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//===- MachineSink.cpp - Sinking for machine instructions -----------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// 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/ADT/DenseSet.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.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/MachineBranchProbabilityInfo.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/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.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>
cl::desc("Split critical edges during machine sinking"),
cl::init(true), cl::Hidden);
static cl::opt<bool>
cl::desc("Use block frequency info to find successors to sink"),
cl::init(true), cl::Hidden);
static cl::opt<unsigned> SplitEdgeProbabilityThreshold(
"Percentage threshold for splitting single-instruction critical edge. "
"If the branch threshold is higher than this threshold, we allow "
"speculative execution of up to 1 instruction to avoid branching to "
"splitted critical edge"),
cl::init(40), cl::Hidden);
STATISTIC(NumSunk, "Number of machine instructions sunk");
STATISTIC(NumSplit, "Number of critical edges split");
STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumPostRACopySink, "Number of copies sunk after RA");
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;
const MachineBranchProbabilityInfo *MBPI;
AliasAnalysis *AA;
// Remember which edges have been considered for breaking.
SmallSet<std::pair<MachineBasicBlock*, MachineBasicBlock*>, 8>
// 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;
using AllSuccsCache =
std::map<MachineBasicBlock *, SmallVector<MachineBasicBlock *, 4>>;
/// DBG_VALUE pointer and flag. The flag is true if this DBG_VALUE is
/// post-dominated by another DBG_VALUE of the same variable location.
/// This is necessary to detect sequences such as:
/// %0 = someinst
/// DBG_VALUE %0, !123, !DIExpression()
/// %1 = anotherinst
/// DBG_VALUE %1, !123, !DIExpression()
/// Where if %0 were to sink, the DBG_VAUE should not sink with it, as that
/// would re-order assignments.
using SeenDbgUser = PointerIntPair<MachineInstr *, 1>;
/// Record of DBG_VALUE uses of vregs in a block, so that we can identify
/// debug instructions to sink.
SmallDenseMap<unsigned, TinyPtrVector<SeenDbgUser>> SeenDbgUsers;
/// Record of debug variables that have had their locations set in the
/// current block.
DenseSet<DebugVariable> SeenDbgVars;
static char ID; // Pass identification
MachineSinking() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (UseBlockFreqInfo)
void releaseMemory() override {
bool ProcessBlock(MachineBasicBlock &MBB);
void ProcessDbgInst(MachineInstr &MI);
bool isWorthBreakingCriticalEdge(MachineInstr &MI,
MachineBasicBlock *From,
MachineBasicBlock *To);
/// 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);
/// If we sink a COPY inst, some debug users of it's destination may no
/// longer be dominated by the COPY, and will eventually be dropped.
/// This is easily rectified by forwarding the non-dominated debug uses
/// to the copy source.
void SalvageUnsunkDebugUsersOfCopy(MachineInstr &,
MachineBasicBlock *TargetBlock);
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;
"Machine code sinking", false, false)
"Machine code sinking", false, false)
bool MachineSinking::PerformTrivialForwardCoalescing(MachineInstr &MI,
MachineBasicBlock *MBB) {
if (!MI.isCopy())
return false;
Register SrcReg = MI.getOperand(1).getReg();
Register DstReg = MI.getOperand(0).getReg();
if (!Register::isVirtualRegister(SrcReg) ||
!Register::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;
LLVM_DEBUG(dbgs() << "Coalescing: " << *DefMI);
LLVM_DEBUG(dbgs() << "*** to: " << MI);
MRI->replaceRegWith(DstReg, SrcReg);
// Conservatively, clear any kill flags, since it's possible that they are no
// longer correct.
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.
MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB,
bool &BreakPHIEdge,
bool &LocalUse) const {
assert(Register::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 = DEC64_32r %reg16437, implicit-def dead %eflags
// ...
// JE_4 <%bb.37>, implicit %eflags
// 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 = 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;
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;
LLVM_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;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
bool EverMadeChange = false;
while (true) {
bool MadeChange = false;
// Process all basic blocks.
