RemovalCallback);
++NumDeadBlocks;
}
// Update SSA form.
if (!SSAUpdateVRs.empty()) {
for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
unsigned VReg = SSAUpdateVRs[i];
SSAUpdate.Initialize(VReg);
// If the original definition is still around, add it as an available
// value.
MachineInstr *DefMI = MRI->getVRegDef(VReg);
MachineBasicBlock *DefBB = nullptr;
if (DefMI) {
DefBB = DefMI->getParent();
SSAUpdate.AddAvailableValue(DefBB, VReg);
}
// Add the new vregs as available values.
DenseMap<Register, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(VReg);
for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = LI->second[j].first;
Register SrcReg = LI->second[j].second;
SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
}
// Rewrite uses that are outside of the original def's block.
MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
while (UI != MRI->use_end()) {
MachineOperand &UseMO = *UI;
MachineInstr *UseMI = UseMO.getParent();
++UI;
if (UseMI->isDebugValue()) {
// SSAUpdate can replace the use with an undef. That creates
// a debug instruction that is a kill.
// FIXME: Should it SSAUpdate job to delete debug instructions
// instead of replacing the use with undef?
UseMI->eraseFromParent();
continue;
}
if (UseMI->getParent() == DefBB && !UseMI->isPHI())
continue;
SSAUpdate.RewriteUse(UseMO);
}
}
SSAUpdateVRs.clear();
SSAUpdateVals.clear();
}
// Eliminate some of the copies inserted by tail duplication to maintain
// SSA form.
for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
MachineInstr *Copy = Copies[i];
if (!Copy->isCopy())
continue;
Register Dst = Copy->getOperand(0).getReg();
Register Src = Copy->getOperand(1).getReg();
if (MRI->hasOneNonDBGUse(Src) &&
MRI->constrainRegClass(Src, MRI->getRegClass(Dst))) {
// Copy is the only use. Do trivial copy propagation here.
MRI->replaceRegWith(Dst, Src);
Copy->eraseFromParent();
}
}
if (NewPHIs.size())
NumAddedPHIs += NewPHIs.size();
if (DuplicatedPreds)
*DuplicatedPreds = std::move(TDBBs);
return true;
}
/// Look for small blocks that are unconditionally branched to and do not fall
/// through. Tail-duplicate their instructions into their predecessors to
/// eliminate (dynamic) branches.
bool TailDuplicator::tailDuplicateBlocks() {
bool MadeChange = false;
if (PreRegAlloc && TailDupVerify) {
LLVM_DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
VerifyPHIs(*MF, true);
}
for (MachineFunction::iterator I = ++MF->begin(), E = MF->end(); I != E;) {
MachineBasicBlock *MBB = &*I++;
if (NumTails == TailDupLimit)
break;
bool IsSimple = isSimpleBB(MBB);
if (!shouldTailDuplicate(IsSimple, *MBB))
continue;
MadeChange |= tailDuplicateAndUpdate(IsSimple, MBB, nullptr);
}
if (PreRegAlloc && TailDupVerify)
VerifyPHIs(*MF, false);
return MadeChange;
}
static bool isDefLiveOut(Register Reg, MachineBasicBlock *BB,
const MachineRegisterInfo *MRI) {
for (MachineInstr &UseMI : MRI->use_instructions(Reg)) {
if (UseMI.isDebugValue())
continue;
if (UseMI.getParent() != BB)
return true;
}
return false;
}
static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
if (MI->getOperand(i + 1).getMBB() == SrcBB)
return i;
return 0;
}
// Remember which registers are used by phis in this block. This is
// used to determine which registers are liveout while modifying the
// block (which is why we need to copy the information).
static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
DenseSet<Register> *UsedByPhi) {
for (const auto &MI : BB) {
if (!MI.isPHI())
break;
for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
Register SrcReg = MI.getOperand(i).getReg();
UsedByPhi->insert(SrcReg);
}
}
}
/// Add a definition and source virtual registers pair for SSA update.
