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llvm-mirror/lib/CodeGen/StrongPHIElimination.cpp
Ahmed Charles 745c53c2a7 Remove dead code. Improve llvm_unreachable text. Simplify some control flow. 
llvm-svn: 150918
2012-02-19 11:37:01 +00:00

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//===- StrongPHIElimination.cpp - Eliminate PHI nodes by inserting copies -===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass eliminates PHI instructions by aggressively coalescing the copies
// that would be inserted by a naive algorithm and only inserting the copies
// that are necessary. The coalescing technique initially assumes that all
// registers appearing in a PHI instruction do not interfere. It then eliminates
// proven interferences, using dominators to only perform a linear number of
// interference tests instead of the quadratic number of interference tests
// that this would naively require. This is a technique derived from:
//
// Budimlic, et al. Fast copy coalescing and live-range identification.
// In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
// Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
// PLDI '02. ACM, New York, NY, 25-32.
//
// The original implementation constructs a data structure they call a dominance
// forest for this purpose. The dominance forest was shown to be unnecessary,
// as it is possible to emulate the creation and traversal of a dominance forest
// by directly using the dominator tree, rather than actually constructing the
// dominance forest. This technique is explained in:
//
// Boissinot, et al. Revisiting Out-of-SSA Translation for Correctness, Code
// Quality and Efficiency,
// In Proceedings of the 7th annual IEEE/ACM International Symposium on Code
// Generation and Optimization (Seattle, Washington, March 22 - 25, 2009).
// CGO '09. IEEE, Washington, DC, 114-125.
//
// Careful implementation allows for all of the dominator forest interference
// checks to be performed at once in a single depth-first traversal of the
// dominator tree, which is what is implemented here.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "strongphielim"
#include "PHIEliminationUtils.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
namespace {
class StrongPHIElimination : public MachineFunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
StrongPHIElimination() : MachineFunctionPass(ID) {
initializeStrongPHIEliminationPass(*PassRegistry::getPassRegistry());
}
virtual void getAnalysisUsage(AnalysisUsage&) const;
bool runOnMachineFunction(MachineFunction&);
private:
/// This struct represents a single node in the union-find data structure
/// representing the variable congruence classes. There is one difference
/// from a normal union-find data structure. We steal two bits from the parent
/// pointer . One of these bits is used to represent whether the register
/// itself has been isolated, and the other is used to represent whether the
/// PHI with that register as its destination has been isolated.
///
/// Note that this leads to the strange situation where the leader of a
/// congruence class may no longer logically be a member, due to being
/// isolated.
struct Node {
enum Flags {
kRegisterIsolatedFlag = 1,
kPHIIsolatedFlag = 2
};
Node(unsigned v) : value(v), rank(0) { parent.setPointer(this); }
Node *getLeader();
PointerIntPair<Node*, 2> parent;
unsigned value;
unsigned rank;
};
/// Add a register in a new congruence class containing only itself.
void addReg(unsigned);
/// Join the congruence classes of two registers. This function is biased
/// towards the left argument, i.e. after
///
/// addReg(r2);
/// unionRegs(r1, r2);
///
/// the leader of the unioned congruence class is the same as the leader of
/// r1's congruence class prior to the union. This is actually relied upon
/// in the copy insertion code.
void unionRegs(unsigned, unsigned);
/// Get the color of a register. The color is 0 if the register has been
/// isolated.
unsigned getRegColor(unsigned);
// Isolate a register.
void isolateReg(unsigned);
/// Get the color of a PHI. The color of a PHI is 0 if the PHI has been
/// isolated. Otherwise, it is the original color of its destination and
/// all of its operands (before they were isolated, if they were).
unsigned getPHIColor(MachineInstr*);
/// Isolate a PHI.
void isolatePHI(MachineInstr*);
/// Traverses a basic block, splitting any interferences found between
/// registers in the same congruence class. It takes two DenseMaps as
/// arguments that it also updates: CurrentDominatingParent, which maps
/// a color to the register in that congruence class whose definition was
/// most recently seen, and ImmediateDominatingParent, which maps a register
/// to the register in the same congruence class that most immediately
/// dominates it.