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) {
LLVM_DEBUG(dbgs() << " *** Splitting critical edge: "
<< printMBBReference(*Pair.first) << " -- "
<< printMBBReference(*NewSucc) << " -- "
<< printMBBReference(*Pair.second) << '\n');
MadeChange = true;
} else
LLVM_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)
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();
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)
if (MI.isDebugInstr()) {
if (MI.isDebugValue())
bool Joined = PerformTrivialForwardCoalescing(MI, &MBB);
if (Joined) {
MadeChange = true;
if (SinkInstruction(MI, SawStore, AllSuccessors)) {
MadeChange = true;
// If we just processed the first instruction in the block, we're done.
} while (!ProcessedBegin);
return MadeChange;
void MachineSinking::ProcessDbgInst(MachineInstr &MI) {
// When we see DBG_VALUEs for registers, record any vreg it reads, so that
// we know what to sink if the vreg def sinks.
assert(MI.isDebugValue() && "Expected DBG_VALUE for processing");
DebugVariable Var(MI.getDebugVariable(), MI.getDebugExpression(),
bool SeenBefore = SeenDbgVars.count(Var) != 0;
MachineOperand &MO = MI.getOperand(0);
if (MO.isReg() && MO.getReg().isVirtual())
SeenDbgUsers[MO.getReg()].push_back(SeenDbgUser(&MI, SeenBefore));
// Record the variable for any DBG_VALUE, to avoid re-ordering any of them.
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;
if (From->isSuccessor(To) && MBPI->getEdgeProbability(From, To) <=
BranchProbability(SplitEdgeProbabilityThreshold, 100))
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())
Register Reg = MO.getReg();
if (Reg == 0)
// We don't move live definitions of physical registers,
// so sinking their uses won't enable any opportunities.
if (Register::isPhysicalRegister(Reg))
// 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) &&
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)
if (!DT->dominates(ToBB, *PI))
return false;
ToSplit.insert(std::make_pair(FromBB, ToBB));
return true;
/// 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(),
// 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 =
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.
// Sort Successors according to their loop depth or block frequency info.
AllSuccs, [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.
Register Reg = MO.getReg();
if (Reg == 0) continue;
if (Register::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))
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;
// 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;
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;
/// 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) {
using MachineBranchPredicate = TargetInstrInfo::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 ||
return false;
const MachineOperand *BaseOp;
int64_t Offset;
if (!TII->getMemOperandWithOffset(MI, BaseOp, Offset, TRI))
return false;
if (!BaseOp->isReg())
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() == BaseOp->getReg();
/// If the sunk instruction is a copy, try to forward the copy instead of
/// leaving an 'undef' DBG_VALUE in the original location. Don't do this if
/// there's any subregister weirdness involved. Returns true if copy
/// propagation occurred.
static bool attemptDebugCopyProp(MachineInstr &SinkInst, MachineInstr &DbgMI) {
const MachineRegisterInfo &MRI = SinkInst.getMF()->getRegInfo();
const TargetInstrInfo &TII = *SinkInst.getMF()->getSubtarget().getInstrInfo();
// Copy DBG_VALUE operand and set the original to undef. We then check to
// see whether this is something that can be copy-forwarded. If it isn't,
// continue around the loop.
MachineOperand DbgMO = DbgMI.getOperand(0);
const MachineOperand *SrcMO = nullptr, *DstMO = nullptr;
auto CopyOperands = TII.isCopyInstr(SinkInst);
if (!CopyOperands)
return false;
SrcMO = CopyOperands->Source;
DstMO = CopyOperands->Destination;
// Check validity of forwarding this copy.
bool PostRA = MRI.getNumVirtRegs() == 0;
// Trying to forward between physical and virtual registers is too hard.
if (DbgMO.getReg().isVirtual() != SrcMO->getReg().isVirtual())
return false;
// Only try virtual register copy-forwarding before regalloc, and physical
// register copy-forwarding after regalloc.
