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llvm/lib/CodeGen/MachineSink.cpp

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//===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass moves instructions into successor blocks when possible, so that
// they aren't executed on paths where their results aren't needed.
//
// This pass is not intended to be a replacement or a complete alternative
// for an LLVM-IR-level sinking pass. It is only designed to sink simple
// constructs that are not exposed before lowering and instruction selection.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SparseBitVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachinePostDominators.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <map>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "machine-sink"
static cl::opt<bool>
SplitEdges("machine-sink-split",
cl::desc("Split critical edges during machine sinking"),
cl::init(true), cl::Hidden);
static cl::opt<bool>
UseBlockFreqInfo("machine-sink-bfi",
cl::desc("Use block frequency info to find successors to sink"),
cl::init(true), cl::Hidden);
STATISTIC(NumSunk, "Number of machine instructions sunk");
STATISTIC(NumSplit, "Number of critical edges split");
STATISTIC(NumCoalesces, "Number of copies coalesced");
namespace {
class MachineSinking : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI; // Machine register information
MachineDominatorTree *DT; // Machine dominator tree
MachinePostDominatorTree *PDT; // Machine post dominator tree
MachineLoopInfo *LI;
const MachineBlockFrequencyInfo *MBFI;
AliasAnalysis *AA;
// Remember which edges have been considered for breaking.
SmallSet<std::pair<MachineBasicBlock*, MachineBasicBlock*>, 8>
CEBCandidates;
// Remember which edges we are about to split.
// This is different from CEBCandidates since those edges
// will be split.
SetVector<std::pair<MachineBasicBlock*, MachineBasicBlock*> > ToSplit;
SparseBitVector<> RegsToClearKillFlags;
typedef std::map<MachineBasicBlock *, SmallVector<MachineBasicBlock *, 4>>
AllSuccsCache;
public:
static char ID; // Pass identification
MachineSinking() : MachineFunctionPass(ID) {
initializeMachineSinkingPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<MachinePostDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineDominatorTree>();
AU.addPreserved<MachinePostDominatorTree>();
AU.addPreserved<MachineLoopInfo>();
if (UseBlockFreqInfo)
AU.addRequired<MachineBlockFrequencyInfo>();
}
void releaseMemory() override {
CEBCandidates.clear();
}
private:
bool ProcessBlock(MachineBasicBlock &MBB);
bool isWorthBreakingCriticalEdge(MachineInstr &MI,
MachineBasicBlock *From,
MachineBasicBlock *To);
/// \brief Postpone the splitting of the given critical
/// edge (\p From, \p To).
///
/// We do not split the edges on the fly. Indeed, this invalidates
/// the dominance information and thus triggers a lot of updates
/// of that information underneath.
/// Instead, we postpone all the splits after each iteration of
/// the main loop. That way, the information is at least valid
/// for the lifetime of an iteration.
///
/// \return True if the edge is marked as toSplit, false otherwise.
/// False can be returned if, for instance, this is not profitable.
bool PostponeSplitCriticalEdge(MachineInstr &MI,
MachineBasicBlock *From,
MachineBasicBlock *To,
bool BreakPHIEdge);
bool SinkInstruction(MachineInstr &MI, bool &SawStore,
AllSuccsCache &AllSuccessors);
bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB,
bool &BreakPHIEdge, bool &LocalUse) const;
MachineBasicBlock *FindSuccToSinkTo(MachineInstr &MI, MachineBasicBlock *MBB,
bool &BreakPHIEdge, AllSuccsCache &AllSuccessors);
bool isProfitableToSinkTo(unsigned Reg, MachineInstr &MI,
MachineBasicBlock *MBB,
MachineBasicBlock *SuccToSinkTo,
AllSuccsCache &AllSuccessors);
bool PerformTrivialForwardCoalescing(MachineInstr &MI,
MachineBasicBlock *MBB);
SmallVector<MachineBasicBlock *, 4> &
GetAllSortedSuccessors(MachineInstr &MI, MachineBasicBlock *MBB,
AllSuccsCache &AllSuccessors) const;
};
} // end anonymous namespace
char MachineSinking::ID = 0;
char &llvm::MachineSinkingID = MachineSinking::ID;
INITIALIZE_PASS_BEGIN(MachineSinking, "machine-sink",
"Machine code sinking", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MachineSinking, "machine-sink",
"Machine code sinking", false, false)
bool MachineSinking::PerformTrivialForwardCoalescing(MachineInstr &MI,
MachineBasicBlock *MBB) {
if (!MI.isCopy())
return false;
unsigned SrcReg = MI.getOperand(1).getReg();
unsigned DstReg = MI.getOperand(0).getReg();
if (!TargetRegisterInfo::isVirtualRegister(SrcReg) ||
!TargetRegisterInfo::isVirtualRegister(DstReg) ||
!MRI->hasOneNonDBGUse(SrcReg))
return false;
const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
const TargetRegisterClass *DRC = MRI->getRegClass(DstReg);
if (SRC != DRC)
return false;
MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
if (DefMI->isCopyLike())
return false;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << MI);
MRI->replaceRegWith(DstReg, SrcReg);
MI.eraseFromParent();
// Conservatively, clear any kill flags, since it's possible that they are no
// longer correct.
