1 //===- StrongPhiElimination.cpp - Eliminate PHI nodes by inserting copies -===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by Owen Anderson and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass eliminates machine instruction PHI nodes by inserting copy
11 // instructions, using an intelligent copy-folding technique based on
12 // dominator information. This is technique is derived from:
14 // Budimlic, et al. Fast copy coalescing and live-range identification.
15 // In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
16 // Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
17 // PLDI '02. ACM, New York, NY, 25-32.
18 // DOI= http://doi.acm.org/10.1145/512529.512534
20 //===----------------------------------------------------------------------===//
22 #define DEBUG_TYPE "strongphielim"
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/CodeGen/BreakCriticalMachineEdge.h"
25 #include "llvm/CodeGen/LiveVariables.h"
26 #include "llvm/CodeGen/MachineDominators.h"
27 #include "llvm/CodeGen/MachineFunctionPass.h"
28 #include "llvm/CodeGen/MachineInstr.h"
29 #include "llvm/CodeGen/SSARegMap.h"
30 #include "llvm/Target/TargetInstrInfo.h"
31 #include "llvm/Target/TargetMachine.h"
32 #include "llvm/ADT/Statistic.h"
33 #include "llvm/Support/Compiler.h"
38 struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
39 static char ID; // Pass identification, replacement for typeid
40 StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
42 DenseMap<MachineBasicBlock*,
43 SmallVector<std::pair<unsigned, unsigned>, 2> > Waiting;
45 bool runOnMachineFunction(MachineFunction &Fn);
47 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
48 AU.addPreserved<LiveVariables>();
49 AU.addPreservedID(PHIEliminationID);
50 AU.addRequired<MachineDominatorTree>();
51 AU.addRequired<LiveVariables>();
53 MachineFunctionPass::getAnalysisUsage(AU);
56 virtual void releaseMemory() {
64 struct DomForestNode {
66 std::vector<DomForestNode*> children;
69 void addChild(DomForestNode* DFN) { children.push_back(DFN); }
72 typedef std::vector<DomForestNode*>::iterator iterator;
74 DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
76 parent->addChild(this);
80 for (iterator I = begin(), E = end(); I != E; ++I)
84 inline unsigned getReg() { return reg; }
86 inline DomForestNode::iterator begin() { return children.begin(); }
87 inline DomForestNode::iterator end() { return children.end(); }
90 DenseMap<MachineBasicBlock*, unsigned> preorder;
91 DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
93 DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > waiting;
96 void computeDFS(MachineFunction& MF);
97 void processBlock(MachineBasicBlock* MBB);
99 std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
100 void processPHIUnion(MachineInstr* Inst,
101 std::set<unsigned>& PHIUnion,
102 std::vector<StrongPHIElimination::DomForestNode*>& DF);
103 void breakCriticalEdges(MachineFunction &Fn);
107 char StrongPHIElimination::ID = 0;
108 RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
109 "Eliminate PHI nodes for register allocation, intelligently");
112 const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
114 /// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
115 /// of the given MachineFunction. These numbers are then used in other parts
116 /// of the PHI elimination process.
117 void StrongPHIElimination::computeDFS(MachineFunction& MF) {
118 SmallPtrSet<MachineDomTreeNode*, 8> frontier;
119 SmallPtrSet<MachineDomTreeNode*, 8> visited;
123 MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
125 MachineDomTreeNode* node = DT.getRootNode();
127 std::vector<MachineDomTreeNode*> worklist;
128 worklist.push_back(node);
130 while (!worklist.empty()) {
131 MachineDomTreeNode* currNode = worklist.back();
133 if (!frontier.count(currNode)) {
134 frontier.insert(currNode);
136 preorder.insert(std::make_pair(currNode->getBlock(), time));
139 bool inserted = false;
140 for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
142 if (!frontier.count(*I) && !visited.count(*I)) {
143 worklist.push_back(*I);
149 frontier.erase(currNode);
150 visited.insert(currNode);
151 maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
158 /// PreorderSorter - a helper class that is used to sort registers
159 /// according to the preorder number of their defining blocks
160 class PreorderSorter {
162 DenseMap<MachineBasicBlock*, unsigned>& preorder;
166 PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
167 LiveVariables& L) : preorder(p), LV(L) { }
169 bool operator()(unsigned A, unsigned B) {
173 MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
174 MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
176 if (preorder[ABlock] < preorder[BBlock])
178 else if (preorder[ABlock] > preorder[BBlock])
181 assert(0 && "Error sorting by dominance!");
186 /// computeDomForest - compute the subforest of the DomTree corresponding
187 /// to the defining blocks of the registers in question
188 std::vector<StrongPHIElimination::DomForestNode*>
189 StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
190 LiveVariables& LV = getAnalysis<LiveVariables>();
192 DomForestNode* VirtualRoot = new DomForestNode(0, 0);
193 maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
195 std::vector<unsigned> worklist;
196 worklist.reserve(regs.size());
197 for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
199 worklist.push_back(*I);
201 PreorderSorter PS(preorder, LV);
202 std::sort(worklist.begin(), worklist.end(), PS);
204 DomForestNode* CurrentParent = VirtualRoot;
205 std::vector<DomForestNode*> stack;
206 stack.push_back(VirtualRoot);
208 for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
210 unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
211 MachineBasicBlock* parentBlock =
212 LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
214 while (pre > maxpreorder[parentBlock]) {
216 CurrentParent = stack.back();
218 parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
221 DomForestNode* child = new DomForestNode(*I, CurrentParent);
222 stack.push_back(child);
223 CurrentParent = child;
226 std::vector<DomForestNode*> ret;
227 ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
231 /// isLiveIn - helper method that determines, from a VarInfo, if a register
232 /// is live into a block
233 bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
234 if (V.AliveBlocks.test(MBB->getNumber()))
237 if (V.DefInst->getParent() != MBB &&
238 V.UsedBlocks.test(MBB->getNumber()))
244 /// isLiveOut - help method that determines, from a VarInfo, if a register is
245 /// live out of a block.