bool arePhysRegs = !DbgMO.getReg().isVirtual();
if (arePhysRegs != PostRA)
return false;
// Pre-regalloc, only forward if all subregisters agree (or there are no
// subregs at all). More analysis might recover some forwardable copies.
if (!PostRA && (DbgMO.getSubReg() != SrcMO->getSubReg() ||
DbgMO.getSubReg() != DstMO->getSubReg()))
return false;
// Post-regalloc, we may be sinking a DBG_VALUE of a sub or super-register
// of this copy. Only forward the copy if the DBG_VALUE operand exactly
// matches the copy destination.
if (PostRA && DbgMO.getReg() != DstMO->getReg())
return false;
return true;
/// Sink an instruction and its associated debug instructions.
static void performSink(MachineInstr &MI, MachineBasicBlock &SuccToSinkTo,
MachineBasicBlock::iterator InsertPos,
SmallVectorImpl<MachineInstr *> &DbgValuesToSink) {
// If we cannot find a location to use (merge with), then we erase the debug
// location to prevent debug-info driven tools from potentially reporting
// wrong location information.
if (!SuccToSinkTo.empty() && InsertPos != SuccToSinkTo.end())
// Move the instruction.
MachineBasicBlock *ParentBlock = MI.getParent();
SuccToSinkTo.splice(InsertPos, ParentBlock, MI,
// Sink a copy of debug users to the insert position. Mark the original
// DBG_VALUE location as 'undef', indicating that any earlier variable
// location should be terminated as we've optimised away the value at this
// point.
for (SmallVectorImpl<MachineInstr *>::iterator DBI = DbgValuesToSink.begin(),
DBE = DbgValuesToSink.end();
DBI != DBE; ++DBI) {
MachineInstr *DbgMI = *DBI;
MachineInstr *NewDbgMI = DbgMI->getMF()->CloneMachineInstr(*DBI);
SuccToSinkTo.insert(InsertPos, NewDbgMI);
if (!attemptDebugCopyProp(MI, *DbgMI))
/// 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;
Register Reg = MO.getReg();
if (Reg == 0 || !Register::isPhysicalRegister(Reg))
if (SuccToSinkTo->isLiveIn(Reg))
return false;
LLVM_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)) {
LLVM_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)) {
LLVM_DEBUG(dbgs() << " *** NOTE: Critical edge found\n");
TryBreak = true;
// Don't sink instructions into a loop.
if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) {
LLVM_DEBUG(dbgs() << " *** NOTE: Loop header found\n");
TryBreak = true;
// Otherwise we are OK with sinking along a critical edge.
if (!TryBreak)
LLVM_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)
LLVM_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)
LLVM_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())
// Collect debug users of any vreg that this inst defines.
SmallVector<MachineInstr *, 4> DbgUsersToSink;
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isDef() || !MO.getReg().isVirtual())
if (!SeenDbgUsers.count(MO.getReg()))
// Sink any users that don't pass any other DBG_VALUEs for this variable.
auto &Users = SeenDbgUsers[MO.getReg()];
for (auto &User : Users) {
MachineInstr *DbgMI = User.getPointer();
if (User.getInt()) {
// This DBG_VALUE would re-order assignments. If we can't copy-propagate
// it, it can't be recovered. Set it undef.
if (!attemptDebugCopyProp(MI, *DbgMI))
} else {
// After sinking, some debug users may not be dominated any more. If possible,
// copy-propagate their operands. As it's expensive, don't do this if there's
// no debuginfo in the program.
if (MI.getMF()->getFunction().getSubprogram() && MI.isCopy())
SalvageUnsunkDebugUsersOfCopy(MI, SuccToSinkTo);
performSink(MI, *SuccToSinkTo, InsertPos, DbgUsersToSink);
// 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;
void MachineSinking::SalvageUnsunkDebugUsersOfCopy(
MachineInstr &MI, MachineBasicBlock *TargetBlock) {
// Enumerate all users of vreg operands that are def'd. Skip those that will
// be sunk. For the rest, if they are not dominated by the block we will sink
// MI into, propagate the copy source to them.