MRI->clearKillFlags(SrcReg);
++NumCoalesces;
return true;
}
/// AllUsesDominatedByBlock - Return true if all uses of the specified register
/// occur in blocks dominated by the specified block. If any use is in the
/// definition block, then return false since it is never legal to move def
/// after uses.
bool
MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB,
bool &BreakPHIEdge,
bool &LocalUse) const {
assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
"Only makes sense for vregs");
// Ignore debug uses because debug info doesn't affect the code.
if (MRI->use_nodbg_empty(Reg))
return true;
// BreakPHIEdge is true if all the uses are in the successor MBB being sunken
// into and they are all PHI nodes. In this case, machine-sink must break
// the critical edge first. e.g.
//
// BB#1: derived from LLVM BB %bb4.preheader
// Predecessors according to CFG: BB#0
// ...
// %reg16385<def> = DEC64_32r %reg16437, %EFLAGS<imp-def,dead>
// ...
// JE_4 <BB#37>, %EFLAGS<imp-use>
// Successors according to CFG: BB#37 BB#2
//
// BB#2: derived from LLVM BB %bb.nph
// Predecessors according to CFG: BB#0 BB#1
// %reg16386<def> = PHI %reg16434, <BB#0>, %reg16385, <BB#1>
BreakPHIEdge = true;
for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
MachineInstr *UseInst = MO.getParent();
unsigned OpNo = &MO - &UseInst->getOperand(0);
MachineBasicBlock *UseBlock = UseInst->getParent();
if (!(UseBlock == MBB && UseInst->isPHI() &&
UseInst->getOperand(OpNo+1).getMBB() == DefMBB)) {
BreakPHIEdge = false;
break;
}
}
if (BreakPHIEdge)
return true;
for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
// Determine the block of the use.
MachineInstr *UseInst = MO.getParent();
unsigned OpNo = &MO - &UseInst->getOperand(0);
MachineBasicBlock *UseBlock = UseInst->getParent();
if (UseInst->isPHI()) {
// PHI nodes use the operand in the predecessor block, not the block with
// the PHI.
UseBlock = UseInst->getOperand(OpNo+1).getMBB();
} else if (UseBlock == DefMBB) {
LocalUse = true;
return false;
}
// Check that it dominates.
if (!DT->dominates(MBB, UseBlock))
return false;
}
return true;
}
bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(*MF.getFunction()))
return false;
DEBUG(dbgs() << "******** Machine Sinking ********\n");
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
DT = &getAnalysis<MachineDominatorTree>();
PDT = &getAnalysis<MachinePostDominatorTree>();
LI = &getAnalysis<MachineLoopInfo>();
MBFI = UseBlockFreqInfo ? &getAnalysis<MachineBlockFrequencyInfo>() : nullptr;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
bool EverMadeChange = false;
while (true) {
bool MadeChange = false;
// Process all basic blocks.
CEBCandidates.clear();
ToSplit.clear();
for (auto &MBB: MF)
MadeChange |= ProcessBlock(MBB);
// If we have anything we marked as toSplit, split it now.
for (auto &Pair : ToSplit) {
auto NewSucc = Pair.first->SplitCriticalEdge(Pair.second, *this);
if (NewSucc != nullptr) {
DEBUG(dbgs() << " *** Splitting critical edge:"
" BB#" << Pair.first->getNumber()
<< " -- BB#" << NewSucc->getNumber()
<< " -- BB#" << Pair.second->getNumber() << '\n');
MadeChange = true;
++NumSplit;
} else
DEBUG(dbgs() << " *** Not legal to break critical edge\n");
}
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
}
// Now clear any kill flags for recorded registers.
for (auto I : RegsToClearKillFlags)
MRI->clearKillFlags(I);
RegsToClearKillFlags.clear();
return EverMadeChange;
}
bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
// Can't sink anything out of a block that has less than two successors.
if (MBB.succ_size() <= 1 || MBB.empty()) return false;
// Don't bother sinking code out of unreachable blocks. In addition to being
// unprofitable, it can also lead to infinite looping, because in an
// unreachable loop there may be nowhere to stop.
if (!DT->isReachableFromEntry(&MBB)) return false;
bool MadeChange = false;
// Cache all successors, sorted by frequency info and loop depth.