///
/// This function assumes that it is being called in a depth-first traversal
/// of the dominator tree.
void SplitInterferencesForBasicBlock(
MachineBasicBlock&,
DenseMap<unsigned, unsigned> &CurrentDominatingParent,
DenseMap<unsigned, unsigned> &ImmediateDominatingParent);
// Lowers a PHI instruction, inserting copies of the source and destination
// registers as necessary.
void InsertCopiesForPHI(MachineInstr*, MachineBasicBlock*);
// Merges the live interval of Reg into NewReg and renames Reg to NewReg
// everywhere that Reg appears. Requires Reg and NewReg to have non-
// overlapping lifetimes.
void MergeLIsAndRename(unsigned Reg, unsigned NewReg);
MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
MachineDominatorTree *DT;
LiveIntervals *LI;
BumpPtrAllocator Allocator;
DenseMap<unsigned, Node*> RegNodeMap;
// Maps a basic block to a list of its defs of registers that appear as PHI
// sources.
DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > PHISrcDefs;
// Maps a color to a pair of a MachineInstr* and a virtual register, which
// is the operand of that PHI corresponding to the current basic block.
DenseMap<unsigned, std::pair<MachineInstr*, unsigned> > CurrentPHIForColor;
// FIXME: Can these two data structures be combined? Would a std::multimap
// be any better?
// Stores pairs of predecessor basic blocks and the source registers of
// inserted copy instructions.
typedef DenseSet<std::pair<MachineBasicBlock*, unsigned> > SrcCopySet;
SrcCopySet InsertedSrcCopySet;
// Maps pairs of predecessor basic blocks and colors to their defining copy
// instructions.
typedef DenseMap<std::pair<MachineBasicBlock*, unsigned>, MachineInstr*>
SrcCopyMap;
SrcCopyMap InsertedSrcCopyMap;
// Maps inserted destination copy registers to their defining copy
// instructions.
typedef DenseMap<unsigned, MachineInstr*> DestCopyMap;
DestCopyMap InsertedDestCopies;
};
struct MIIndexCompare {
MIIndexCompare(LiveIntervals *LiveIntervals) : LI(LiveIntervals) { }
bool operator()(const MachineInstr *LHS, const MachineInstr *RHS) const {
return LI->getInstructionIndex(LHS) < LI->getInstructionIndex(RHS);
}
LiveIntervals *LI;
};
} // namespace
STATISTIC(NumPHIsLowered, "Number of PHIs lowered");
STATISTIC(NumDestCopiesInserted, "Number of destination copies inserted");
STATISTIC(NumSrcCopiesInserted, "Number of source copies inserted");
char StrongPHIElimination::ID = 0;
INITIALIZE_PASS_BEGIN(StrongPHIElimination, "strong-phi-node-elimination",
"Eliminate PHI nodes for register allocation, intelligently", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_END(StrongPHIElimination, "strong-phi-node-elimination",
"Eliminate PHI nodes for register allocation, intelligently", false, false)
char &llvm::StrongPHIEliminationID = StrongPHIElimination::ID;
void StrongPHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<SlotIndexes>();
AU.addPreserved<SlotIndexes>();
AU.addRequired<LiveIntervals>();
AU.addPreserved<LiveIntervals>();
MachineFunctionPass::getAnalysisUsage(AU);
}
static MachineOperand *findLastUse(MachineBasicBlock *MBB, unsigned Reg) {
// FIXME: This only needs to check from the first terminator, as only the
// first terminator can use a virtual register.