void TailDuplicator::addSSAUpdateEntry(Register OrigReg, Register NewReg,
MachineBasicBlock *BB) {
DenseMap<Register, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(OrigReg);
if (LI != SSAUpdateVals.end())
LI->second.push_back(std::make_pair(BB, NewReg));
else {
AvailableValsTy Vals;
Vals.push_back(std::make_pair(BB, NewReg));
SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
SSAUpdateVRs.push_back(OrigReg);
}
}
/// Process PHI node in TailBB by turning it into a copy in PredBB. Remember the
/// source register that's contributed by PredBB and update SSA update map.
void TailDuplicator::processPHI(
MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
DenseMap<Register, RegSubRegPair> &LocalVRMap,
SmallVectorImpl<std::pair<Register, RegSubRegPair>> &Copies,
const DenseSet<Register> &RegsUsedByPhi, bool Remove) {
Register DefReg = MI->getOperand(0).getReg();
unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
assert(SrcOpIdx && "Unable to find matching PHI source?");
Register SrcReg = MI->getOperand(SrcOpIdx).getReg();
unsigned SrcSubReg = MI->getOperand(SrcOpIdx).getSubReg();
const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
LocalVRMap.insert(std::make_pair(DefReg, RegSubRegPair(SrcReg, SrcSubReg)));
// Insert a copy from source to the end of the block. The def register is the
// available value liveout of the block.
Register NewDef = MRI->createVirtualRegister(RC);
Copies.push_back(std::make_pair(NewDef, RegSubRegPair(SrcReg, SrcSubReg)));
if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
addSSAUpdateEntry(DefReg, NewDef, PredBB);
if (!Remove)
return;
// Remove PredBB from the PHI node.
MI->RemoveOperand(SrcOpIdx + 1);
MI->RemoveOperand(SrcOpIdx);
if (MI->getNumOperands() == 1)
MI->eraseFromParent();
}
/// Duplicate a TailBB instruction to PredBB and update
/// the source operands due to earlier PHI translation.
void TailDuplicator::duplicateInstruction(
MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
DenseMap<Register, RegSubRegPair> &LocalVRMap,
const DenseSet<Register> &UsedByPhi) {
// Allow duplication of CFI instructions.
if (MI->isCFIInstruction()) {
BuildMI(*PredBB, PredBB->end(), PredBB->findDebugLoc(PredBB->begin()),
TII->get(TargetOpcode::CFI_INSTRUCTION)).addCFIIndex(
MI->getOperand(0).getCFIIndex());
return;
}
MachineInstr &NewMI = TII->duplicate(*PredBB, PredBB->end(), *MI);
if (PreRegAlloc) {
for (unsigned i = 0, e = NewMI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = NewMI.getOperand(i);
if (!MO.isReg())
continue;
Register Reg = MO.getReg();
if (!Register::isVirtualRegister(Reg))
continue;
if (MO.isDef()) {
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
Register NewReg = MRI->createVirtualRegister(RC);
MO.setReg(NewReg);
LocalVRMap.insert(std::make_pair(Reg, RegSubRegPair(NewReg, 0)));
if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
addSSAUpdateEntry(Reg, NewReg, PredBB);
} else {
auto VI = LocalVRMap.find(Reg);
if (VI != LocalVRMap.end()) {
// Need to make sure that the register class of the mapped register
// will satisfy the constraints of the class of the register being
// replaced.
auto *OrigRC = MRI->getRegClass(Reg);
auto *MappedRC = MRI->getRegClass(VI->second.Reg);
const TargetRegisterClass *ConstrRC;
if (VI->second.SubReg != 0) {
ConstrRC = TRI->getMatchingSuperRegClass(MappedRC, OrigRC,
VI->second.SubReg);
if (ConstrRC) {
// The actual constraining (as in "find appropriate new class")
// is done by getMatchingSuperRegClass, so now we only need to
// change the class of the mapped register.
MRI->setRegClass(VI->second.Reg, ConstrRC);
}
} else {
// For mapped registers that do not have sub-registers, simply
// restrict their class to match the original one.
ConstrRC = MRI->constrainRegClass(VI->second.Reg, OrigRC);
}
if (ConstrRC) {
// If the class constraining succeeded, we can simply replace
// the old register with the mapped one.
MO.setReg(VI->second.Reg);
// We have Reg -> VI.Reg:VI.SubReg, so if Reg is used with a
// sub-register, we need to compose the sub-register indices.
