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

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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 machine instruction PHI nodes by inserting copy
// instructions, using an intelligent copy-folding technique based on
// dominator information. This is technique is 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.
// DOI= http://doi.acm.org/10.1145/512529.512534
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "strongphielim"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;
namespace {
struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
static char ID; // Pass identification, replacement for typeid
StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
DenseMap<MachineBasicBlock*,
std::map<unsigned, unsigned> > Waiting;
std::map<unsigned, std::vector<unsigned> > Stacks;
std::set<unsigned> UsedByAnother;
bool runOnMachineFunction(MachineFunction &Fn);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addPreserved<LiveVariables>();
AU.addPreservedID(PHIEliminationID);
AU.addRequired<MachineDominatorTree>();
AU.addRequired<LiveVariables>();
AU.setPreservesAll();
MachineFunctionPass::getAnalysisUsage(AU);
}
virtual void releaseMemory() {
preorder.clear();
maxpreorder.clear();
Waiting.clear();
}
private:
struct DomForestNode {
private:
std::vector<DomForestNode*> children;
unsigned reg;
void addChild(DomForestNode* DFN) { children.push_back(DFN); }
public:
typedef std::vector<DomForestNode*>::iterator iterator;
DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
if (parent)
parent->addChild(this);
}
~DomForestNode() {
for (iterator I = begin(), E = end(); I != E; ++I)
delete *I;
}
inline unsigned getReg() { return reg; }
inline DomForestNode::iterator begin() { return children.begin(); }
inline DomForestNode::iterator end() { return children.end(); }
};
DenseMap<MachineBasicBlock*, unsigned> preorder;
DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
void computeDFS(MachineFunction& MF);
void processBlock(MachineBasicBlock* MBB);
std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
void processPHIUnion(MachineInstr* Inst,
std::set<unsigned>& PHIUnion,
std::vector<StrongPHIElimination::DomForestNode*>& DF,
std::vector<std::pair<unsigned, unsigned> >& locals);
void ScheduleCopies(MachineBasicBlock* MBB, std::set<unsigned>& pushed);
void InsertCopies(MachineBasicBlock* MBB);
};
char StrongPHIElimination::ID = 0;
RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
"Eliminate PHI nodes for register allocation, intelligently");
}
const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
/// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
/// of the given MachineFunction. These numbers are then used in other parts
/// of the PHI elimination process.
void StrongPHIElimination::computeDFS(MachineFunction& MF) {
SmallPtrSet<MachineDomTreeNode*, 8> frontier;
SmallPtrSet<MachineDomTreeNode*, 8> visited;
unsigned time = 0;
MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
MachineDomTreeNode* node = DT.getRootNode();
std::vector<MachineDomTreeNode*> worklist;
worklist.push_back(node);
while (!worklist.empty()) {
MachineDomTreeNode* currNode = worklist.back();
if (!frontier.count(currNode)) {
frontier.insert(currNode);
++time;
preorder.insert(std::make_pair(currNode->getBlock(), time));
}
bool inserted = false;
for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
I != E; ++I)
if (!frontier.count(*I) && !visited.count(*I)) {
worklist.push_back(*I);
inserted = true;
break;
}
if (!inserted) {
frontier.erase(currNode);
visited.insert(currNode);
maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
worklist.pop_back();
}
}
}
/// PreorderSorter - a helper class that is used to sort registers
/// according to the preorder number of their defining blocks
class PreorderSorter {
private:
DenseMap<MachineBasicBlock*, unsigned>& preorder;
LiveVariables& LV;
public:
PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
LiveVariables& L) : preorder(p), LV(L) { }