for (MachineBasicBlock::reverse_iterator RI = MBB->rbegin(); ; ++RI) {
assert (RI != MBB->rend());
MachineInstr *MI = &*RI;
for (MachineInstr::mop_iterator OI = MI->operands_begin(),
OE = MI->operands_end(); OI != OE; ++OI) {
MachineOperand &MO = *OI;
if (MO.isReg() && MO.isUse() && MO.getReg() == Reg)
return &MO;
}
}
}
bool StrongPHIElimination::runOnMachineFunction(MachineFunction &MF) {
MRI = &MF.getRegInfo();
TII = MF.getTarget().getInstrInfo();
DT = &getAnalysis<MachineDominatorTree>();
LI = &getAnalysis<LiveIntervals>();
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I) {
for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
BBI != BBE && BBI->isPHI(); ++BBI) {
unsigned DestReg = BBI->getOperand(0).getReg();
addReg(DestReg);
PHISrcDefs[I].push_back(BBI);
for (unsigned i = 1; i < BBI->getNumOperands(); i += 2) {
MachineOperand &SrcMO = BBI->getOperand(i);
unsigned SrcReg = SrcMO.getReg();
addReg(SrcReg);
unionRegs(DestReg, SrcReg);
MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
if (DefMI)
PHISrcDefs[DefMI->getParent()].push_back(DefMI);
}
}
}
// Perform a depth-first traversal of the dominator tree, splitting
// interferences amongst PHI-congruence classes.
DenseMap<unsigned, unsigned> CurrentDominatingParent;
DenseMap<unsigned, unsigned> ImmediateDominatingParent;
for (df_iterator<MachineDomTreeNode*> DI = df_begin(DT->getRootNode()),
DE = df_end(DT->getRootNode()); DI != DE; ++DI) {
SplitInterferencesForBasicBlock(*DI->getBlock(),
CurrentDominatingParent,
ImmediateDominatingParent);
}
// Insert copies for all PHI source and destination registers.
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I) {
for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
BBI != BBE && BBI->isPHI(); ++BBI) {
InsertCopiesForPHI(BBI, I);
}
}
// FIXME: Preserve the equivalence classes during copy insertion and use
// the preversed equivalence classes instead of recomputing them.
RegNodeMap.clear();
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I) {
for (MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
BBI != BBE && BBI->isPHI(); ++BBI) {
unsigned DestReg = BBI->getOperand(0).getReg();
addReg(DestReg);
for (unsigned i = 1; i < BBI->getNumOperands(); i += 2) {
unsigned SrcReg = BBI->getOperand(i).getReg();
addReg(SrcReg);
unionRegs(DestReg, SrcReg);
}
}
}
DenseMap<unsigned, unsigned> RegRenamingMap;
bool Changed = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I) {
MachineBasicBlock::iterator BBI = I->begin(), BBE = I->end();
while (BBI != BBE && BBI->isPHI()) {
MachineInstr *PHI = BBI;
assert(PHI->getNumOperands() > 0);
unsigned SrcReg = PHI->getOperand(1).getReg();
unsigned SrcColor = getRegColor(SrcReg);
unsigned NewReg = RegRenamingMap[SrcColor];
if (!NewReg) {
NewReg = SrcReg;
RegRenamingMap[SrcColor] = SrcReg;
}
MergeLIsAndRename(SrcReg, NewReg);
unsigned DestReg = PHI->getOperand(0).getReg();
if (!InsertedDestCopies.count(DestReg))
MergeLIsAndRename(DestReg, NewReg);
for (unsigned i = 3; i < PHI->getNumOperands(); i += 2) {
unsigned SrcReg = PHI->getOperand(i).getReg();
MergeLIsAndRename(SrcReg, NewReg);
}
++BBI;
LI->RemoveMachineInstrFromMaps(PHI);
PHI->eraseFromParent();
Changed = true;
}
}
// Due to the insertion of copies to split live ranges, the live intervals are
// guaranteed to not overlap, except in one case: an original PHI source and a
// PHI destination copy. In this case, they have the same value and thus don't
// truly intersect, so we merge them into the value live at that point.