MO.setSubReg(TRI->composeSubRegIndices(MO.getSubReg(),
VI->second.SubReg));
} else {
// The direct replacement is not possible, due to failing register
// class constraints. An explicit COPY is necessary. Create one
// that can be reused
auto *NewRC = MI->getRegClassConstraint(i, TII, TRI);
if (NewRC == nullptr)
NewRC = OrigRC;
Register NewReg = MRI->createVirtualRegister(NewRC);
BuildMI(*PredBB, NewMI, NewMI.getDebugLoc(),
TII->get(TargetOpcode::COPY), NewReg)
.addReg(VI->second.Reg, 0, VI->second.SubReg);
LocalVRMap.erase(VI);
LocalVRMap.insert(std::make_pair(Reg, RegSubRegPair(NewReg, 0)));
MO.setReg(NewReg);
// The composed VI.Reg:VI.SubReg is replaced with NewReg, which
// is equivalent to the whole register Reg. Hence, Reg:subreg
// is same as NewReg:subreg, so keep the sub-register index
// unchanged.
}
// Clear any kill flags from this operand. The new register could
// have uses after this one, so kills are not valid here.
MO.setIsKill(false);
}
}
}
}
}
/// After FromBB is tail duplicated into its predecessor blocks, the successors
/// have gained new predecessors. Update the PHI instructions in them
/// accordingly.
void TailDuplicator::updateSuccessorsPHIs(
MachineBasicBlock *FromBB, bool isDead,
SmallVectorImpl<MachineBasicBlock *> &TDBBs,
SmallSetVector<MachineBasicBlock *, 8> &Succs) {
for (MachineBasicBlock *SuccBB : Succs) {
for (MachineInstr &MI : *SuccBB) {
if (!MI.isPHI())
break;
MachineInstrBuilder MIB(*FromBB->getParent(), MI);
unsigned Idx = 0;
for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
MachineOperand &MO = MI.getOperand(i + 1);
if (MO.getMBB() == FromBB) {
Idx = i;
break;
}
}
assert(Idx != 0);
MachineOperand &MO0 = MI.getOperand(Idx);
Register Reg = MO0.getReg();
if (isDead) {
// Folded into the previous BB.
// There could be duplicate phi source entries. FIXME: Should sdisel
// or earlier pass fixed this?
for (unsigned i = MI.getNumOperands() - 2; i != Idx; i -= 2) {
MachineOperand &MO = MI.getOperand(i + 1);
if (MO.getMBB() == FromBB) {
MI.RemoveOperand(i + 1);
MI.RemoveOperand(i);
}
}
} else
Idx = 0;
// If Idx is set, the operands at Idx and Idx+1 must be removed.
// We reuse the location to avoid expensive RemoveOperand calls.
DenseMap<Register, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(Reg);
if (LI != SSAUpdateVals.end()) {
// This register is defined in the tail block.
for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = LI->second[j].first;
// If we didn't duplicate a bb into a particular predecessor, we
// might still have added an entry to SSAUpdateVals to correcly
// recompute SSA. If that case, avoid adding a dummy extra argument
// this PHI.
if (!SrcBB->isSuccessor(SuccBB))
continue;
Register SrcReg = LI->second[j].second;
if (Idx != 0) {
MI.getOperand(Idx).setReg(SrcReg);
MI.getOperand(Idx + 1).setMBB(SrcBB);
Idx = 0;
} else {
MIB.addReg(SrcReg).addMBB(SrcBB);
}
}
} else {
// Live in tail block, must also be live in predecessors.
for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = TDBBs[j];
if (Idx != 0) {
MI.getOperand(Idx).setReg(Reg);
MI.getOperand(Idx + 1).setMBB(SrcBB);
Idx = 0;
} else {
MIB.addReg(Reg).addMBB(SrcBB);
}
}
}
if (Idx != 0) {
MI.RemoveOperand(Idx + 1);
MI.RemoveOperand(Idx);
}
}
}
}
/// Determine if it is profitable to duplicate this block.
bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
MachineBasicBlock &TailBB) {
// When doing tail-duplication during layout, the block ordering is in flux,
// so canFallThrough returns a result based on incorrect information and
// should just be ignored.
