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llvm/lib/CodeGen/MachineSink.cpp
Owen Anderson 2ab36d3502 Begin adding static dependence information to passes, which will allow us to 
perform initialization without static constructors AND without explicit initialization
by the client.  For the moment, passes are required to initialize both their
(potential) dependencies and any passes they preserve.  I hope to be able to relax
the latter requirement in the future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116334 91177308-0d34-0410-b5e6-96231b3b80d8
2010-10-12 19:48:12 +00:00

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//===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass moves instructions into successor blocks when possible, so that
// they aren't executed on paths where their results aren't needed.
//
// This pass is not intended to be a replacement or a complete alternative
// for an LLVM-IR-level sinking pass. It is only designed to sink simple
// constructs that are not exposed before lowering and instruction selection.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "machine-sink"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
static cl::opt<bool>
SplitEdges("machine-sink-split",
cl::desc("Split critical edges during machine sinking"),
cl::init(true), cl::Hidden);
STATISTIC(NumSunk, "Number of machine instructions sunk");
STATISTIC(NumSplit, "Number of critical edges split");
STATISTIC(NumCoalesces, "Number of copies coalesced");
namespace {
class MachineSinking : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI; // Machine register information
MachineDominatorTree *DT; // Machine dominator tree
MachineLoopInfo *LI;
AliasAnalysis *AA;
BitVector AllocatableSet; // Which physregs are allocatable?
// Remember which edges have been considered for breaking.
SmallSet<std::pair<MachineBasicBlock*,MachineBasicBlock*>, 8>
CEBCandidates;
public:
static char ID; // Pass identification
MachineSinking() : MachineFunctionPass(ID) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AliasAnalysis>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineDominatorTree>();
AU.addPreserved<MachineLoopInfo>();
}
virtual void releaseMemory() {
CEBCandidates.clear();
}
private:
bool ProcessBlock(MachineBasicBlock &MBB);
bool isWorthBreakingCriticalEdge(MachineInstr *MI,
MachineBasicBlock *From,
MachineBasicBlock *To);
MachineBasicBlock *SplitCriticalEdge(MachineInstr *MI,
MachineBasicBlock *From,
MachineBasicBlock *To,
bool BreakPHIEdge);
bool SinkInstruction(MachineInstr *MI, bool &SawStore);
bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB,
bool &BreakPHIEdge, bool &LocalUse) const;
bool PerformTrivialForwardCoalescing(MachineInstr *MI,
MachineBasicBlock *MBB);
};
} // end anonymous namespace
char MachineSinking::ID = 0;
INITIALIZE_PASS_BEGIN(MachineSinking, "machine-sink",
"Machine code sinking", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(MachineSinking, "machine-sink",
"Machine code sinking", false, false)
FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
bool MachineSinking::PerformTrivialForwardCoalescing(MachineInstr *MI,
MachineBasicBlock *MBB) {
if (!MI->isCopy())
return false;
unsigned SrcReg = MI->getOperand(1).getReg();
unsigned DstReg = MI->getOperand(0).getReg();
if (!TargetRegisterInfo::isVirtualRegister(SrcReg) ||
!TargetRegisterInfo::isVirtualRegister(DstReg) ||
!MRI->hasOneNonDBGUse(SrcReg))
return false;
const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
const TargetRegisterClass *DRC = MRI->getRegClass(DstReg);
if (SRC != DRC)
return false;
MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
if (DefMI->isCopyLike())
return false;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << *MI);
MRI->replaceRegWith(DstReg, SrcReg);
MI->eraseFromParent();
++NumCoalesces;
return true;
}
/// AllUsesDominatedByBlock - Return true if all uses of the specified register
/// occur in blocks dominated by the specified block. If any use is in the
/// definition block, then return false since it is never legal to move def
/// after uses.
bool
MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
MachineBasicBlock *MBB,
MachineBasicBlock *DefMBB,
bool &BreakPHIEdge,
bool &LocalUse) const {
assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
"Only makes sense for vregs");
if (MRI->use_nodbg_empty(Reg))
return true;
// Ignoring debug uses is necessary so debug info doesn't affect the code.
// This may leave a referencing dbg_value in the original block, before
// the definition of the vreg. Dwarf generator handles this although the
// user might not get the right info at runtime.
// BreakPHIEdge is true if all the uses are in the successor MBB being sunken
// into and they are all PHI nodes. In this case, machine-sink must break
// the critical edge first. e.g.
//
// BB#1: derived from LLVM BB %bb4.preheader
// Predecessors according to CFG: BB#0
// ...
// %reg16385<def> = DEC64_32r %reg16437, %EFLAGS<imp-def,dead>
// ...