// FIXME: Is there some better way we can handle this?
for (DestCopyMap::iterator I = InsertedDestCopies.begin(),
E = InsertedDestCopies.end(); I != E; ++I) {
unsigned DestReg = I->first;
unsigned DestColor = getRegColor(DestReg);
unsigned NewReg = RegRenamingMap[DestColor];
LiveInterval &DestLI = LI->getInterval(DestReg);
LiveInterval &NewLI = LI->getInterval(NewReg);
assert(DestLI.ranges.size() == 1
&& "PHI destination copy's live interval should be a single live "
"range from the beginning of the BB to the copy instruction.");
LiveRange *DestLR = DestLI.begin();
VNInfo *NewVNI = NewLI.getVNInfoAt(DestLR->start);
if (!NewVNI) {
NewVNI = NewLI.createValueCopy(DestLR->valno, LI->getVNInfoAllocator());
MachineInstr *CopyInstr = I->second;
CopyInstr->getOperand(1).setIsKill(true);
}
LiveRange NewLR(DestLR->start, DestLR->end, NewVNI);
NewLI.addRange(NewLR);
LI->removeInterval(DestReg);
MRI->replaceRegWith(DestReg, NewReg);
}
// Adjust the live intervals of all PHI source registers to handle the case
// where the PHIs in successor blocks were the only later uses of the source
// register.
for (SrcCopySet::iterator I = InsertedSrcCopySet.begin(),
E = InsertedSrcCopySet.end(); I != E; ++I) {
MachineBasicBlock *MBB = I->first;
unsigned SrcReg = I->second;
if (unsigned RenamedRegister = RegRenamingMap[getRegColor(SrcReg)])
SrcReg = RenamedRegister;
LiveInterval &SrcLI = LI->getInterval(SrcReg);
bool isLiveOut = false;
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
SE = MBB->succ_end(); SI != SE; ++SI) {
if (SrcLI.liveAt(LI->getMBBStartIdx(*SI))) {
isLiveOut = true;
break;
}
}
if (isLiveOut)
continue;
MachineOperand *LastUse = findLastUse(MBB, SrcReg);
assert(LastUse);
SlotIndex LastUseIndex = LI->getInstructionIndex(LastUse->getParent());
SrcLI.removeRange(LastUseIndex.getRegSlot(), LI->getMBBEndIdx(MBB));
LastUse->setIsKill(true);
}
Allocator.Reset();
RegNodeMap.clear();
PHISrcDefs.clear();
InsertedSrcCopySet.clear();
InsertedSrcCopyMap.clear();
InsertedDestCopies.clear();
return Changed;
}
void StrongPHIElimination::addReg(unsigned Reg) {
if (RegNodeMap.count(Reg))
return;
RegNodeMap[Reg] = new (Allocator) Node(Reg);
}
StrongPHIElimination::Node*
StrongPHIElimination::Node::getLeader() {
Node *N = this;
Node *Parent = parent.getPointer();
Node *Grandparent = Parent->parent.getPointer();
while (Parent != Grandparent) {
N->parent.setPointer(Grandparent);
N = Grandparent;
Parent = Parent->parent.getPointer();
Grandparent = Parent->parent.getPointer();
}
return Parent;
}
unsigned StrongPHIElimination::getRegColor(unsigned Reg) {
DenseMap<unsigned, Node*>::iterator RI = RegNodeMap.find(Reg);
if (RI == RegNodeMap.end())
return 0;
Node *Node = RI->second;
if (Node->parent.getInt() & Node::kRegisterIsolatedFlag)
return 0;
return Node->getLeader()->value;
}
void StrongPHIElimination::unionRegs(unsigned Reg1, unsigned Reg2) {
Node *Node1 = RegNodeMap[Reg1]->getLeader();
Node *Node2 = RegNodeMap[Reg2]->getLeader();
if (Node1->rank > Node2->rank) {
Node2->parent.setPointer(Node1->getLeader());
} else if (Node1->rank < Node2->rank) {
Node1->parent.setPointer(Node2->getLeader());
} else if (Node1 != Node2) {
Node2->parent.setPointer(Node1->getLeader());
Node1->rank++;
}
}
void StrongPHIElimination::isolateReg(unsigned Reg) {
Node *Node = RegNodeMap[Reg];
Node->parent.setInt(Node->parent.getInt() | Node::kRegisterIsolatedFlag);
}
unsigned StrongPHIElimination::getPHIColor(MachineInstr *PHI) {
assert(PHI->isPHI());
unsigned DestReg = PHI->getOperand(0).getReg();
Node *DestNode = RegNodeMap[DestReg];
if (DestNode->parent.getInt() & Node::kPHIIsolatedFlag)