if (!LayoutMode && TailBB.canFallThrough())
return false;
// Don't try to tail-duplicate single-block loops.
if (TailBB.isSuccessor(&TailBB))
return false;
// Set the limit on the cost to duplicate. When optimizing for size,
// duplicate only one, because one branch instruction can be eliminated to
// compensate for the duplication.
unsigned MaxDuplicateCount;
bool OptForSize = MF->getFunction().hasOptSize() ||
llvm::shouldOptimizeForSize(&TailBB, PSI, MBFI);
if (TailDupSize == 0)
MaxDuplicateCount = TailDuplicateSize;
else
MaxDuplicateCount = TailDupSize;
if (OptForSize)
MaxDuplicateCount = 1;
// If the block to be duplicated ends in an unanalyzable fallthrough, don't
// duplicate it.
// A similar check is necessary in MachineBlockPlacement to make sure pairs of
// blocks with unanalyzable fallthrough get layed out contiguously.
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond) &&
TailBB.canFallThrough())
return false;
// If the target has hardware branch prediction that can handle indirect
// branches, duplicating them can often make them predictable when there
// are common paths through the code. The limit needs to be high enough
// to allow undoing the effects of tail merging and other optimizations
// that rearrange the predecessors of the indirect branch.
bool HasIndirectbr = false;
if (!TailBB.empty())
HasIndirectbr = TailBB.back().isIndirectBranch();
if (HasIndirectbr && PreRegAlloc)
MaxDuplicateCount = TailDupIndirectBranchSize;
// Check the instructions in the block to determine whether tail-duplication
// is invalid or unlikely to be profitable.
unsigned InstrCount = 0;
for (MachineInstr &MI : TailBB) {
// Non-duplicable things shouldn't be tail-duplicated.
// CFI instructions are marked as non-duplicable, because Darwin compact
// unwind info emission can't handle multiple prologue setups. In case of
// DWARF, allow them be duplicated, so that their existence doesn't prevent
// tail duplication of some basic blocks, that would be duplicated otherwise.
if (MI.isNotDuplicable() &&
(TailBB.getParent()->getTarget().getTargetTriple().isOSDarwin() ||
!MI.isCFIInstruction()))
return false;
// Convergent instructions can be duplicated only if doing so doesn't add
// new control dependencies, which is what we're going to do here.
if (MI.isConvergent())
return false;
// Do not duplicate 'return' instructions if this is a pre-regalloc run.
// A return may expand into a lot more instructions (e.g. reload of callee
// saved registers) after PEI.
if (PreRegAlloc && MI.isReturn())
return false;
// Avoid duplicating calls before register allocation. Calls presents a
// barrier to register allocation so duplicating them may end up increasing
// spills.
if (PreRegAlloc && MI.isCall())
return false;
// TailDuplicator::appendCopies will erroneously place COPYs after
// INLINEASM_BR instructions after 4b0aa5724fea, which demonstrates the same
// bug that was fixed in f7a53d82c090.
// FIXME: Use findPHICopyInsertPoint() to find the correct insertion point
// for the COPY when replacing PHIs.
if (MI.getOpcode() == TargetOpcode::INLINEASM_BR)
return false;
if (MI.isBundle())
InstrCount += MI.getBundleSize();
else if (!MI.isPHI() && !MI.isMetaInstruction())
InstrCount += 1;
if (InstrCount > MaxDuplicateCount)
return false;
}
// Check if any of the successors of TailBB has a PHI node in which the
// value corresponding to TailBB uses a subregister.
// If a phi node uses a register paired with a subregister, the actual
// "value type" of the phi may differ from the type of the register without
// any subregisters. Due to a bug, tail duplication may add a new operand
// without a necessary subregister, producing an invalid code. This is
// demonstrated by test/CodeGen/Hexagon/tail-dup-subreg-abort.ll.
