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

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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 https://llvm.org/LICENSE.txt 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/MapVector.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/RegisterClassInfo.h"
#include "llvm/CodeGen/RegisterPressure.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>
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);
static cl::opt<unsigned> SplitEdgeProbabilityThreshold(
"machine-sink-split-probability-threshold",
cl::desc(
"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);
static cl::opt<unsigned> SinkLoadInstsPerBlockThreshold(
"machine-sink-load-instrs-threshold",
cl::desc("Do not try to find alias store for a load if there is a in-path "
"block whose instruction number is higher than this threshold."),
cl::init(2000), cl::Hidden);
static cl::opt<unsigned> SinkLoadBlocksThreshold(
"machine-sink-load-blocks-threshold",
cl::desc("Do not try to find alias store for a load if the block number in "
"the straight line is higher than this threshold."),
cl::init(20), cl::Hidden);
static cl::opt<bool>
SinkInstsIntoLoop("sink-insts-to-avoid-spills",
cl::desc("Sink instructions into loops to avoid "
"register spills"),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> SinkIntoLoopLimit(
"machine-sink-loop-limit",
cl::desc("The maximum number of instructions considered for loop sinking."),
cl::init(50), cl::Hidden);
STATISTIC(NumSunk, "Number of machine instructions sunk");
STATISTIC(NumLoopSunk, "Number of machine instructions sunk into a loop");
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;
MachineBlockFrequencyInfo *MBFI;
const MachineBranchProbabilityInfo *MBPI;
AliasAnalysis *AA;
RegisterClassInfo RegClassInfo;
// 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;
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;
std::map<std::pair<MachineBasicBlock *, MachineBasicBlock *>, bool>
HasStoreCache;
std::map<std::pair<MachineBasicBlock *, MachineBasicBlock *>,
std::vector<MachineInstr *>>
StoreInstrCache;
/// Cached BB's register pressure.
std::map<MachineBasicBlock *, std::vector<unsigned>> CachedRegisterPressure;
public:
static char ID; // Pass identification
MachineSinking() : MachineFunctionPass(ID) {
initializeMachineSinkingPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<MachinePostDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addPreserved<MachineLoopInfo>();
if (UseBlockFreqInfo)
AU.addRequired<MachineBlockFrequencyInfo>();
}
void releaseMemory() override {
CEBCandidates.clear();
}
private:
bool ProcessBlock(MachineBasicBlock &MBB);
void ProcessDbgInst(MachineInstr &MI);
bool isWorthBreakingCriticalEdge(MachineInstr &MI,
MachineBasicBlock *From,
MachineBasicBlock *To);
bool hasStoreBetween(MachineBasicBlock *From, MachineBasicBlock *To,
MachineInstr &MI);
/// 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(Register Reg, MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB, bool &BreakPHIEdge,
bool &LocalUse) const;
MachineBasicBlock *FindSuccToSinkTo(MachineInstr &MI, MachineBasicBlock *MBB,
bool &BreakPHIEdge, AllSuccsCache &AllSuccessors);
void FindLoopSinkCandidates(MachineLoop *L, MachineBasicBlock *BB,
SmallVectorImpl<MachineInstr *> &Candidates);
bool SinkIntoLoop(MachineLoop *L, MachineInstr &I);
bool isProfitableToSinkTo(Register 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;
std::vector<unsigned> &getBBRegisterPressure(MachineBasicBlock &MBB);
};
} // end anonymous namespace
char MachineSinking::ID = 0;
char &llvm::MachineSinkingID = MachineSinking::ID;
INITIALIZE_PASS_BEGIN(MachineSinking, DEBUG_TYPE,
"Machine code sinking", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MachineSinking, DEBUG_TYPE,
"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);
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(Register 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:
// Predecessors according to CFG: %bb.0
// ...
// %def = DEC64_32r %x, implicit-def dead %eflags
// ...