bool operator()(unsigned A, unsigned B) {
if (A == B)
return false;
MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
if (preorder[ABlock] < preorder[BBlock])
return true;
else if (preorder[ABlock] > preorder[BBlock])
return false;
assert(0 && "Error sorting by dominance!");
return false;
}
};
/// computeDomForest - compute the subforest of the DomTree corresponding
/// to the defining blocks of the registers in question
std::vector<StrongPHIElimination::DomForestNode*>
StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
LiveVariables& LV = getAnalysis<LiveVariables>();
DomForestNode* VirtualRoot = new DomForestNode(0, 0);
maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
std::vector<unsigned> worklist;
worklist.reserve(regs.size());
for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
I != E; ++I)
worklist.push_back(*I);
PreorderSorter PS(preorder, LV);
std::sort(worklist.begin(), worklist.end(), PS);
DomForestNode* CurrentParent = VirtualRoot;
std::vector<DomForestNode*> stack;
stack.push_back(VirtualRoot);
for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
I != E; ++I) {
unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
MachineBasicBlock* parentBlock =
LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
while (pre > maxpreorder[parentBlock]) {
stack.pop_back();
CurrentParent = stack.back();
parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
}
DomForestNode* child = new DomForestNode(*I, CurrentParent);
stack.push_back(child);
CurrentParent = child;
}
std::vector<DomForestNode*> ret;
ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
return ret;
}
/// isLiveIn - helper method that determines, from a VarInfo, if a register
/// is live into a block
bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
if (V.AliveBlocks.test(MBB->getNumber()))
return true;
if (V.DefInst->getParent() != MBB &&
V.UsedBlocks.test(MBB->getNumber()))
return true;
return false;
}
/// isLiveOut - help method that determines, from a VarInfo, if a register is
/// live out of a block.
bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
if (MBB == V.DefInst->getParent() ||
V.UsedBlocks.test(MBB->getNumber())) {
for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
E = V.Kills.end(); I != E; ++I)
if ((*I)->getParent() == MBB)
return false;
return true;
}
return false;
}
/// isKillInst - helper method that determines, from a VarInfo, if an
/// instruction kills a given register
bool isKillInst(LiveVariables::VarInfo& V, MachineInstr* MI) {
return std::find(V.Kills.begin(), V.Kills.end(), MI) != V.Kills.end();
}
/// interferes - checks for local interferences by scanning a block. The only
/// trick parameter is 'mode' which tells it the relationship of the two
/// registers. 0 - defined in the same block, 1 - first properly dominates
/// second, 2 - second properly dominates first
bool interferes(LiveVariables::VarInfo& First, LiveVariables::VarInfo& Second,
MachineBasicBlock* scan, unsigned mode) {
MachineInstr* def = 0;
MachineInstr* kill = 0;
bool interference = false;
// Wallk the block, checking for interferences
for (MachineBasicBlock::iterator MBI = scan->begin(), MBE = scan->end();
MBI != MBE; ++MBI) {
MachineInstr* curr = MBI;
// Same defining block...
if (mode == 0) {
if (curr == First.DefInst) {
// If we find our first DefInst, save it
if (!def) {
def = curr;
// If there's already an unkilled DefInst, then
// this is an interference
} else if (!kill) {
interference = true;
break;
// If there's a DefInst followed by a KillInst, then
// they can't interfere
} else {
interference = false;
break;
}
// Symmetric with the above
} else if (curr == Second.DefInst ) {
if (!def) {
def = curr;
} else if (!kill) {
interference = true;
break;
} else {
interference = false;
break;
}
// Store KillInsts if they match up with the DefInst
} else if (isKillInst(First, curr)) {
if (def == First.DefInst) {
kill = curr;
} else if (isKillInst(Second, curr)) {
if (def == Second.DefInst) {
kill = curr;
}
}
}
// First properly dominates second...