// Disable tail duplication for this case for now, until the problem is
// fixed.
for (auto SB : TailBB.successors()) {
for (auto &I : *SB) {
if (!I.isPHI())
break;
unsigned Idx = getPHISrcRegOpIdx(&I, &TailBB);
assert(Idx != 0);
MachineOperand &PU = I.getOperand(Idx);
if (PU.getSubReg() != 0)
return false;
}
}
if (HasIndirectbr && PreRegAlloc)
return true;
if (IsSimple)
return true;
if (!PreRegAlloc)
return true;
return canCompletelyDuplicateBB(TailBB);
}
/// True if this BB has only one unconditional jump.
bool TailDuplicator::isSimpleBB(MachineBasicBlock *TailBB) {
if (TailBB->succ_size() != 1)
return false;
if (TailBB->pred_empty())
return false;
MachineBasicBlock::iterator I = TailBB->getFirstNonDebugInstr(true);
if (I == TailBB->end())
return true;
return I->isUnconditionalBranch();
}
static bool bothUsedInPHI(const MachineBasicBlock &A,
const SmallPtrSet<MachineBasicBlock *, 8> &SuccsB) {
for (MachineBasicBlock *BB : A.successors())
if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
return true;
return false;
}
bool TailDuplicator::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
for (MachineBasicBlock *PredBB : BB.predecessors()) {
if (PredBB->succ_size() > 1)
return false;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
return false;
if (!PredCond.empty())
return false;
}
return true;
}
bool TailDuplicator::duplicateSimpleBB(
MachineBasicBlock *TailBB, SmallVectorImpl<MachineBasicBlock *> &TDBBs,
const DenseSet<Register> &UsedByPhi,
SmallVectorImpl<MachineInstr *> &Copies) {
SmallPtrSet<MachineBasicBlock *, 8> Succs(TailBB->succ_begin(),
TailBB->succ_end());
SmallVector<MachineBasicBlock *, 8> Preds(TailBB->predecessors());
bool Changed = false;
for (MachineBasicBlock *PredBB : Preds) {
if (PredBB->hasEHPadSuccessor() || PredBB->mayHaveInlineAsmBr())
continue;
if (bothUsedInPHI(*PredBB, Succs))
continue;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
continue;
Changed = true;
LLVM_DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
<< "From simple Succ: " << *TailBB);
MachineBasicBlock *NewTarget = *TailBB->succ_begin();
MachineBasicBlock *NextBB = PredBB->getNextNode();
// Make PredFBB explicit.
if (PredCond.empty())
PredFBB = PredTBB;
// Make fall through explicit.
if (!PredTBB)
PredTBB = NextBB;
if (!PredFBB)
PredFBB = NextBB;
// Redirect
if (PredFBB == TailBB)
PredFBB = NewTarget;
if (PredTBB == TailBB)
PredTBB = NewTarget;
// Make the branch unconditional if possible
if (PredTBB == PredFBB) {
PredCond.clear();
PredFBB = nullptr;
}
// Avoid adding fall through branches.
if (PredFBB == NextBB)
PredFBB = nullptr;
if (PredTBB == NextBB && PredFBB == nullptr)
PredTBB = nullptr;
auto DL = PredBB->findBranchDebugLoc();
TII->removeBranch(*PredBB);
if (!PredBB->isSuccessor(NewTarget))
PredBB->replaceSuccessor(TailBB, NewTarget);
else {
PredBB->removeSuccessor(TailBB, true);
assert(PredBB->succ_size() <= 1);
}
// For AutoFDO, since BB is going to be removed, we won't be able to sample
// it. To avoid assigning a zero weight for BB, move all its pseudo probes
// into Succ and mark them dangling. This should allow the counts inference
// a chance to get a more reasonable weight for BB.
TailBB->moveAndDanglePseudoProbes(PredBB);
if (PredTBB)
TII->insertBranch(*PredBB, PredTBB, PredFBB, PredCond, DL);
TDBBs.push_back(PredBB);
}
return Changed;
}
bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
MachineBasicBlock *PredBB) {
// EH edges are ignored by analyzeBranch.
if (PredBB->succ_size() > 1)
return false;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
return false;
if (!PredCond.empty())
return false;
return true;
}
/// If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
/// \p IsSimple result of isSimpleBB
/// \p TailBB Block to be duplicated.
/// \p ForcedLayoutPred When non-null, use this block as the layout predecessor
/// instead of the previous block in MF's order.
/// \p TDBBs A vector to keep track of all blocks tail-duplicated
/// into.
