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/LiveVariables.h"
25 #include "llvm/CodeGen/MachineDominators.h"
26 #include "llvm/CodeGen/MachineFunctionPass.h"
27 #include "llvm/CodeGen/MachineInstr.h"
28 #include "llvm/CodeGen/SSARegMap.h"
29 #include "llvm/Target/TargetInstrInfo.h"
30 #include "llvm/Target/TargetMachine.h"
31 #include "llvm/ADT/Statistic.h"
32 #include "llvm/Support/Compiler.h"
37 struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
38 static char ID; // Pass identification, replacement for typeid
39 StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
41 DenseMap<MachineBasicBlock*,
42 SmallVector<std::pair<unsigned, unsigned>, 2> > Waiting;
44 bool runOnMachineFunction(MachineFunction &Fn);
46 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 AU.addPreserved<LiveVariables>();
48 AU.addPreservedID(PHIEliminationID);
49 AU.addRequired<MachineDominatorTree>();
50 AU.addRequired<LiveVariables>();
52 MachineFunctionPass::getAnalysisUsage(AU);
55 virtual void releaseMemory() {
63 struct DomForestNode {
65 std::vector<DomForestNode*> children;
68 void addChild(DomForestNode* DFN) { children.push_back(DFN); }
71 typedef std::vector<DomForestNode*>::iterator iterator;
73 DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
75 parent->addChild(this);
79 for (iterator I = begin(), E = end(); I != E; ++I)
83 inline unsigned getReg() { return reg; }
85 inline DomForestNode::iterator begin() { return children.begin(); }
86 inline DomForestNode::iterator end() { return children.end(); }
89 DenseMap<MachineBasicBlock*, unsigned> preorder;
90 DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
92 DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > waiting;
95 void computeDFS(MachineFunction& MF);
96 void processBlock(MachineBasicBlock* MBB);
98 std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
99 void processPHIUnion(MachineInstr* Inst,
100 std::set<unsigned>& PHIUnion,
101 std::vector<StrongPHIElimination::DomForestNode*>& DF,
102 std::vector<std::pair<unsigned, unsigned> >& locals);
106 char StrongPHIElimination::ID = 0;
107 RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
108 "Eliminate PHI nodes for register allocation, intelligently");
111 const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
113 /// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
114 /// of the given MachineFunction. These numbers are then used in other parts
115 /// of the PHI elimination process.
116 void StrongPHIElimination::computeDFS(MachineFunction& MF) {
117 SmallPtrSet<MachineDomTreeNode*, 8> frontier;
118 SmallPtrSet<MachineDomTreeNode*, 8> visited;
122 MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
124 MachineDomTreeNode* node = DT.getRootNode();
126 std::vector<MachineDomTreeNode*> worklist;
127 worklist.push_back(node);
129 while (!worklist.empty()) {
130 MachineDomTreeNode* currNode = worklist.back();
132 if (!frontier.count(currNode)) {
133 frontier.insert(currNode);
135 preorder.insert(std::make_pair(currNode->getBlock(), time));
138 bool inserted = false;
139 for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
141 if (!frontier.count(*I) && !visited.count(*I)) {
142 worklist.push_back(*I);
148 frontier.erase(currNode);
149 visited.insert(currNode);
150 maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
157 /// PreorderSorter - a helper class that is used to sort registers
158 /// according to the preorder number of their defining blocks
159 class PreorderSorter {
161 DenseMap<MachineBasicBlock*, unsigned>& preorder;
165 PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
166 LiveVariables& L) : preorder(p), LV(L) { }
168 bool operator()(unsigned A, unsigned B) {
172 MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
173 MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
175 if (preorder[ABlock] < preorder[BBlock])
177 else if (preorder[ABlock] > preorder[BBlock])
180 assert(0 && "Error sorting by dominance!");
185 /// computeDomForest - compute the subforest of the DomTree corresponding
186 /// to the defining blocks of the registers in question
187 std::vector<StrongPHIElimination::DomForestNode*>
188 StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
189 LiveVariables& LV = getAnalysis<LiveVariables>();
191 DomForestNode* VirtualRoot = new DomForestNode(0, 0);
192 maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
194 std::vector<unsigned> worklist;
195 worklist.reserve(regs.size());
196 for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
198 worklist.push_back(*I);
200 PreorderSorter PS(preorder, LV);
201 std::sort(worklist.begin(), worklist.end(), PS);
203 DomForestNode* CurrentParent = VirtualRoot;
204 std::vector<DomForestNode*> stack;
205 stack.push_back(VirtualRoot);
207 for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
209 unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
210 MachineBasicBlock* parentBlock =
211 LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
213 while (pre > maxpreorder[parentBlock]) {
215 CurrentParent = stack.back();
217 parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
220 DomForestNode* child = new DomForestNode(*I, CurrentParent);
221 stack.push_back(child);
222 CurrentParent = child;
225 std::vector<DomForestNode*> ret;
226 ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
230 /// isLiveIn - helper method that determines, from a VarInfo, if a register
231 /// is live into a block
232 bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
233 if (V.AliveBlocks.test(MBB->getNumber()))
236 if (V.DefInst->getParent() != MBB &&
237 V.UsedBlocks.test(MBB->getNumber()))
243 /// isLiveOut - help method that determines, from a VarInfo, if a register is
244 /// live out of a block.