SmallVector<MachineInstr *, 4> DbgDefUsers;
const MachineRegisterInfo &MRI = MI.getMF()->getRegInfo();
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isDef() || !MO.getReg().isVirtual())
for (auto &User : MRI.use_instructions(MO.getReg())) {
if (!User.isDebugValue() || DT->dominates(TargetBlock, User.getParent()))
// If is in same block, will either sink or be use-before-def.
if (User.getParent() == MI.getParent())
assert(User.getOperand(0).isReg() &&
"DBG_VALUE user of vreg, but non reg operand?");
// Point the users of this copy that are no longer dominated, at the source
// of the copy.
for (auto *User : DbgDefUsers) {
// This pass is not intended to be a replacement or a complete alternative
// for the pre-ra machine sink pass. It is only designed to sink COPY
// instructions which should be handled after RA.
// This pass sinks COPY instructions into a successor block, if the COPY is not
// used in the current block and the COPY is live-in to a single successor
// (i.e., doesn't require the COPY to be duplicated). This avoids executing the
// copy on paths where their results aren't needed. This also exposes
// additional opportunites for dead copy elimination and shrink wrapping.
// These copies were either not handled by or are inserted after the MachineSink
// pass. As an example of the former case, the MachineSink pass cannot sink
// COPY instructions with allocatable source registers; for AArch64 these type
// of copy instructions are frequently used to move function parameters (PhyReg)
// into virtual registers in the entry block.
// For the machine IR below, this pass will sink %w19 in the entry into its
// successor (%bb.1) because %w19 is only live-in in %bb.1.
// %bb.0:
// %wzr = SUBSWri %w1, 1
// %w19 = COPY %w0
// Bcc 11, %bb.2
// %bb.1:
// Live Ins: %w19
// BL @fun
// %w0 = ADDWrr %w0, %w19
// RET %w0
// %bb.2:
// %w0 = COPY %wzr
// RET %w0
// As we sink %w19 (CSR in AArch64) into %bb.1, the shrink-wrapping pass will be
// able to see %bb.0 as a candidate.
namespace {
class PostRAMachineSinking : public MachineFunctionPass {
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID;
PostRAMachineSinking() : MachineFunctionPass(ID) {}
StringRef getPassName() const override { return "PostRA Machine Sink"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
/// Track which register units have been modified and used.
LiveRegUnits ModifiedRegUnits, UsedRegUnits;
/// Track DBG_VALUEs of (unmodified) register units. Each DBG_VALUE has an
/// entry in this map for each unit it touches.
DenseMap<unsigned, TinyPtrVector<MachineInstr *>> SeenDbgInstrs;
/// Sink Copy instructions unused in the same block close to their uses in
/// successors.
bool tryToSinkCopy(MachineBasicBlock &BB, MachineFunction &MF,
const TargetRegisterInfo *TRI, const TargetInstrInfo *TII);
} // namespace
char PostRAMachineSinking::ID = 0;
char &llvm::PostRAMachineSinkingID = PostRAMachineSinking::ID;
INITIALIZE_PASS(PostRAMachineSinking, "postra-machine-sink",
"PostRA Machine Sink", false, false)
static bool aliasWithRegsInLiveIn(MachineBasicBlock &MBB, unsigned Reg,
const TargetRegisterInfo *TRI) {
LiveRegUnits LiveInRegUnits(*TRI);
return !LiveInRegUnits.available(Reg);
static MachineBasicBlock *
getSingleLiveInSuccBB(MachineBasicBlock &CurBB,
const SmallPtrSetImpl<MachineBasicBlock *> &SinkableBBs,
unsigned Reg, const TargetRegisterInfo *TRI) {
// Try to find a single sinkable successor in which Reg is live-in.