return 0;
for (unsigned i = 1; i < PHI->getNumOperands(); i += 2) {
unsigned SrcColor = getRegColor(PHI->getOperand(i).getReg());
if (SrcColor)
return SrcColor;
}
return 0;
}
void StrongPHIElimination::isolatePHI(MachineInstr *PHI) {
assert(PHI->isPHI());
Node *Node = RegNodeMap[PHI->getOperand(0).getReg()];
Node->parent.setInt(Node->parent.getInt() | Node::kPHIIsolatedFlag);
}
/// SplitInterferencesForBasicBlock - traverses a basic block, splitting any
/// interferences found between registers in the same congruence class. It
/// takes two DenseMaps as arguments that it also updates:
///
/// 1) CurrentDominatingParent, which maps a color to the register in that
/// congruence class whose definition was most recently seen.
///
/// 2) ImmediateDominatingParent, which maps a register to the register in the
/// same congruence class that most immediately dominates it.
///
/// This function assumes that it is being called in a depth-first traversal
/// of the dominator tree.
///
/// The algorithm used here is a generalization of the dominance-based SSA test
/// for two variables. If there are variables a_1, ..., a_n such that
///
/// def(a_1) dom ... dom def(a_n),
///
/// then we can test for an interference between any two a_i by only using O(n)
/// interference tests between pairs of variables. If i < j and a_i and a_j
/// interfere, then a_i is alive at def(a_j), so it is also alive at def(a_i+1).
/// Thus, in order to test for an interference involving a_i, we need only check
/// for a potential interference with a_i+1.
///
/// This method can be generalized to arbitrary sets of variables by performing
/// a depth-first traversal of the dominator tree. As we traverse down a branch
/// of the dominator tree, we keep track of the current dominating variable and
/// only perform an interference test with that variable. However, when we go to
/// another branch of the dominator tree, the definition of the current dominating
/// variable may no longer dominate the current block. In order to correct this,
/// we need to use a stack of past choices of the current dominating variable
/// and pop from this stack until we find a variable whose definition actually
/// dominates the current block.
///
/// There will be one push on this stack for each variable that has become the
/// current dominating variable, so instead of using an explicit stack we can
/// simply associate the previous choice for a current dominating variable with
/// the new choice. This works better in our implementation, where we test for
/// interference in multiple distinct sets at once.
void
StrongPHIElimination::SplitInterferencesForBasicBlock(
MachineBasicBlock &MBB,
DenseMap<unsigned, unsigned> &CurrentDominatingParent,
DenseMap<unsigned, unsigned> &ImmediateDominatingParent) {
// Sort defs by their order in the original basic block, as the code below
// assumes that it is processing definitions in dominance order.
std::vector<MachineInstr*> &DefInstrs = PHISrcDefs[&MBB];
std::sort(DefInstrs.begin(), DefInstrs.end(), MIIndexCompare(LI));
for (std::vector<MachineInstr*>::const_iterator BBI = DefInstrs.begin(),
BBE = DefInstrs.end(); BBI != BBE; ++BBI) {
for (MachineInstr::const_mop_iterator I = (*BBI)->operands_begin(),
E = (*BBI)->operands_end(); I != E; ++I) {
const MachineOperand &MO = *I;
// FIXME: This would be faster if it were possible to bail out of checking
// an instruction's operands after the explicit defs, but this is incorrect
// for variadic instructions, which may appear before register allocation
// in the future.