246 bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
247 if (MBB == V.DefInst->getParent() ||
248 V.UsedBlocks.test(MBB->getNumber())) {
249 for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
250 E = V.Kills.end(); I != E; ++I)
251 if ((*I)->getParent() == MBB)
260 /// processBlock - Eliminate PHIs in the given block
261 void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
262 LiveVariables& LV = getAnalysis<LiveVariables>();
264 // Holds names that have been added to a set in any PHI within this block
265 // before the current one.
266 std::set<unsigned> ProcessedNames;
268 MachineBasicBlock::iterator P = MBB->begin();
269 while (P->getOpcode() == TargetInstrInfo::PHI) {
270 LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());
272 unsigned DestReg = P->getOperand(0).getReg();
274 // Hold the names that are currently in the candidate set.
275 std::set<unsigned> PHIUnion;
276 std::set<MachineBasicBlock*> UnionedBlocks;
278 for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
279 unsigned SrcReg = P->getOperand(i-1).getReg();
280 LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
282 // Check for trivial interferences
283 if (isLiveIn(SrcInfo, P->getParent()) ||
284 isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
285 ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
286 isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
287 ProcessedNames.count(SrcReg) ||
288 UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
290 // add a copy from a_i to p in Waiting[From[a_i]]
291 MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
292 Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
294 PHIUnion.insert(SrcReg);
295 UnionedBlocks.insert(SrcInfo.DefInst->getParent());
299 std::vector<StrongPHIElimination::DomForestNode*> DF =
300 computeDomForest(PHIUnion);
302 // Walk DomForest to resolve interferences
304 ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
309 void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
310 std::set<unsigned>& PHIUnion,
311 std::vector<StrongPHIElimination::DomForestNode*>& DF) {
313 std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
314 SmallPtrSet<DomForestNode*, 4> visited;
316 LiveVariables& LV = getAnalysis<LiveVariables>();
317 unsigned DestReg = Inst->getOperand(0).getReg();
319 while (!worklist.empty()) {
320 DomForestNode* DFNode = worklist.back();
322 LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
323 visited.insert(DFNode);
325 bool inserted = false;
326 SmallPtrSet<DomForestNode*, 4> interferences;
327 for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
329 DomForestNode* child = *CI;
330 LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
332 if (isLiveOut(Info, CInfo.DefInst->getParent())) {
333 // Insert copies for parent
334 for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
335 if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
336 unsigned SrcReg = DFNode->getReg();
337 MachineBasicBlock* From = Inst->getOperand(i).getMBB();
339 Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
340 PHIUnion.erase(SrcReg);
343 } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
344 Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
345 // FIXME: Add (p, c) to possible local interferences
348 if (!visited.count(child)) {
349 worklist.push_back(child);
354 if (!inserted) worklist.pop_back();
358 /// breakCriticalEdges - Break critical edges coming into blocks with PHI
359 /// nodes, preserving dominator and livevariable info.
360 void StrongPHIElimination::breakCriticalEdges(MachineFunction &Fn) {
361 typedef std::pair<MachineBasicBlock*, MachineBasicBlock*> MBB_pair;
363 MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
364 LiveVariables& LV = getAnalysis<LiveVariables>();
366 // Find critical edges
367 std::vector<MBB_pair> criticals;
368 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
370 I->begin()->getOpcode() == TargetInstrInfo::PHI &&
372 for (MachineBasicBlock::pred_iterator PI = I->pred_begin(),
373 PE = I->pred_end(); PI != PE; ++PI)
374 if ((*PI)->succ_size() > 1)
375 criticals.push_back(std::make_pair(*PI, I));
377 for (std::vector<MBB_pair>::iterator I = criticals.begin(),
378 E = criticals.end(); I != E; ++I) {
380 MachineBasicBlock* new_bb = SplitCriticalMachineEdge(I->first, I->second);
383 MDT.splitBlock(I->first);
385 // Update livevariables
386 for (unsigned var = 1024; var < Fn.getSSARegMap()->getLastVirtReg(); ++var)
387 if (isLiveOut(LV.getVarInfo(var), I->first))
388 LV.getVarInfo(var).AliveBlocks.set(new_bb->getNumber());
392 bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
393 breakCriticalEdges(Fn);
396 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
398 I->begin()->getOpcode() == TargetInstrInfo::PHI)