// JE_4 <BB#37>, %EFLAGS<imp-use>
// Successors according to CFG: BB#37 BB#2
//
// BB#2: derived from LLVM BB %bb.nph
// Predecessors according to CFG: BB#0 BB#1
// %reg16386<def> = PHI %reg16434, <BB#0>, %reg16385, <BB#1>
BreakPHIEdge = true;
for (MachineRegisterInfo::use_nodbg_iterator
I = MRI->use_nodbg_begin(Reg), E = MRI->use_nodbg_end();
I != E; ++I) {
MachineInstr *UseInst = &*I;
MachineBasicBlock *UseBlock = UseInst->getParent();
if (!(UseBlock == MBB && UseInst->isPHI() &&
UseInst->getOperand(I.getOperandNo()+1).getMBB() == DefMBB)) {
BreakPHIEdge = false;
break;
}
}
if (BreakPHIEdge)
return true;
for (MachineRegisterInfo::use_nodbg_iterator
I = MRI->use_nodbg_begin(Reg), E = MRI->use_nodbg_end();
I != E; ++I) {
// Determine the block of the use.
MachineInstr *UseInst = &*I;
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(I.getOperandNo()+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) {
DEBUG(dbgs() << "******** Machine Sinking ********\n");
const TargetMachine &TM = MF.getTarget();
TII = TM.getInstrInfo();
TRI = TM.getRegisterInfo();
MRI = &MF.getRegInfo();
DT = &getAnalysis<MachineDominatorTree>();
LI = &getAnalysis<MachineLoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
AllocatableSet = TRI->getAllocatableSet(MF);
bool EverMadeChange = false;
while (1) {
bool MadeChange = false;
// Process all basic blocks.
CEBCandidates.clear();
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I)
MadeChange |= ProcessBlock(*I);
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
}
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;
// Walk the basic block bottom-up. Remember if we saw a store.
MachineBasicBlock::iterator I = MBB.end();
--I;
bool ProcessedBegin, SawStore = false;
do {
MachineInstr *MI = I; // The instruction to sink.
// Predecrement I (if it's not begin) so that it isn't invalidated by
// sinking.
ProcessedBegin = I == MBB.begin();
if (!ProcessedBegin)
--I;
if (MI->isDebugValue())
continue;
if (PerformTrivialForwardCoalescing(MI, &MBB))
continue;
if (SinkInstruction(MI, SawStore))
++NumSunk, MadeChange = true;
// If we just processed the first instruction in the block, we're done.
} while (!ProcessedBegin);
return MadeChange;
}
bool MachineSinking::isWorthBreakingCriticalEdge(MachineInstr *MI,
MachineBasicBlock *From,
MachineBasicBlock *To) {
// FIXME: Need much better heuristics.
// If the pass has already considered breaking this edge (during this pass
// through the function), then let's go ahead and break it. This means
// sinking multiple "cheap" instructions into the same block.
if (!CEBCandidates.insert(std::make_pair(From, To)))
return true;
if (!MI->isCopy() && !MI->getDesc().isAsCheapAsAMove())
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()) continue;
unsigned Reg = MO.getReg();
if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
if (MRI->hasOneNonDBGUse(Reg))
return true;
}
return false;
}
MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineInstr *MI,
MachineBasicBlock *FromBB,
MachineBasicBlock *ToBB,
bool BreakPHIEdge) {
if (!isWorthBreakingCriticalEdge(MI, FromBB, ToBB))
return 0;
// Avoid breaking back edge. From == To means backedge for single BB loop.
if (!SplitEdges || FromBB == ToBB)
return 0;
// Check for backedges of more "complex" loops.
if (LI->getLoopFor(FromBB) == LI->getLoopFor(ToBB) &&
LI->isLoopHeader(ToBB))
return 0;
// 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 0;
}
}
return FromBB->SplitCriticalEdge(ToBB, this);
}
static bool AvoidsSinking(MachineInstr *MI, MachineRegisterInfo *MRI) {
return MI->isInsertSubreg() || MI->isSubregToReg() || MI->isRegSequence();
}
/// 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) {
// Don't sink insert_subreg, subreg_to_reg, reg_sequence. These are meant to
// be close to the source to make it easier to coalesce.
if (AvoidsSinking(MI, MRI))
return false;
// Check if it's safe to move the instruction.
if (!MI->isSafeToMove(TII, AA, SawStore))
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.
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
MachineBasicBlock *ParentBlock = MI->getParent();
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
MachineBasicBlock *SuccToSinkTo = 0;
bool BreakPHIEdge = false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg()) continue; // Ignore non-register operands.