AllSuccsCache AllSuccessors;
// Walk the basic block bottom-up. Remember if we saw a store.
MachineBasicBlock::iterator I = MBB.end();
--I;
bool ProcessedBegin, SawStore = false;
do {
MachineInstr &MI = *I; // The instruction to sink.
// Predecrement I (if it's not begin) so that it isn't invalidated by
// sinking.
ProcessedBegin = I == MBB.begin();
if (!ProcessedBegin)
--I;
if (MI.isDebugValue())
continue;
bool Joined = PerformTrivialForwardCoalescing(MI, &MBB);
if (Joined) {
MadeChange = true;
continue;
}
if (SinkInstruction(MI, SawStore, AllSuccessors)) {
++NumSunk;
MadeChange = true;
}
// If we just processed the first instruction in the block, we're done.
} while (!ProcessedBegin);
return MadeChange;
}
bool MachineSinking::isWorthBreakingCriticalEdge(MachineInstr &MI,
MachineBasicBlock *From,
MachineBasicBlock *To) {
// FIXME: Need much better heuristics.
// If the pass has already considered breaking this edge (during this pass
// through the function), then let's go ahead and break it. This means
// sinking multiple "cheap" instructions into the same block.
if (!CEBCandidates.insert(std::make_pair(From, To)).second)
return true;
if (!MI.isCopy() && !TII->isAsCheapAsAMove(MI))
return true;
// MI is cheap, we probably don't want to break the critical edge for it.
// However, if this would allow some definitions of its source operands
// to be sunk then it's probably worth it.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg() || !MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (Reg == 0)
continue;
// We don't move live definitions of physical registers,
// so sinking their uses won't enable any opportunities.
if (TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
// If this instruction is the only user of a virtual register,
// check if breaking the edge will enable sinking
// both this instruction and the defining instruction.
if (MRI->hasOneNonDBGUse(Reg)) {
// If the definition resides in same MBB,
// claim it's likely we can sink these together.
// If definition resides elsewhere, we aren't
// blocking it from being sunk so don't break the edge.
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (DefMI->getParent() == MI.getParent())
return true;
}
}
return false;
}
bool MachineSinking::PostponeSplitCriticalEdge(MachineInstr &MI,
MachineBasicBlock *FromBB,
MachineBasicBlock *ToBB,
bool BreakPHIEdge) {
if (!isWorthBreakingCriticalEdge(MI, FromBB, ToBB))
return false;
// Avoid breaking back edge. From == To means backedge for single BB loop.
if (!SplitEdges || FromBB == ToBB)
return false;
// Check for backedges of more "complex" loops.
if (LI->getLoopFor(FromBB) == LI->getLoopFor(ToBB) &&
LI->isLoopHeader(ToBB))
return false;
// It's not always legal to break critical edges and sink the computation
// to the edge.
//
// BB#1:
// v1024
// Beq BB#3
// <fallthrough>
// BB#2:
// ... no uses of v1024
// <fallthrough>
// BB#3:
// ...
// = v1024
//
// If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
//
// BB#1:
// ...
// Bne BB#2
// BB#4:
// v1024 =
// B BB#3
// BB#2:
// ... no uses of v1024
// <fallthrough>
// BB#3:
// ...
// = v1024
//
// This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
// flow. We need to ensure the new basic block where the computation is
// sunk to dominates all the uses.
// It's only legal to break critical edge and sink the computation to the
// new block if all the predecessors of "To", except for "From", are
// not dominated by "From". Given SSA property, this means these
// predecessors are dominated by "To".