// JE_4 <%bb.37>, implicit %eflags
// Successors according to CFG: %bb.37 %bb.2
//
// %bb.2:
// %p = PHI %y, %bb.0, %def, %bb.1
if (all_of(MRI->use_nodbg_operands(Reg), [&](MachineOperand &MO) {
MachineInstr *UseInst = MO.getParent();
unsigned OpNo = UseInst->getOperandNo(&MO);
MachineBasicBlock *UseBlock = UseInst->getParent();
return UseBlock == MBB && UseInst->isPHI() &&
UseInst->getOperand(OpNo + 1).getMBB() == DefMBB;
})) {
BreakPHIEdge = true;
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;
}
/// Return true if this machine instruction loads from global offset table or
/// constant pool.
static bool mayLoadFromGOTOrConstantPool(MachineInstr &MI) {
assert(MI.mayLoad() && "Expected MI that loads!");
// If we lost memory operands, conservatively assume that the instruction
// reads from everything..
if (MI.memoperands_empty())
return true;
for (MachineMemOperand *MemOp : MI.memoperands())
if (const PseudoSourceValue *PSV = MemOp->getPseudoValue())
if (PSV->isGOT() || PSV->isConstantPool())
return true;
return false;
}
void MachineSinking::FindLoopSinkCandidates(MachineLoop *L, MachineBasicBlock *BB,
SmallVectorImpl<MachineInstr *> &Candidates) {
for (auto &MI : *BB) {
LLVM_DEBUG(dbgs() << "LoopSink: Analysing candidate: " << MI);
if (!TII->shouldSink(MI)) {
LLVM_DEBUG(dbgs() << "LoopSink: Instruction not a candidate for this "
"target\n");
continue;
}
if (!L->isLoopInvariant(MI)) {
LLVM_DEBUG(dbgs() << "LoopSink: Instruction is not loop invariant\n");
continue;
}
bool DontMoveAcrossStore = true;
if (!MI.isSafeToMove(AA, DontMoveAcrossStore)) {
LLVM_DEBUG(dbgs() << "LoopSink: Instruction not safe to move.\n");
continue;
}
if (MI.mayLoad() && !mayLoadFromGOTOrConstantPool(MI)) {
LLVM_DEBUG(dbgs() << "LoopSink: Dont sink GOT or constant pool loads\n");
continue;
}
if (MI.isConvergent())
continue;
const MachineOperand &MO = MI.getOperand(0);
if (!MO.isReg() || !MO.getReg() || !MO.isDef())
continue;
if (!MRI->hasOneDef(MO.getReg()))
continue;
LLVM_DEBUG(dbgs() << "LoopSink: Instruction added as candidate.\n");
Candidates.push_back(&MI);
}
}
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();
RegClassInfo.runOnMachineFunction(MF);
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) {
LLVM_DEBUG(dbgs() << " *** Splitting critical edge: "
<< printMBBReference(*Pair.first) << " -- "
<< printMBBReference(*NewSucc) << " -- "
<< printMBBReference(*Pair.second) << '\n');
if (MBFI)
MBFI->onEdgeSplit(*Pair.first, *NewSucc, *MBPI);
MadeChange = true;
++NumSplit;
} 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;
}
if (SinkInstsIntoLoop) {
SmallVector<MachineLoop *, 8> Loops(LI->begin(), LI->end());
for (auto *L : Loops) {
MachineBasicBlock *Preheader = LI->findLoopPreheader(L);
if (!Preheader) {
LLVM_DEBUG(dbgs() << "LoopSink: Can't find preheader\n");
continue;
}
SmallVector<MachineInstr *, 8> Candidates;
FindLoopSinkCandidates(L, Preheader, Candidates);
// Walk the candidates in reverse order so that we start with the use
// of a def-use chain, if there is any.