/// \p Copies A vector of copy instructions inserted. Used later to
/// walk all the inserted copies and remove redundant ones.
bool TailDuplicator::tailDuplicate(bool IsSimple, MachineBasicBlock *TailBB,
MachineBasicBlock *ForcedLayoutPred,
SmallVectorImpl<MachineBasicBlock *> &TDBBs,
SmallVectorImpl<MachineInstr *> &Copies,
SmallVectorImpl<MachineBasicBlock *> *CandidatePtr) {
LLVM_DEBUG(dbgs() << "\n*** Tail-duplicating " << printMBBReference(*TailBB)
<< '\n');
bool ShouldUpdateTerminators = TailBB->canFallThrough();
DenseSet<Register> UsedByPhi;
getRegsUsedByPHIs(*TailBB, &UsedByPhi);
if (IsSimple)
return duplicateSimpleBB(TailBB, TDBBs, UsedByPhi, Copies);
// Iterate through all the unique predecessors and tail-duplicate this
// block into them, if possible. Copying the list ahead of time also
// avoids trouble with the predecessor list reallocating.
bool Changed = false;
SmallSetVector<MachineBasicBlock *, 8> Preds;
if (CandidatePtr)
Preds.insert(CandidatePtr->begin(), CandidatePtr->end());
else
Preds.insert(TailBB->pred_begin(), TailBB->pred_end());
for (MachineBasicBlock *PredBB : Preds) {
assert(TailBB != PredBB &&
"Single-block loop should have been rejected earlier!");
if (!canTailDuplicate(TailBB, PredBB))
continue;
// Don't duplicate into a fall-through predecessor (at least for now).
// If profile is available, findDuplicateCandidates can choose better
// fall-through predecessor.
if (!(MF->getFunction().hasProfileData() && LayoutMode)) {
bool IsLayoutSuccessor = false;
if (ForcedLayoutPred)
IsLayoutSuccessor = (ForcedLayoutPred == PredBB);
else if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
IsLayoutSuccessor = true;
if (IsLayoutSuccessor)
continue;
}
LLVM_DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
<< "From Succ: " << *TailBB);
TDBBs.push_back(PredBB);
// Remove PredBB's unconditional branch.
TII->removeBranch(*PredBB);
// Clone the contents of TailBB into PredBB.
DenseMap<Register, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<Register, RegSubRegPair>, 4> CopyInfos;
for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
I != E; /* empty */) {
MachineInstr *MI = &*I;
++I;
if (MI->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
} else {
// Replace def of virtual registers with new registers, and update
// uses with PHI source register or the new registers.
duplicateInstruction(MI, TailBB, PredBB, LocalVRMap, UsedByPhi);
}
}
appendCopies(PredBB, CopyInfos, Copies);
NumTailDupAdded += TailBB->size() - 1; // subtract one for removed branch
// Update the CFG.
PredBB->removeSuccessor(PredBB->succ_begin());
assert(PredBB->succ_empty() &&
"TailDuplicate called on block with multiple successors!");
for (MachineBasicBlock *Succ : TailBB->successors())
PredBB->addSuccessor(Succ, MBPI->getEdgeProbability(TailBB, Succ));
// Update branches in pred to jump to tail's layout successor if needed.
if (ShouldUpdateTerminators)
PredBB->updateTerminator(TailBB->getNextNode());
Changed = true;
++NumTailDups;
}
// If TailBB was duplicated into all its predecessors except for the prior
// block, which falls through unconditionally, move the contents of this
// block into the prior block.
MachineBasicBlock *PrevBB = ForcedLayoutPred;
if (!PrevBB)
PrevBB = &*std::prev(TailBB->getIterator());
MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
SmallVector<MachineOperand, 4> PriorCond;
// This has to check PrevBB->succ_size() because EH edges are ignored by
// analyzeBranch.
if (PrevBB->succ_size() == 1 &&
// Layout preds are not always CFG preds. Check.