} else if (mode == 1) {
if (curr == Second.DefInst) {
// DefInst of second without kill of first is an interference
if (!kill) {
interference = true;
break;
// DefInst after a kill is a non-interference
} else {
interference = false;
break;
}
// Save KillInsts of First
} else if (isKillInst(First, curr)) {
kill = curr;
}
// Symmetric with the above
} else if (mode == 2) {
if (curr == First.DefInst) {
if (!kill) {
interference = true;
break;
} else {
interference = false;
break;
}
} else if (isKillInst(Second, curr)) {
kill = curr;
}
}
}
return interference;
}
/// processBlock - Eliminate PHIs in the given block
void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
LiveVariables& LV = getAnalysis<LiveVariables>();
// Holds names that have been added to a set in any PHI within this block
// before the current one.
std::set<unsigned> ProcessedNames;
MachineBasicBlock::iterator P = MBB->begin();
while (P->getOpcode() == TargetInstrInfo::PHI) {
LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());
unsigned DestReg = P->getOperand(0).getReg();
// Hold the names that are currently in the candidate set.
std::set<unsigned> PHIUnion;
std::set<MachineBasicBlock*> UnionedBlocks;
for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
unsigned SrcReg = P->getOperand(i-1).getReg();
LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
// Check for trivial interferences
if (isLiveIn(SrcInfo, P->getParent()) ||
isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
ProcessedNames.count(SrcReg) ||
UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
// add a copy from a_i to p in Waiting[From[a_i]]
MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
Waiting[From].insert(std::make_pair(SrcReg, DestReg));
UsedByAnother.insert(SrcReg);
} else {
PHIUnion.insert(SrcReg);
UnionedBlocks.insert(SrcInfo.DefInst->getParent());
}
}
std::vector<StrongPHIElimination::DomForestNode*> DF =
computeDomForest(PHIUnion);
// Walk DomForest to resolve interferences
std::vector<std::pair<unsigned, unsigned> > localInterferences;
processPHIUnion(P, PHIUnion, DF, localInterferences);
// Check for local interferences
for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
std::pair<unsigned, unsigned> p = *I;
LiveVariables::VarInfo& FirstInfo = LV.getVarInfo(p.first);
LiveVariables::VarInfo& SecondInfo = LV.getVarInfo(p.second);
MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
// Determine the block we need to scan and the relationship between
// the two registers
MachineBasicBlock* scan = 0;
unsigned mode = 0;
if (FirstInfo.DefInst->getParent() == SecondInfo.DefInst->getParent()) {
scan = FirstInfo.DefInst->getParent();
mode = 0; // Same block
} else if (MDT.dominates(FirstInfo.DefInst->getParent(),
SecondInfo.DefInst->getParent())) {
scan = SecondInfo.DefInst->getParent();
mode = 1; // First dominates second
} else {
scan = FirstInfo.DefInst->getParent();
mode = 2; // Second dominates first
}
// If there's an interference, we need to insert copies
if (interferes(FirstInfo, SecondInfo, scan, mode)) {
// Insert copies for First
for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
if (P->getOperand(i-1).getReg() == p.first) {
unsigned SrcReg = p.first;
MachineBasicBlock* From = P->getOperand(i).getMBB();
Waiting[From].insert(std::make_pair(SrcReg,
P->getOperand(0).getReg()));
UsedByAnother.insert(SrcReg);
PHIUnion.erase(SrcReg);
}
}
}
}
// FIXME: Cache renaming information
ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
++P;
}
}
/// processPHIUnion - Take a set of candidate registers to be coallesced when
/// decomposing the PHI instruction. Use the DominanceForest to remove the ones
/// that are known to interfere, and flag others that need to be checked for
/// local interferences.
void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
std::set<unsigned>& PHIUnion,
std::vector<StrongPHIElimination::DomForestNode*>& DF,
std::vector<std::pair<unsigned, unsigned> >& locals) {
std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
SmallPtrSet<DomForestNode*, 4> visited;
LiveVariables& LV = getAnalysis<LiveVariables>();
unsigned DestReg = Inst->getOperand(0).getReg();
// DF walk on the DomForest
while (!worklist.empty()) {
DomForestNode* DFNode = worklist.back();
LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
visited.insert(DFNode);
bool inserted = false;
for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
CI != CE; ++CI) {
DomForestNode* child = *CI;
LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
if (isLiveOut(Info, CInfo.DefInst->getParent())) {
// Insert copies for parent
for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
unsigned SrcReg = DFNode->getReg();
MachineBasicBlock* From = Inst->getOperand(i).getMBB();
Waiting[From].insert(std::make_pair(SrcReg, DestReg));
UsedByAnother.insert(SrcReg);
PHIUnion.erase(SrcReg);
}
}
} else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
// Add (p, c) to possible local interferences
locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
}
if (!visited.count(child)) {
worklist.push_back(child);
inserted = true;
}
}
if (!inserted) worklist.pop_back();
}
}
/// ScheduleCopies - Insert copies into predecessor blocks, scheduling
/// them properly so as to avoid the 'lost copy' and the 'virtual swap'
/// problems.