// TODO: Sort the candidates using a cost-model.
unsigned i = 0;
for (auto It = Candidates.rbegin(); It != Candidates.rend(); ++It) {
if (i++ == SinkIntoLoopLimit) {
LLVM_DEBUG(dbgs() << "LoopSink: Limit reached of instructions to "
"be analysed.");
break;
}
MachineInstr *I = *It;
if (!SinkIntoLoop(L, *I))
break;
EverMadeChange = true;
++NumLoopSunk;
}
}
}
HasStoreCache.clear();
StoreInstrCache.clear();
// 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.isDebugOrPseudoInstr()) {
if (MI.isDebugValue())
ProcessDbgInst(MI);
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);
SeenDbgUsers.clear();
SeenDbgVars.clear();
// recalculate the bb register pressure after sinking one BB.
CachedRegisterPressure.clear();
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(),
MI.getDebugLoc()->getInlinedAt());
bool SeenBefore = SeenDbgVars.contains(Var);
for (MachineOperand &MO : MI.debug_operands()) {
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.
SeenDbgVars.insert(Var);
}
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())
continue;
Register 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 (Register::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;
}
std::vector<unsigned> &
MachineSinking::getBBRegisterPressure(MachineBasicBlock &MBB) {
// Currently to save compiling time, MBB's register pressure will not change
// in one ProcessBlock iteration because of CachedRegisterPressure. but MBB's
// register pressure is changed after sinking any instructions into it.
// FIXME: need a accurate and cheap register pressure estiminate model here.
auto RP = CachedRegisterPressure.find(&MBB);
if (RP != CachedRegisterPressure.end())
return RP->second;
RegionPressure Pressure;
RegPressureTracker RPTracker(Pressure);
// Initialize the register pressure tracker.
RPTracker.init(MBB.getParent(), &RegClassInfo, nullptr, &MBB, MBB.end(),
/*TrackLaneMasks*/ false, /*TrackUntiedDefs=*/true);
for (MachineBasicBlock::iterator MII = MBB.instr_end(),
MIE = MBB.instr_begin();
MII != MIE; --MII) {
MachineInstr &MI = *std::prev(MII);
if (MI.isDebugValue() || MI.isDebugLabel() || MI.isPseudoProbe())
continue;
RegisterOperands RegOpers;
RegOpers.collect(MI, *TRI, *MRI, false, false);
RPTracker.recedeSkipDebugValues();
assert(&*RPTracker.getPos() == &MI && "RPTracker sync error!");
RPTracker.recede(RegOpers);
}
RPTracker.closeRegion();
auto It = CachedRegisterPressure.insert(
std::make_pair(&MBB, RPTracker.getPressure().MaxSetPressure));
return It.first->second;
}
/// isProfitableToSinkTo - Return true if it is profitable to sink MI.
bool MachineSinking::isProfitableToSinkTo(Register 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);
MachineLoop *ML = LI->getLoopFor(MBB);
// If the instruction is not inside a loop, it is not profitable to sink MI to
// a post dominate block SuccToSinkTo.
if (!ML)
return false;
auto isRegisterPressureSetExceedLimit = [&](const TargetRegisterClass *RC) {
unsigned Weight = TRI->getRegClassWeight(RC).RegWeight;
const int *PS = TRI->getRegClassPressureSets(RC);
// Get register pressure for block SuccToSinkTo.
std::vector<unsigned> BBRegisterPressure =
getBBRegisterPressure(*SuccToSinkTo);
for (; *PS != -1; PS++)
// check if any register pressure set exceeds limit in block SuccToSinkTo
// after sinking.
if (Weight + BBRegisterPressure[*PS] >=
TRI->getRegPressureSetLimit(*MBB->getParent(), *PS))
return true;
return false;
};
// If this instruction is inside a loop and sinking this instruction can make
// more registers live range shorten, it is still prifitable.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI.getOperand(i);
// Ignore non-register operands.
if (!MO.isReg())
continue;
Register Reg = MO.getReg();
if (Reg == 0)
continue;
// Don't handle physical register.
if (Register::isPhysicalRegister(Reg))
return false;
// Users for the defs are all dominated by SuccToSinkTo.
if (MO.isDef()) {
// This def register's live range is shortened after sinking.
bool LocalUse = false;
if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, MBB, BreakPHIEdge,
LocalUse))
return false;
} else {
MachineInstr *DefMI = MRI->getVRegDef(Reg);
// DefMI is defined outside of loop. There should be no live range
// impact for this operand. Defination outside of loop means:
// 1: defination is outside of loop.