//
// There is no need to do this check if all the uses are PHI nodes. PHI
// sources are only defined on the specific predecessor edges.
if (!BreakPHIEdge) {
for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(),
E = ToBB->pred_end(); PI != E; ++PI) {
if (*PI == FromBB)
continue;
if (!DT->dominates(ToBB, *PI))
return false;
}
}
ToSplit.insert(std::make_pair(FromBB, ToBB));
return true;
}
/// collectDebgValues - Scan instructions following MI and collect any
/// matching DBG_VALUEs.
static void collectDebugValues(MachineInstr &MI,
SmallVectorImpl<MachineInstr *> &DbgValues) {
DbgValues.clear();
if (!MI.getOperand(0).isReg())
return;
MachineBasicBlock::iterator DI = MI; ++DI;
for (MachineBasicBlock::iterator DE = MI.getParent()->end();
DI != DE; ++DI) {
if (!DI->isDebugValue())
return;
if (DI->getOperand(0).isReg() &&
DI->getOperand(0).getReg() == MI.getOperand(0).getReg())
DbgValues.push_back(&*DI);
}
}
/// isProfitableToSinkTo - Return true if it is profitable to sink MI.
bool MachineSinking::isProfitableToSinkTo(unsigned Reg, MachineInstr &MI,
MachineBasicBlock *MBB,
MachineBasicBlock *SuccToSinkTo,
AllSuccsCache &AllSuccessors) {
assert (SuccToSinkTo && "Invalid SinkTo Candidate BB");
if (MBB == SuccToSinkTo)
return false;
// It is profitable if SuccToSinkTo does not post dominate current block.
if (!PDT->dominates(SuccToSinkTo, MBB))
return true;
// It is profitable to sink an instruction from a deeper loop to a shallower
// loop, even if the latter post-dominates the former (PR21115).
if (LI->getLoopDepth(MBB) > LI->getLoopDepth(SuccToSinkTo))
return true;
// Check if only use in post dominated block is PHI instruction.
bool NonPHIUse = false;
for (MachineInstr &UseInst : MRI->use_nodbg_instructions(Reg)) {
MachineBasicBlock *UseBlock = UseInst.getParent();
if (UseBlock == SuccToSinkTo && !UseInst.isPHI())
NonPHIUse = true;
}
if (!NonPHIUse)
return true;
// If SuccToSinkTo post dominates then also it may be profitable if MI
// can further profitably sinked into another block in next round.
bool BreakPHIEdge = false;
// FIXME - If finding successor is compile time expensive then cache results.
if (MachineBasicBlock *MBB2 =
FindSuccToSinkTo(MI, SuccToSinkTo, BreakPHIEdge, AllSuccessors))
return isProfitableToSinkTo(Reg, MI, SuccToSinkTo, MBB2, AllSuccessors);
// If SuccToSinkTo is final destination and it is a post dominator of current
// block then it is not profitable to sink MI into SuccToSinkTo block.
return false;
}
/// Get the sorted sequence of successors for this MachineBasicBlock, possibly
/// computing it if it was not already cached.
SmallVector<MachineBasicBlock *, 4> &
MachineSinking::GetAllSortedSuccessors(MachineInstr &MI, MachineBasicBlock *MBB,
AllSuccsCache &AllSuccessors) const {
// Do we have the sorted successors in cache ?
auto Succs = AllSuccessors.find(MBB);
if (Succs != AllSuccessors.end())
return Succs->second;
SmallVector<MachineBasicBlock *, 4> AllSuccs(MBB->succ_begin(),
MBB->succ_end());
// Handle cases where sinking can happen but where the sink point isn't a
// successor. For example:
//
// x = computation
// if () {} else {}
// use x
//
const std::vector<MachineDomTreeNode *> &Children =
DT->getNode(MBB)->getChildren();
for (const auto &DTChild : Children)
// DomTree children of MBB that have MBB as immediate dominator are added.
if (DTChild->getIDom()->getBlock() == MI.getParent() &&
// Skip MBBs already added to the AllSuccs vector above.
!MBB->isSuccessor(DTChild->getBlock()))
AllSuccs.push_back(DTChild->getBlock());
// Sort Successors according to their loop depth or block frequency info.
std::stable_sort(
AllSuccs.begin(), AllSuccs.end(),
[this](const MachineBasicBlock *L, const MachineBasicBlock *R) {
uint64_t LHSFreq = MBFI ? MBFI->getBlockFreq(L).getFrequency() : 0;
uint64_t RHSFreq = MBFI ? MBFI->getBlockFreq(R).getFrequency() : 0;
bool HasBlockFreq = LHSFreq != 0 && RHSFreq != 0;
return HasBlockFreq ? LHSFreq < RHSFreq
: LI->getLoopDepth(L) < LI->getLoopDepth(R);
});
auto it = AllSuccessors.insert(std::make_pair(MBB, AllSuccs));
return it.first->second;
}
/// FindSuccToSinkTo - Find a successor to sink this instruction to.