*PrevBB->succ_begin() == TailBB &&
!TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond) &&
PriorCond.empty() &&
(!PriorTBB || PriorTBB == TailBB) &&
TailBB->pred_size() == 1 &&
!TailBB->hasAddressTaken()) {
LLVM_DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
<< "From MBB: " << *TailBB);
// There may be a branch to the layout successor. This is unlikely but it
// happens. The correct thing to do is to remove the branch before
// duplicating the instructions in all cases.
TII->removeBranch(*PrevBB);
if (PreRegAlloc) {
DenseMap<Register, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<Register, RegSubRegPair>, 4> CopyInfos;
MachineBasicBlock::iterator I = TailBB->begin();
// Process PHI instructions first.
while (I != TailBB->end() && I->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
processPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
}
// Now copy the non-PHI instructions.
while (I != TailBB->end()) {
// Replace def of virtual registers with new registers, and update
// uses with PHI source register or the new registers.
MachineInstr *MI = &*I++;
assert(!MI->isBundle() && "Not expecting bundles before regalloc!");
duplicateInstruction(MI, TailBB, PrevBB, LocalVRMap, UsedByPhi);
MI->eraseFromParent();
}
appendCopies(PrevBB, CopyInfos, Copies);
} else {
TII->removeBranch(*PrevBB);
// No PHIs to worry about, just splice the instructions over.
PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
}
PrevBB->removeSuccessor(PrevBB->succ_begin());
assert(PrevBB->succ_empty());
PrevBB->transferSuccessors(TailBB);
// Update branches in PrevBB based on Tail's layout successor.
if (ShouldUpdateTerminators)
PrevBB->updateTerminator(TailBB->getNextNode());
TDBBs.push_back(PrevBB);
Changed = true;
}
// If this is after register allocation, there are no phis to fix.
if (!PreRegAlloc)
return Changed;
// If we made no changes so far, we are safe.
if (!Changed)
return Changed;
// Handle the nasty case in that we duplicated a block that is part of a loop
// into some but not all of its predecessors. For example:
// 1 -> 2 <-> 3 |
// \ |
// \---> rest |
// if we duplicate 2 into 1 but not into 3, we end up with
// 12 -> 3 <-> 2 -> rest |
// \ / |
// \----->-----/ |
// If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
// with a phi in 3 (which now dominates 2).
// What we do here is introduce a copy in 3 of the register defined by the
// phi, just like when we are duplicating 2 into 3, but we don't copy any
// real instructions or remove the 3 -> 2 edge from the phi in 2.
for (MachineBasicBlock *PredBB : Preds) {
if (is_contained(TDBBs, PredBB))
continue;
// EH edges
if (PredBB->succ_size() != 1)
continue;
DenseMap<Register, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<Register, RegSubRegPair>, 4> CopyInfos;
MachineBasicBlock::iterator I = TailBB->begin();
// Process PHI instructions first.
while (I != TailBB->end() && I->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
}
appendCopies(PredBB, CopyInfos, Copies);
}
return Changed;
}
/// At the end of the block \p MBB generate COPY instructions between registers
/// described by \p CopyInfos. Append resulting instructions to \p Copies.
void TailDuplicator::appendCopies(MachineBasicBlock *MBB,
SmallVectorImpl<std::pair<Register, RegSubRegPair>> &CopyInfos,
SmallVectorImpl<MachineInstr*> &Copies) {
MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
const MCInstrDesc &CopyD = TII->get(TargetOpcode::COPY);
for (auto &CI : CopyInfos) {
auto C = BuildMI(*MBB, Loc, DebugLoc(), CopyD, CI.first)
.addReg(CI.second.Reg, 0, CI.second.SubReg);
Copies.push_back(C);
}
}
/// Remove the specified dead machine basic block from the function, updating
/// the CFG.
void TailDuplicator::removeDeadBlock(
MachineBasicBlock *MBB,
function_ref<void(MachineBasicBlock *)> *RemovalCallback) {
assert(MBB->pred_empty() && "MBB must be dead!");
LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
MachineFunction *MF = MBB->getParent();
// Update the call site info.
for (const MachineInstr &MI : *MBB)
if (MI.shouldUpdateCallSiteInfo())
MF->eraseCallSiteInfo(&MI);
if (RemovalCallback)
(*RemovalCallback)(MBB);
// Remove all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end() - 1);
// Remove the block.
MBB->eraseFromParent();
}
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