// 2: defination is in this loop, but it is a PHI in the loop header.
if (LI->getLoopFor(DefMI->getParent()) != ML ||
(DefMI->isPHI() && LI->isLoopHeader(DefMI->getParent())))
continue;
// The DefMI is defined inside the loop.
// If sinking this operand makes some register pressure set exceed limit,
// it is not profitable.
if (isRegisterPressureSetExceedLimit(MRI->getRegClass(Reg))) {
LLVM_DEBUG(dbgs() << "register pressure exceed limit, not profitable.");
return false;
}
}
}
// If MI is in loop and all its operands are alive across the whole loop or if
// no operand sinking make register pressure set exceed limit, it is
// profitable to sink MI.
return true;
}
/// 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->successors());
// Handle cases where sinking can happen but where the sink point isn't a
// successor. For example:
//
// x = computation
// if () {} else {}
// use x
//
for (MachineDomTreeNode *DTChild : DT->getNode(MBB)->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.
llvm::stable_sort(
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;
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;
// It ought to be okay to sink instructions into an INLINEASM_BR target, but
// only if we make sure that MI occurs _before_ an INLINEASM_BR instruction in
// the source block (which this code does not yet do). So for now, forbid
// doing so.
if (SuccToSinkTo && SuccToSinkTo->isInlineAsmBrIndirectTarget())
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 ||
!PredBB->getTerminator()->getMetadata(LLVMContext::MD_make_implicit))
return false;
const MachineOperand *BaseOp;
int64_t Offset;
bool OffsetIsScalable;
if (!TII->getMemOperandWithOffset(MI, BaseOp, Offset, OffsetIsScalable, 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,
Register Reg) {
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.
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 (Reg.isVirtual() != SrcMO->getReg().isVirtual())
return false;
// Only try virtual register copy-forwarding before regalloc, and physical
// register copy-forwarding after regalloc.
bool arePhysRegs = !Reg.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)
for (auto &DbgMO : DbgMI.getDebugOperandsForReg(Reg))
if (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 && Reg != DstMO->getReg())
return false;
for (auto &DbgMO : DbgMI.getDebugOperandsForReg(Reg)) {
DbgMO.setReg(SrcMO->getReg());
DbgMO.setSubReg(SrcMO->getSubReg());
}
return true;
}
using MIRegs = std::pair<MachineInstr *, SmallVector<unsigned, 2>>;
/// Sink an instruction and its associated debug instructions.
static void performSink(MachineInstr &MI, MachineBasicBlock &SuccToSinkTo,
MachineBasicBlock::iterator InsertPos,
SmallVectorImpl<MIRegs> &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())
MI.setDebugLoc(DILocation::getMergedLocation(MI.getDebugLoc(),
InsertPos->getDebugLoc()));
else
MI.setDebugLoc(DebugLoc());
// Move the instruction.
MachineBasicBlock *ParentBlock = MI.getParent();
SuccToSinkTo.splice(InsertPos, ParentBlock, MI,
++MachineBasicBlock::iterator(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 (auto DbgValueToSink : DbgValuesToSink) {
MachineInstr *DbgMI = DbgValueToSink.first;
MachineInstr *NewDbgMI = DbgMI->getMF()->CloneMachineInstr(DbgMI);
SuccToSinkTo.insert(InsertPos, NewDbgMI);
bool PropagatedAllSunkOps = true;
for (unsigned Reg : DbgValueToSink.second) {
if (DbgMI->hasDebugOperandForReg(Reg)) {
if (!attemptDebugCopyProp(MI, *DbgMI, Reg)) {
PropagatedAllSunkOps = false;
break;
}
}
}
if (!PropagatedAllSunkOps)
DbgMI->setDebugValueUndef();
}
}
/// hasStoreBetween - check if there is store betweeen straight line blocks From
/// and To.