MachineBasicBlock *
MachineSinking::FindSuccToSinkTo(MachineInstr &MI, MachineBasicBlock *MBB,
bool &BreakPHIEdge,
AllSuccsCache &AllSuccessors) {
assert (MBB && "Invalid MachineBasicBlock!");
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
MachineBasicBlock *SuccToSinkTo = nullptr;
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg()) continue; // Ignore non-register operands.
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
if (MO.isUse()) {
// If the physreg has no defs anywhere, it's just an ambient register
// and we can freely move its uses. Alternatively, if it's allocatable,
// it could get allocated to something with a def during allocation.
if (!MRI->isConstantPhysReg(Reg, *MBB->getParent()))
return nullptr;
} else if (!MO.isDead()) {
// A def that isn't dead. We can't move it.
return nullptr;
}
} else {
// Virtual register uses are always safe to sink.
if (MO.isUse()) continue;
// If it's not safe to move defs of the register class, then abort.
if (!TII->isSafeToMoveRegClassDefs(MRI->getRegClass(Reg)))
return nullptr;
// Virtual register defs can only be sunk if all their uses are in blocks
// dominated by one of the successors.
if (SuccToSinkTo) {
// If a previous operand picked a block to sink to, then this operand
// must be sinkable to the same block.
bool LocalUse = false;
if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, MBB,
BreakPHIEdge, LocalUse))
return nullptr;
continue;
}
// Otherwise, we should look at all the successors and decide which one
// we should sink to. If we have reliable block frequency information
// (frequency != 0) available, give successors with smaller frequencies
// higher priority, otherwise prioritize smaller loop depths.
for (MachineBasicBlock *SuccBlock :
GetAllSortedSuccessors(MI, MBB, AllSuccessors)) {
bool LocalUse = false;
if (AllUsesDominatedByBlock(Reg, SuccBlock, MBB,
BreakPHIEdge, LocalUse)) {
SuccToSinkTo = SuccBlock;
break;
}
if (LocalUse)
// Def is used locally, it's never safe to move this def.
return nullptr;
}
// If we couldn't find a block to sink to, ignore this instruction.
if (!SuccToSinkTo)
return nullptr;
if (!isProfitableToSinkTo(Reg, MI, MBB, SuccToSinkTo, AllSuccessors))
return nullptr;
}
}
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (MBB == SuccToSinkTo)
return nullptr;
// It's not safe to sink instructions to EH landing pad. Control flow into
// landing pad is implicitly defined.
if (SuccToSinkTo && SuccToSinkTo->isEHPad())
return nullptr;
return SuccToSinkTo;
}
/// \brief Return true if MI is likely to be usable as a memory operation by the
/// implicit null check optimization.
///
/// This is a "best effort" heuristic, and should not be relied upon for
/// correctness. This returning true does not guarantee that the implicit null
/// check optimization is legal over MI, and this returning false does not
/// guarantee MI cannot possibly be used to do a null check.
static bool SinkingPreventsImplicitNullCheck(MachineInstr &MI,
const TargetInstrInfo *TII,
const TargetRegisterInfo *TRI) {
typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
auto *MBB = MI.getParent();
if (MBB->pred_size() != 1)
return false;
auto *PredMBB = *MBB->pred_begin();
auto *PredBB = PredMBB->getBasicBlock();
// Frontends that don't use implicit null checks have no reason to emit
// branches with make.implicit metadata, and this function should always
// return false for them.
if (!PredBB ||
!PredBB->getTerminator()->getMetadata(LLVMContext::MD_make_implicit))
return false;
unsigned BaseReg;
int64_t Offset;
if (!TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI))
return false;
if (!(MI.mayLoad() && !MI.isPredicable()))
return false;
MachineBranchPredicate MBP;
if (TII->analyzeBranchPredicate(*PredMBB, MBP, false))
return false;
return MBP.LHS.isReg() && MBP.RHS.isImm() && MBP.RHS.getImm() == 0 &&
(MBP.Predicate == MachineBranchPredicate::PRED_NE ||
MBP.Predicate == MachineBranchPredicate::PRED_EQ) &&
MBP.LHS.getReg() == BaseReg;
}
/// SinkInstruction - Determine whether it is safe to sink the specified machine
/// instruction out of its current block into a successor.