if (!MO.isReg() || !MO.isDef())
continue;
unsigned DestReg = MO.getReg();
if (!DestReg || !TargetRegisterInfo::isVirtualRegister(DestReg))
continue;
// If the virtual register being defined is not used in any PHI or has
// already been isolated, then there are no more interferences to check.
unsigned DestColor = getRegColor(DestReg);
if (!DestColor)
continue;
// The input to this pass sometimes is not in SSA form in every basic
// block, as some virtual registers have redefinitions. We could eliminate
// this by fixing the passes that generate the non-SSA code, or we could
// handle it here by tracking defining machine instructions rather than
// virtual registers. For now, we just handle the situation conservatively
// in a way that will possibly lead to false interferences.
unsigned &CurrentParent = CurrentDominatingParent[DestColor];
unsigned NewParent = CurrentParent;
if (NewParent == DestReg)
continue;
// Pop registers from the stack represented by ImmediateDominatingParent
// until we find a parent that dominates the current instruction.
while (NewParent && (!DT->dominates(MRI->getVRegDef(NewParent), *BBI)
|| !getRegColor(NewParent)))
NewParent = ImmediateDominatingParent[NewParent];
// If NewParent is nonzero, then its definition dominates the current
// instruction, so it is only necessary to check for the liveness of
// NewParent in order to check for an interference.
if (NewParent
&& LI->getInterval(NewParent).liveAt(LI->getInstructionIndex(*BBI))) {
// If there is an interference, always isolate the new register. This
// could be improved by using a heuristic that decides which of the two
// registers to isolate.
isolateReg(DestReg);
CurrentParent = NewParent;
} else {
// If there is no interference, update ImmediateDominatingParent and set
// the CurrentDominatingParent for this color to the current register.
ImmediateDominatingParent[DestReg] = NewParent;
CurrentParent = DestReg;
}
}
}
// We now walk the PHIs in successor blocks and check for interferences. This
// is necessary because the use of a PHI's operands are logically contained in
// the predecessor block. The def of a PHI's destination register is processed
// along with the other defs in a basic block.
CurrentPHIForColor.clear();
for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
SE = MBB.succ_end(); SI != SE; ++SI) {
for (MachineBasicBlock::iterator BBI = (*SI)->begin(), BBE = (*SI)->end();
BBI != BBE && BBI->isPHI(); ++BBI) {
MachineInstr *PHI = BBI;
// If a PHI is already isolated, either by being isolated directly or
// having all of its operands isolated, ignore it.
unsigned Color = getPHIColor(PHI);
if (!Color)
continue;
// Find the index of the PHI operand that corresponds to this basic block.
unsigned PredIndex;
for (PredIndex = 1; PredIndex < PHI->getNumOperands(); PredIndex += 2) {
if (PHI->getOperand(PredIndex + 1).getMBB() == &MBB)
break;
}
assert(PredIndex < PHI->getNumOperands());
unsigned PredOperandReg = PHI->getOperand(PredIndex).getReg();
// Pop registers from the stack represented by ImmediateDominatingParent
// until we find a parent that dominates the current instruction.
unsigned &CurrentParent = CurrentDominatingParent[Color];
unsigned NewParent = CurrentParent;
while (NewParent
&& (!DT->dominates(MRI->getVRegDef(NewParent)->getParent(), &MBB)
|| !getRegColor(NewParent)))
NewParent = ImmediateDominatingParent[NewParent];
CurrentParent = NewParent;
// If there is an interference with a register, always isolate the
// register rather than the PHI. It is also possible to isolate the
// PHI, but that introduces copies for all of the registers involved
// in that PHI.
if (NewParent && LI->isLiveOutOfMBB(LI->getInterval(NewParent), &MBB)
&& NewParent != PredOperandReg)
isolateReg(NewParent);
std::pair<MachineInstr*, unsigned>
&CurrentPHI = CurrentPHIForColor[Color];
// If two PHIs have the same operand from every shared predecessor, then
// they don't actually interfere. Otherwise, isolate the current PHI. This
// could possibly be improved, e.g. we could isolate the PHI with the
// fewest operands.