bool MachineSinking::hasStoreBetween(MachineBasicBlock *From,
MachineBasicBlock *To, MachineInstr &MI) {
// Make sure From and To are in straight line which means From dominates To
// and To post dominates From.
if (!DT->dominates(From, To) || !PDT->dominates(To, From))
return true;
auto BlockPair = std::make_pair(From, To);
// Does these two blocks pair be queried before and have a definite cached
// result?
if (HasStoreCache.find(BlockPair) != HasStoreCache.end())
return HasStoreCache[BlockPair];
if (StoreInstrCache.find(BlockPair) != StoreInstrCache.end())
return llvm::any_of(StoreInstrCache[BlockPair], [&](MachineInstr *I) {
return I->mayAlias(AA, MI, false);
});
bool SawStore = false;
bool HasAliasedStore = false;
DenseSet<MachineBasicBlock *> HandledBlocks;
DenseSet<MachineBasicBlock *> HandledDomBlocks;
// Go through all reachable blocks from From.
for (MachineBasicBlock *BB : depth_first(From)) {
// We insert the instruction at the start of block To, so no need to worry
// about stores inside To.
// Store in block From should be already considered when just enter function
// SinkInstruction.
if (BB == To || BB == From)
continue;
// We already handle this BB in previous iteration.
if (HandledBlocks.count(BB))
continue;
HandledBlocks.insert(BB);
// To post dominates BB, it must be a path from block From.
if (PDT->dominates(To, BB)) {
if (!HandledDomBlocks.count(BB))
HandledDomBlocks.insert(BB);
// If this BB is too big or the block number in straight line between From
// and To is too big, stop searching to save compiling time.
if (BB->size() > SinkLoadInstsPerBlockThreshold ||
HandledDomBlocks.size() > SinkLoadBlocksThreshold) {
for (auto *DomBB : HandledDomBlocks) {
if (DomBB != BB && DT->dominates(DomBB, BB))
HasStoreCache[std::make_pair(DomBB, To)] = true;
else if(DomBB != BB && DT->dominates(BB, DomBB))
HasStoreCache[std::make_pair(From, DomBB)] = true;
}
HasStoreCache[BlockPair] = true;
return true;
}
for (MachineInstr &I : *BB) {
// Treat as alias conservatively for a call or an ordered memory
// operation.
if (I.isCall() || I.hasOrderedMemoryRef()) {
for (auto *DomBB : HandledDomBlocks) {
if (DomBB != BB && DT->dominates(DomBB, BB))
HasStoreCache[std::make_pair(DomBB, To)] = true;
else if(DomBB != BB && DT->dominates(BB, DomBB))
HasStoreCache[std::make_pair(From, DomBB)] = true;
}
HasStoreCache[BlockPair] = true;
return true;
}
if (I.mayStore()) {
SawStore = true;
// We still have chance to sink MI if all stores between are not
// aliased to MI.
// Cache all store instructions, so that we don't need to go through
// all From reachable blocks for next load instruction.
if (I.mayAlias(AA, MI, false))
HasAliasedStore = true;
StoreInstrCache[BlockPair].push_back(&I);
}
}
}
}
// If there is no store at all, cache the result.
if (!SawStore)
HasStoreCache[BlockPair] = false;
return HasAliasedStore;
}
/// Sink instructions into loops if profitable. This especially tries to prevent
/// register spills caused by register pressure if there is little to no
/// overhead moving instructions into loops.
bool MachineSinking::SinkIntoLoop(MachineLoop *L, MachineInstr &I) {
LLVM_DEBUG(dbgs() << "LoopSink: Finding sink block for: " << I);
MachineBasicBlock *Preheader = L->getLoopPreheader();
assert(Preheader && "Loop sink needs a preheader block");
MachineBasicBlock *SinkBlock = nullptr;
bool CanSink = true;
const MachineOperand &MO = I.getOperand(0);
for (MachineInstr &MI : MRI->use_instructions(MO.getReg())) {
LLVM_DEBUG(dbgs() << "LoopSink: Analysing use: " << MI);
if (!L->contains(&MI)) {
LLVM_DEBUG(dbgs() << "LoopSink: Use not in loop, can't sink.\n");
CanSink = false;
break;
}
// FIXME: Come up with a proper cost model that estimates whether sinking
// the instruction (and thus possibly executing it on every loop
// iteration) is more expensive than a register.