bool MachineSinking::SinkInstruction(MachineInstr &MI, bool &SawStore,
AllSuccsCache &AllSuccessors) {
// Don't sink instructions that the target prefers not to sink.
if (!TII->shouldSink(MI))
return false;
// Check if it's safe to move the instruction.
if (!MI.isSafeToMove(AA, SawStore))
return false;
// Convergent operations may not be made control-dependent on additional
// values.
if (MI.isConvergent())
return false;
// Don't break implicit null checks. This is a performance heuristic, and not
// required for correctness.
if (SinkingPreventsImplicitNullCheck(MI, TII, TRI))
return false;
// FIXME: This should include support for sinking instructions within the
// block they are currently in to shorten the live ranges. We often get
// instructions sunk into the top of a large block, but it would be better to
// also sink them down before their first use in the block. This xform has to
// be careful not to *increase* register pressure though, e.g. sinking
// "x = y + z" down if it kills y and z would increase the live ranges of y
// and z and only shrink the live range of x.
bool BreakPHIEdge = false;
MachineBasicBlock *ParentBlock = MI.getParent();
MachineBasicBlock *SuccToSinkTo =
FindSuccToSinkTo(MI, ParentBlock, BreakPHIEdge, AllSuccessors);
// If there are no outputs, it must have side-effects.
if (!SuccToSinkTo)
return false;
// If the instruction to move defines a dead physical register which is live
// when leaving the basic block, don't move it because it could turn into a
// "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>)
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 == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg)) continue;
if (SuccToSinkTo->isLiveIn(Reg))
return false;
}
DEBUG(dbgs() << "Sink instr " << MI << "\tinto block " << *SuccToSinkTo);
// If the block has multiple predecessors, this is a critical edge.
// Decide if we can sink along it or need to break the edge.
if (SuccToSinkTo->pred_size() > 1) {
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
bool TryBreak = false;
bool store = true;
if (!MI.isSafeToMove(AA, store)) {
DEBUG(dbgs() << " *** NOTE: Won't sink load along critical edge.\n");
TryBreak = true;
}
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!TryBreak && !DT->dominates(ParentBlock, SuccToSinkTo)) {
DEBUG(dbgs() << " *** NOTE: Critical edge found\n");
TryBreak = true;
}
// Don't sink instructions into a loop.
if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) {
DEBUG(dbgs() << " *** NOTE: Loop header found\n");
TryBreak = true;
}
// Otherwise we are OK with sinking along a critical edge.
if (!TryBreak)
DEBUG(dbgs() << "Sinking along critical edge.\n");
else {
// Mark this edge as to be split.
// If the edge can actually be split, the next iteration of the main loop
// will sink MI in the newly created block.
bool Status =
PostponeSplitCriticalEdge(MI, ParentBlock, SuccToSinkTo, BreakPHIEdge);
if (!Status)
DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
"break critical edge\n");
// The instruction will not be sunk this time.
return false;
}
}
if (BreakPHIEdge) {
// BreakPHIEdge is true if all the uses are in the successor MBB being
// sunken into and they are all PHI nodes. In this case, machine-sink must
// break the critical edge first.
bool Status = PostponeSplitCriticalEdge(MI, ParentBlock,
SuccToSinkTo, BreakPHIEdge);
if (!Status)
DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
"break critical edge\n");
// The instruction will not be sunk this time.
return false;
}
// Determine where to insert into. Skip phi nodes.
MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI())
++InsertPos;
// collect matching debug values.
SmallVector<MachineInstr *, 2> DbgValuesToSink;
collectDebugValues(MI, DbgValuesToSink);
// Move the instruction.
SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
++MachineBasicBlock::iterator(MI));
// Move debug values.
for (SmallVectorImpl<MachineInstr *>::iterator DBI = DbgValuesToSink.begin(),
DBE = DbgValuesToSink.end(); DBI != DBE; ++DBI) {
MachineInstr *DbgMI = *DBI;
SuccToSinkTo->splice(InsertPos, ParentBlock, DbgMI,
++MachineBasicBlock::iterator(DbgMI));
}
// Conservatively, clear any kill flags, since it's possible that they are no
// longer correct.
// Note that we have to clear the kill flags for any register this instruction
// uses as we may sink over another instruction which currently kills the
// used registers.
for (MachineOperand &MO : MI.operands()) {
if (MO.isReg() && MO.isUse())
RegsToClearKillFlags.set(MO.getReg()); // Remember to clear kill flags.
}
return true;
}
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