if (CurrentPHI.first && CurrentPHI.second != PredOperandReg)
isolatePHI(PHI);
else
CurrentPHI = std::make_pair(PHI, PredOperandReg);
}
}
}
void StrongPHIElimination::InsertCopiesForPHI(MachineInstr *PHI,
MachineBasicBlock *MBB) {
assert(PHI->isPHI());
++NumPHIsLowered;
unsigned PHIColor = getPHIColor(PHI);
for (unsigned i = 1; i < PHI->getNumOperands(); i += 2) {
MachineOperand &SrcMO = PHI->getOperand(i);
// If a source is defined by an implicit def, there is no need to insert a
// copy in the predecessor.
if (SrcMO.isUndef())
continue;
unsigned SrcReg = SrcMO.getReg();
assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
"Machine PHI Operands must all be virtual registers!");
MachineBasicBlock *PredBB = PHI->getOperand(i + 1).getMBB();
unsigned SrcColor = getRegColor(SrcReg);
// If neither the PHI nor the operand were isolated, then we only need to
// set the phi-kill flag on the VNInfo at this PHI.
if (PHIColor && SrcColor == PHIColor) {
LiveInterval &SrcInterval = LI->getInterval(SrcReg);
SlotIndex PredIndex = LI->getMBBEndIdx(PredBB);
VNInfo *SrcVNI = SrcInterval.getVNInfoBefore(PredIndex);
assert(SrcVNI);
SrcVNI->setHasPHIKill(true);
continue;
}
unsigned CopyReg = 0;
if (PHIColor) {
SrcCopyMap::const_iterator I
= InsertedSrcCopyMap.find(std::make_pair(PredBB, PHIColor));
CopyReg
= I != InsertedSrcCopyMap.end() ? I->second->getOperand(0).getReg() : 0;
}
if (!CopyReg) {
const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
CopyReg = MRI->createVirtualRegister(RC);
MachineBasicBlock::iterator
CopyInsertPoint = findPHICopyInsertPoint(PredBB, MBB, SrcReg);
unsigned SrcSubReg = SrcMO.getSubReg();
MachineInstr *CopyInstr = BuildMI(*PredBB,
CopyInsertPoint,
PHI->getDebugLoc(),
TII->get(TargetOpcode::COPY),
CopyReg).addReg(SrcReg, 0, SrcSubReg);
LI->InsertMachineInstrInMaps(CopyInstr);
++NumSrcCopiesInserted;
// addLiveRangeToEndOfBlock() also adds the phikill flag to the VNInfo for
// the newly added range.
LI->addLiveRangeToEndOfBlock(CopyReg, CopyInstr);
InsertedSrcCopySet.insert(std::make_pair(PredBB, SrcReg));
addReg(CopyReg);
if (PHIColor) {
unionRegs(PHIColor, CopyReg);
assert(getRegColor(CopyReg) != CopyReg);
} else {
PHIColor = CopyReg;
assert(getRegColor(CopyReg) == CopyReg);
}
if (!InsertedSrcCopyMap.count(std::make_pair(PredBB, PHIColor)))
InsertedSrcCopyMap[std::make_pair(PredBB, PHIColor)] = CopyInstr;
}
SrcMO.setReg(CopyReg);
// If SrcReg is not live beyond the PHI, trim its interval so that it is no
// longer live-in to MBB. Note that SrcReg may appear in other PHIs that are
// processed later, but this is still correct to do at this point because we
// never rely on LiveIntervals being correct while inserting copies.
// FIXME: Should this just count uses at PHIs like the normal PHIElimination
// pass does?