// For now assumes that copies are cheap and thus almost always worth it.
if (!MI.isCopy()) {
LLVM_DEBUG(dbgs() << "LoopSink: Use is not a copy\n");
CanSink = false;
break;
}
if (!SinkBlock) {
SinkBlock = MI.getParent();
LLVM_DEBUG(dbgs() << "LoopSink: Setting sink block to: "
<< printMBBReference(*SinkBlock) << "\n");
continue;
}
SinkBlock = DT->findNearestCommonDominator(SinkBlock, MI.getParent());
if (!SinkBlock) {
LLVM_DEBUG(dbgs() << "LoopSink: Can't find nearest dominator\n");
CanSink = false;
break;
}
LLVM_DEBUG(dbgs() << "LoopSink: Setting nearest common dom block: " <<
printMBBReference(*SinkBlock) << "\n");
}
if (!CanSink) {
LLVM_DEBUG(dbgs() << "LoopSink: Can't sink instruction.\n");
return false;
}
if (!SinkBlock) {
LLVM_DEBUG(dbgs() << "LoopSink: Not sinking, can't find sink block.\n");
return false;
}
if (SinkBlock == Preheader) {
LLVM_DEBUG(dbgs() << "LoopSink: Not sinking, sink block is the preheader\n");
return false;
}
if (SinkBlock->size() > SinkLoadInstsPerBlockThreshold) {
LLVM_DEBUG(dbgs() << "LoopSink: Not Sinking, block too large to analyse.\n");
return false;
}
LLVM_DEBUG(dbgs() << "LoopSink: Sinking instruction!\n");
SinkBlock->splice(SinkBlock->getFirstNonPHI(), Preheader, I);
// The instruction is moved from its basic block, so do not retain the
// debug information.
assert(!I.isDebugInstr() && "Should not sink debug inst");
I.setDebugLoc(DebugLoc());
return true;
}
/// 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))
continue;
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 =
MI.mayLoad() ? hasStoreBetween(ParentBlock, SuccToSinkTo, MI) : 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())
++InsertPos;
// Collect debug users of any vreg that this inst defines.
SmallVector<MIRegs, 4> DbgUsersToSink;
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isDef() || !MO.getReg().isVirtual())
continue;
if (!SeenDbgUsers.count(MO.getReg()))
continue;
// 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, MO.getReg()))
DbgMI->setDebugValueUndef();
} else {
DbgUsersToSink.push_back(
{DbgMI, SmallVector<unsigned, 2>(1, MO.getReg())});
}
}
}
// 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) {
assert(MI.isCopy());
assert(MI.getOperand(1).isReg());
// 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;
SmallVector<Register, 4> DbgUseRegs;
const MachineRegisterInfo &MRI = MI.getMF()->getRegInfo();
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isDef() || !MO.getReg().isVirtual())
continue;
DbgUseRegs.push_back(MO.getReg());
for (auto &User : MRI.use_instructions(MO.getReg())) {
if (!User.isDebugValue() || DT->dominates(TargetBlock, User.getParent()))
continue;
// If is in same block, will either sink or be use-before-def.
if (User.getParent() == MI.getParent())
continue;
assert(User.hasDebugOperandForReg(MO.getReg()) &&
"DBG_VALUE user of vreg, but has no operand for it?");
DbgDefUsers.push_back(&User);
}
}
// Point the users of this copy that are no longer dominated, at the source
// of the copy.
for (auto *User : DbgDefUsers) {
for (auto &Reg : DbgUseRegs) {
for (auto &DbgOp : User->getDebugOperandsForReg(Reg)) {
DbgOp.setReg(MI.getOperand(1).getReg());
DbgOp.setSubReg(MI.getOperand(1).getSubReg());
}
}
}
}
//===----------------------------------------------------------------------===//
// 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 {
public:
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 {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
private:
/// 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. The DBG_VALUE's entry
/// consists of a pointer to the instruction itself, and a vector of registers
/// referred to by the instruction that overlap the key register unit.