///
/// Based on "Practical Improvements to the Construction and Destruction
/// of Static Single Assignment Form" by Briggs, et al.
void StrongPHIElimination::ScheduleCopies(MachineBasicBlock* MBB,
std::set<unsigned>& pushed) {
std::map<unsigned, unsigned>& copy_set= Waiting[MBB];
std::map<unsigned, unsigned> worklist;
std::map<unsigned, unsigned> map;
// Setup worklist of initial copies
for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
E = copy_set.end(); I != E; ) {
map.insert(std::make_pair(I->first, I->first));
map.insert(std::make_pair(I->second, I->second));
if (!UsedByAnother.count(I->first)) {
worklist.insert(*I);
// Avoid iterator invalidation
unsigned first = I->first;
++I;
copy_set.erase(first);
} else {
++I;
}
}
LiveVariables& LV = getAnalysis<LiveVariables>();
// Iterate over the worklist, inserting copies
while (!worklist.empty() || !copy_set.empty()) {
while (!worklist.empty()) {
std::pair<unsigned, unsigned> curr = *worklist.begin();
worklist.erase(curr.first);
if (isLiveOut(LV.getVarInfo(curr.second), MBB)) {
// Insert copy from curr.second to a temporary
// Push temporary on Stacks
// Insert temporary in pushed
}
// Insert copy from map[curr.first] to curr.second
map[curr.first] = curr.second;
// If curr.first is a destination in copy_set...
for (std::map<unsigned, unsigned>::iterator I = copy_set.begin(),
E = copy_set.end(); I != E; )
if (curr.first == I->second) {
std::pair<unsigned, unsigned> temp = *I;
// Avoid iterator invalidation
++I;
copy_set.erase(temp.first);
worklist.insert(temp);
break;
} else {
++I;
}
}
if (!copy_set.empty()) {
std::pair<unsigned, unsigned> curr = *copy_set.begin();
copy_set.erase(curr.first);
// Insert a copy from dest to a new temporary t at the end of b
// map[curr.second] = t;
worklist.insert(curr);
}
}
}
/// InsertCopies - insert copies into MBB and all of its successors
void StrongPHIElimination::InsertCopies(MachineBasicBlock* MBB) {
std::set<unsigned> pushed;
// Rewrite register uses from Stacks
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I)
for (unsigned i = 0; i < I->getNumOperands(); ++i)
if (I->getOperand(i).isRegister() &&
Stacks[I->getOperand(i).getReg()].size()) {
I->getOperand(i).setReg(Stacks[I->getOperand(i).getReg()].back());
}
// Schedule the copies for this block
ScheduleCopies(MBB, pushed);
// Recur to our successors
for (GraphTraits<MachineBasicBlock*>::ChildIteratorType I =
GraphTraits<MachineBasicBlock*>::child_begin(MBB), E =
GraphTraits<MachineBasicBlock*>::child_end(MBB); I != E; ++I)
InsertCopies(*I);
// As we exit this block, pop the names we pushed while processing it
for (std::set<unsigned>::iterator I = pushed.begin(),
E = pushed.end(); I != E; ++I)
Stacks[*I].pop_back();
}
bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
// Compute DFS numbers of each block
computeDFS(Fn);
// Determine which phi node operands need copies
for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
if (!I->empty() &&
I->begin()->getOpcode() == TargetInstrInfo::PHI)
processBlock(I);
// Insert copies
// FIXME: This process should probably preserve LiveVariables
InsertCopies(Fn.begin());
// FIXME: Perform renaming
return false;
}
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