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
if (MO.isUse()) {
// If the physreg has no defs anywhere, it's just an ambient register
// and we can freely move its uses. Alternatively, if it's allocatable,
// it could get allocated to something with a def during allocation.
if (!MRI->def_empty(Reg))
return false;
if (AllocatableSet.test(Reg))
return false;
// Check for a def among the register's aliases too.
for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
unsigned AliasReg = *Alias;
if (!MRI->def_empty(AliasReg))
return false;
if (AllocatableSet.test(AliasReg))
return false;
}
} else if (!MO.isDead()) {
// A def that isn't dead. We can't move it.
return false;
}
} 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 false;
// FIXME: This picks a successor to sink into based on having one
// successor that dominates all the uses. However, there are cases where
// sinking can happen but where the sink point isn't a successor. For
// example:
//
// x = computation
// if () {} else {}
// use x
//
// the instruction could be sunk over the whole diamond for the
// if/then/else (or loop, etc), allowing it to be sunk into other blocks
// after that.
// 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, ParentBlock,
BreakPHIEdge, LocalUse))
return false;
continue;
}
// Otherwise, we should look at all the successors and decide which one
// we should sink to.
for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
E = ParentBlock->succ_end(); SI != E; ++SI) {
bool LocalUse = false;
if (AllUsesDominatedByBlock(Reg, *SI, ParentBlock,
BreakPHIEdge, LocalUse)) {
SuccToSinkTo = *SI;
break;
}
if (LocalUse)
// Def is used locally, it's never safe to move this def.
return false;
}
// If we couldn't find a block to sink to, ignore this instruction.
if (SuccToSinkTo == 0)
return false;
}
}
// If there are no outputs, it must have side-effects.
if (SuccToSinkTo == 0)
return false;
// It's not safe to sink instructions to EH landing pad. Control flow into
// landing pad is implicitly defined.
if (SuccToSinkTo->isLandingPad())
return false;
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (MI->getParent() == SuccToSinkTo)
return false;
// If the instruction to move defines a dead physical register which is live
// when leaving the basic block, don't move it because it could turn into a
// "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>)
for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
const MachineOperand &MO = MI->getOperand(I);
if (!MO.isReg()) continue;
unsigned Reg = MO.getReg();
if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg)) continue;
if (SuccToSinkTo->isLiveIn(Reg))
return false;
}
DEBUG(dbgs() << "Sink instr " << *MI << "\tinto block " << *SuccToSinkTo);
// If the block has multiple predecessors, this would introduce computation on
// a path that it doesn't already exist. We could split the critical edge,
// but for now we just punt.
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(TII, AA, store)) {
DEBUG(dbgs() << " *** NOTE: Won't sink load along critical edge.\n");
TryBreak = true;
}
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!TryBreak && !DT->dominates(ParentBlock, SuccToSinkTo)) {
DEBUG(dbgs() << " *** NOTE: Critical edge found\n");
TryBreak = true;
}
// Don't sink instructions into a loop.
if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) {
DEBUG(dbgs() << " *** NOTE: Loop header found\n");
TryBreak = true;
}
// Otherwise we are OK with sinking along a critical edge.
if (!TryBreak)
DEBUG(dbgs() << "Sinking along critical edge.\n");
else {
MachineBasicBlock *NewSucc =
SplitCriticalEdge(MI, ParentBlock, SuccToSinkTo, BreakPHIEdge);
if (!NewSucc) {
DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
"break critical edge\n");
return false;
} else {
DEBUG(dbgs() << " *** Splitting critical edge:"
" BB#" << ParentBlock->getNumber()
<< " -- BB#" << NewSucc->getNumber()
<< " -- BB#" << SuccToSinkTo->getNumber() << '\n');
SuccToSinkTo = NewSucc;
++NumSplit;
BreakPHIEdge = 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.
MachineBasicBlock *NewSucc = SplitCriticalEdge(MI, ParentBlock,
SuccToSinkTo, BreakPHIEdge);
if (!NewSucc) {
DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
"break critical edge\n");
return false;
}
DEBUG(dbgs() << " *** Splitting critical edge:"
" BB#" << ParentBlock->getNumber()
<< " -- BB#" << NewSucc->getNumber()
<< " -- BB#" << SuccToSinkTo->getNumber() << '\n');
SuccToSinkTo = NewSucc;
++NumSplit;
}
// Determine where to insert into. Skip phi nodes.
MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI())
++InsertPos;
// Move the instruction.
SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
++MachineBasicBlock::iterator(MI));
// Conservatively, clear any kill flags, since it's possible that they are no
// longer correct.
MI->clearKillInfo();
return true;
}
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