MachineBasicBlock *BB = nullptr;
for (auto *SI : SinkableBBs) {
if (aliasWithRegsInLiveIn(*SI, Reg, TRI)) {
// If BB is set here, Reg is live-in to at least two sinkable successors,
// so quit.
if (BB)
return nullptr;
BB = SI;
// Reg is not live-in to any sinkable successors.
if (!BB)
return nullptr;
// Check if any register aliased with Reg is live-in in other successors.
for (auto *SI : CurBB.successors()) {
if (!SinkableBBs.count(SI) && aliasWithRegsInLiveIn(*SI, Reg, TRI))
return nullptr;
return BB;
static MachineBasicBlock *
getSingleLiveInSuccBB(MachineBasicBlock &CurBB,
const SmallPtrSetImpl<MachineBasicBlock *> &SinkableBBs,
ArrayRef<unsigned> DefedRegsInCopy,
const TargetRegisterInfo *TRI) {
MachineBasicBlock *SingleBB = nullptr;
for (auto DefReg : DefedRegsInCopy) {
MachineBasicBlock *BB =
getSingleLiveInSuccBB(CurBB, SinkableBBs, DefReg, TRI);
if (!BB || (SingleBB && SingleBB != BB))
return nullptr;
SingleBB = BB;
return SingleBB;
static void clearKillFlags(MachineInstr *MI, MachineBasicBlock &CurBB,
SmallVectorImpl<unsigned> &UsedOpsInCopy,
LiveRegUnits &UsedRegUnits,
const TargetRegisterInfo *TRI) {
for (auto U : UsedOpsInCopy) {
MachineOperand &MO = MI->getOperand(U);
Register SrcReg = MO.getReg();
if (!UsedRegUnits.available(SrcReg)) {
MachineBasicBlock::iterator NI = std::next(MI->getIterator());
for (MachineInstr &UI : make_range(NI, CurBB.end())) {
if (UI.killsRegister(SrcReg, TRI)) {
UI.clearRegisterKills(SrcReg, TRI);
static void updateLiveIn(MachineInstr *MI, MachineBasicBlock *SuccBB,
SmallVectorImpl<unsigned> &UsedOpsInCopy,
SmallVectorImpl<unsigned> &DefedRegsInCopy) {
MachineFunction &MF = *SuccBB->getParent();
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
for (unsigned DefReg : DefedRegsInCopy)
for (MCSubRegIterator S(DefReg, TRI, true); S.isValid(); ++S)
for (auto U : UsedOpsInCopy) {
Register SrcReg = MI->getOperand(U).getReg();
LaneBitmask Mask;
for (MCRegUnitMaskIterator S(SrcReg, TRI); S.isValid(); ++S) {
Mask |= (*S).second;
SuccBB->addLiveIn(SrcReg, Mask.any() ? Mask : LaneBitmask::getAll());
static bool hasRegisterDependency(MachineInstr *MI,
SmallVectorImpl<unsigned> &UsedOpsInCopy,
SmallVectorImpl<unsigned> &DefedRegsInCopy,
LiveRegUnits &ModifiedRegUnits,
LiveRegUnits &UsedRegUnits) {
bool HasRegDependency = false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg)
if (MO.isDef()) {
if (!ModifiedRegUnits.available(Reg) || !UsedRegUnits.available(Reg)) {
HasRegDependency = true;
// FIXME: instead of isUse(), readsReg() would be a better fix here,
// For example, we can ignore modifications in reg with undef. However,
// it's not perfectly clear if skipping the internal read is safe in all
// other targets.
} else if (MO.isUse()) {
if (!ModifiedRegUnits.available(Reg)) {
HasRegDependency = true;
return HasRegDependency;
static SmallSet<unsigned, 4> getRegUnits(unsigned Reg,
const TargetRegisterInfo *TRI) {
SmallSet<unsigned, 4> RegUnits;
for (auto RI = MCRegUnitIterator(Reg, TRI); RI.isValid(); ++RI)
return RegUnits;
bool PostRAMachineSinking::tryToSinkCopy(MachineBasicBlock &CurBB,
MachineFunction &MF,
const TargetRegisterInfo *TRI,
const TargetInstrInfo *TII) {
SmallPtrSet<MachineBasicBlock *, 2> SinkableBBs;
// FIXME: For now, we sink only to a successor which has a single predecessor
// so that we can directly sink COPY instructions to the successor without
// adding any new block or branch instruction.
for (MachineBasicBlock *SI : CurBB.successors())
if (!SI->livein_empty() && SI->pred_size() == 1)
if (SinkableBBs.empty())
return false;
bool Changed = false;
// Track which registers have been modified and used between the end of the
// block and the current instruction.
for (auto I = CurBB.rbegin(), E = CurBB.rend(); I != E;) {
MachineInstr *MI = &*I;
// Track the operand index for use in Copy.