DenseMap<unsigned, SmallVector<MIRegs, 2>> 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);
LiveInRegUnits.addLiveIns(MBB);
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);
MO.setIsKill(true);
break;
}
}
}
}
}
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)
SuccBB->removeLiveIn(*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());
}
SuccBB->sortUniqueLiveIns();
}
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())
continue;
Register Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isDef()) {
if (!ModifiedRegUnits.available(Reg) || !UsedRegUnits.available(Reg)) {
HasRegDependency = true;
break;
}
DefedRegsInCopy.push_back(Reg);
// 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;
break;
}
UsedOpsInCopy.push_back(i);
}
}
return HasRegDependency;
}
static SmallSet<MCRegister, 4> getRegUnits(MCRegister Reg,
const TargetRegisterInfo *TRI) {
SmallSet<MCRegister, 4> RegUnits;
for (auto RI = MCRegUnitIterator(Reg, TRI); RI.isValid(); ++RI)
RegUnits.insert(*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)
SinkableBBs.insert(SI);
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.
ModifiedRegUnits.clear();
UsedRegUnits.clear();
SeenDbgInstrs.clear();
for (auto I = CurBB.rbegin(), E = CurBB.rend(); I != E;) {
MachineInstr *MI = &*I;
++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()) {
SmallDenseMap<MCRegister, SmallVector<unsigned, 2>, 4> MIUnits;
bool IsValid = true;
for (MachineOperand &MO : MI->debug_operands()) {
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)) {
IsValid = false;
break;
}
// Record debug use of each reg unit.
SmallSet<MCRegister, 4> RegUnits = getRegUnits(MO.getReg(), TRI);
for (MCRegister Reg : RegUnits)
MIUnits[Reg].push_back(MO.getReg());
}
}
if (IsValid) {
for (auto RegOps : MIUnits)
SeenDbgInstrs[RegOps.first].push_back({MI, RegOps.second});
}
continue;
}
if (MI->isDebugOrPseudoInstr())
continue;
// 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,
TRI);
continue;
}
// Don't sink the COPY if it would violate a register dependency.
if (hasRegisterDependency(MI, UsedOpsInCopy, DefedRegsInCopy,
ModifiedRegUnits, UsedRegUnits)) {
LiveRegUnits::accumulateUsedDefed(*MI, ModifiedRegUnits, UsedRegUnits,
TRI);
continue;
}
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,
TRI);
continue;
}
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.
MapVector<MachineInstr *, MIRegs::second_type> DbgValsToSinkMap;
for (auto &MO : MI->operands()) {
if (!MO.isReg() || !MO.isDef())
continue;
SmallSet<MCRegister, 4> Units = getRegUnits(MO.getReg(), TRI);
for (MCRegister Reg : Units) {
for (auto MIRegs : SeenDbgInstrs.lookup(Reg)) {
auto &Regs = DbgValsToSinkMap[MIRegs.first];
for (unsigned Reg : MIRegs.second)
Regs.push_back(Reg);
}
}
}
SmallVector<MIRegs, 4> DbgValsToSink(DbgValsToSinkMap.begin(),
DbgValsToSinkMap.end());
// 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;
++NumPostRACopySink;
}
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();
ModifiedRegUnits.init(*TRI);
UsedRegUnits.init(*TRI);
for (auto &BB : MF)
Changed |= tryToSinkCopy(BB, MF, TRI, TII);
return Changed;
}
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