LiveInterval &SrcLI = LI->getInterval(SrcReg);
SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
SlotIndex NextInstrIndex = PHIIndex.getNextIndex();
if (SrcLI.liveAt(MBBStartIndex) && SrcLI.expiredAt(NextInstrIndex))
SrcLI.removeRange(MBBStartIndex, PHIIndex, true);
}
unsigned DestReg = PHI->getOperand(0).getReg();
unsigned DestColor = getRegColor(DestReg);
if (PHIColor && DestColor == PHIColor) {
LiveInterval &DestLI = LI->getInterval(DestReg);
// Set the phi-def flag for the VN at this PHI.
SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
VNInfo *DestVNI = DestLI.getVNInfoAt(PHIIndex.getRegSlot());
assert(DestVNI);
DestVNI->setIsPHIDef(true);
// Prior to PHI elimination, the live ranges of PHIs begin at their defining
// instruction. After PHI elimination, PHI instructions are replaced by VNs
// with the phi-def flag set, and the live ranges of these VNs start at the
// beginning of the basic block.
SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
DestVNI->def = MBBStartIndex;
DestLI.addRange(LiveRange(MBBStartIndex,
PHIIndex.getRegSlot(),
DestVNI));
return;
}
const TargetRegisterClass *RC = MRI->getRegClass(DestReg);
unsigned CopyReg = MRI->createVirtualRegister(RC);
MachineInstr *CopyInstr = BuildMI(*MBB,
MBB->SkipPHIsAndLabels(MBB->begin()),
PHI->getDebugLoc(),
TII->get(TargetOpcode::COPY),
DestReg).addReg(CopyReg);
LI->InsertMachineInstrInMaps(CopyInstr);
PHI->getOperand(0).setReg(CopyReg);
++NumDestCopiesInserted;
// Add the region from the beginning of MBB to the copy instruction to
// CopyReg's live interval, and give the VNInfo the phidef flag.
LiveInterval &CopyLI = LI->getOrCreateInterval(CopyReg);
SlotIndex MBBStartIndex = LI->getMBBStartIdx(MBB);
SlotIndex DestCopyIndex = LI->getInstructionIndex(CopyInstr);
VNInfo *CopyVNI = CopyLI.getNextValue(MBBStartIndex,
LI->getVNInfoAllocator());
CopyVNI->setIsPHIDef(true);
CopyLI.addRange(LiveRange(MBBStartIndex,
DestCopyIndex.getRegSlot(),
CopyVNI));
// Adjust DestReg's live interval to adjust for its new definition at
// CopyInstr.
LiveInterval &DestLI = LI->getOrCreateInterval(DestReg);
SlotIndex PHIIndex = LI->getInstructionIndex(PHI);
DestLI.removeRange(PHIIndex.getRegSlot(), DestCopyIndex.getRegSlot());
VNInfo *DestVNI = DestLI.getVNInfoAt(DestCopyIndex.getRegSlot());
assert(DestVNI);
DestVNI->def = DestCopyIndex.getRegSlot();
InsertedDestCopies[CopyReg] = CopyInstr;
}
void StrongPHIElimination::MergeLIsAndRename(unsigned Reg, unsigned NewReg) {
if (Reg == NewReg)
return;
LiveInterval &OldLI = LI->getInterval(Reg);
LiveInterval &NewLI = LI->getInterval(NewReg);
// Merge the live ranges of the two registers.
DenseMap<VNInfo*, VNInfo*> VNMap;
for (LiveInterval::iterator LRI = OldLI.begin(), LRE = OldLI.end();
LRI != LRE; ++LRI) {
LiveRange OldLR = *LRI;
VNInfo *OldVN = OldLR.valno;
VNInfo *&NewVN = VNMap[OldVN];
if (!NewVN) {
NewVN = NewLI.createValueCopy(OldVN, LI->getVNInfoAllocator());
VNMap[OldVN] = NewVN;
}
LiveRange LR(OldLR.start, OldLR.end, NewVN);
NewLI.addRange(LR);
}
// Remove the LiveInterval for the register being renamed and replace all
// of its defs and uses with the new register.
LI->removeInterval(Reg);
MRI->replaceRegWith(Reg, NewReg);
}
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