SmallVector<unsigned, 2> UsedOpsInCopy;
// Track the register number defed in Copy.
SmallVector<unsigned, 2> DefedRegsInCopy;
// We must sink this DBG_VALUE if its operand is sunk. To avoid searching
// for DBG_VALUEs later, record them when they're encountered.
if (MI->isDebugValue()) {
auto &MO = MI->getOperand(0);
if (MO.isReg() && Register::isPhysicalRegister(MO.getReg())) {
// Bail if we can already tell the sink would be rejected, rather
// than needlessly accumulating lots of DBG_VALUEs.
if (hasRegisterDependency(MI, UsedOpsInCopy, DefedRegsInCopy,
ModifiedRegUnits, UsedRegUnits))
// Record debug use of each reg unit.
SmallSet<unsigned, 4> Units = getRegUnits(MO.getReg(), TRI);
for (unsigned Reg : Units)
if (MI->isDebugInstr())
// Do not move any instruction across function call.
if (MI->isCall())
return false;
if (!MI->isCopy() || !MI->getOperand(0).isRenamable()) {
LiveRegUnits::accumulateUsedDefed(*MI, ModifiedRegUnits, UsedRegUnits,
// Don't sink the COPY if it would violate a register dependency.
if (hasRegisterDependency(MI, UsedOpsInCopy, DefedRegsInCopy,
ModifiedRegUnits, UsedRegUnits)) {
LiveRegUnits::accumulateUsedDefed(*MI, ModifiedRegUnits, UsedRegUnits,
assert((!UsedOpsInCopy.empty() && !DefedRegsInCopy.empty()) &&
"Unexpect SrcReg or DefReg");
MachineBasicBlock *SuccBB =
getSingleLiveInSuccBB(CurBB, SinkableBBs, DefedRegsInCopy, TRI);
// Don't sink if we cannot find a single sinkable successor in which Reg
// is live-in.
if (!SuccBB) {
LiveRegUnits::accumulateUsedDefed(*MI, ModifiedRegUnits, UsedRegUnits,
assert((SuccBB->pred_size() == 1 && *SuccBB->pred_begin() == &CurBB) &&
"Unexpected predecessor");
// Collect DBG_VALUEs that must sink with this copy. We've previously
// recorded which reg units that DBG_VALUEs read, if this instruction
// writes any of those units then the corresponding DBG_VALUEs must sink.
SetVector<MachineInstr *> DbgValsToSinkSet;
SmallVector<MachineInstr *, 4> DbgValsToSink;
for (auto &MO : MI->operands()) {
if (!MO.isReg() || !MO.isDef())
SmallSet<unsigned, 4> Units = getRegUnits(MO.getReg(), TRI);
for (unsigned Reg : Units)
for (auto *MI : SeenDbgInstrs.lookup(Reg))
DbgValsToSink.insert(DbgValsToSink.begin(), DbgValsToSinkSet.begin(),
// Clear the kill flag if SrcReg is killed between MI and the end of the
// block.
clearKillFlags(MI, CurBB, UsedOpsInCopy, UsedRegUnits, TRI);
MachineBasicBlock::iterator InsertPos = SuccBB->getFirstNonPHI();
performSink(*MI, *SuccBB, InsertPos, DbgValsToSink);
updateLiveIn(MI, SuccBB, UsedOpsInCopy, DefedRegsInCopy);
Changed = true;
return Changed;
bool PostRAMachineSinking::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
bool Changed = false;
const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
for (auto &BB : MF)
Changed |= tryToSinkCopy(BB, MF, TRI, TII);
return Changed;