245 bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
246 if (MBB == V.DefInst->getParent() ||
247 V.UsedBlocks.test(MBB->getNumber())) {
248 for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
249 E = V.Kills.end(); I != E; ++I)
250 if ((*I)->getParent() == MBB)
259 /// isKillInst - helper method that determines, from a VarInfo, if an
260 /// instruction kills a given register
261 bool isKillInst(LiveVariables::VarInfo& V, MachineInstr* MI) {
262 return std::find(V.Kills.begin(), V.Kills.end(), MI) != V.Kills.end();
265 /// interferes - checks for local interferences by scanning a block. The only
266 /// trick parameter is 'mode' which tells it the relationship of the two
267 /// registers. 0 - defined in the same block, 1 - first properly dominates
268 /// second, 2 - second properly dominates first
269 bool interferes(LiveVariables::VarInfo& First, LiveVariables::VarInfo& Second,
270 MachineBasicBlock* scan, unsigned mode) {
271 MachineInstr* def = 0;
272 MachineInstr* kill = 0;
274 bool interference = false;
276 // Wallk the block, checking for interferences
277 for (MachineBasicBlock::iterator MBI = scan->begin(), MBE = scan->end();
279 MachineInstr* curr = MBI;
281 // Same defining block...
283 if (curr == First.DefInst) {
284 // If we find our first DefInst, save it
287 // If there's already an unkilled DefInst, then
288 // this is an interference
292 // If there's a DefInst followed by a KillInst, then
293 // they can't interfere
295 interference = false;
298 // Symmetric with the above
299 } else if (curr == Second.DefInst ) {
306 interference = false;
309 // Store KillInsts if they match up with the DefInst
310 } else if (isKillInst(First, curr)) {
311 if (def == First.DefInst) {
313 } else if (isKillInst(Second, curr)) {
314 if (def == Second.DefInst) {
319 // First properly dominates second...
320 } else if (mode == 1) {
321 if (curr == Second.DefInst) {
322 // DefInst of second without kill of first is an interference
326 // DefInst after a kill is a non-interference
328 interference = false;
331 // Save KillInsts of First
332 } else if (isKillInst(First, curr)) {
335 // Symmetric with the above
336 } else if (mode == 2) {
337 if (curr == First.DefInst) {
342 interference = false;
345 } else if (isKillInst(Second, curr)) {
354 /// processBlock - Eliminate PHIs in the given block
355 void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
356 LiveVariables& LV = getAnalysis<LiveVariables>();
358 // Holds names that have been added to a set in any PHI within this block
359 // before the current one.
360 std::set<unsigned> ProcessedNames;
362 MachineBasicBlock::iterator P = MBB->begin();
363 while (P->getOpcode() == TargetInstrInfo::PHI) {
364 LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());
366 unsigned DestReg = P->getOperand(0).getReg();
368 // Hold the names that are currently in the candidate set.
369 std::set<unsigned> PHIUnion;
370 std::set<MachineBasicBlock*> UnionedBlocks;
372 for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
373 unsigned SrcReg = P->getOperand(i-1).getReg();
374 LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
376 // Check for trivial interferences
377 if (isLiveIn(SrcInfo, P->getParent()) ||
378 isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
379 ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
380 isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
381 ProcessedNames.count(SrcReg) ||
382 UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
384 // add a copy from a_i to p in Waiting[From[a_i]]
385 MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
386 Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
388 PHIUnion.insert(SrcReg);
389 UnionedBlocks.insert(SrcInfo.DefInst->getParent());
393 std::vector<StrongPHIElimination::DomForestNode*> DF =
394 computeDomForest(PHIUnion);
396 // Walk DomForest to resolve interferences
397 std::vector<std::pair<unsigned, unsigned> > localInterferences;
398 processPHIUnion(P, PHIUnion, DF, localInterferences);
400 // Check for local interferences
401 for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
402 localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
403 std::pair<unsigned, unsigned> p = *I;
405 LiveVariables::VarInfo& FirstInfo = LV.getVarInfo(p.first);
406 LiveVariables::VarInfo& SecondInfo = LV.getVarInfo(p.second);
408 MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
410 // Determine the block we need to scan and the relationship between
412 MachineBasicBlock* scan = 0;
414 if (FirstInfo.DefInst->getParent() == SecondInfo.DefInst->getParent()) {
415 scan = FirstInfo.DefInst->getParent();
416 mode = 0; // Same block
417 } else if (MDT.dominates(FirstInfo.DefInst->getParent(),
418 SecondInfo.DefInst->getParent())) {
419 scan = SecondInfo.DefInst->getParent();
420 mode = 1; // First dominates second
422 scan = FirstInfo.DefInst->getParent();
423 mode = 2; // Second dominates first
426 // If there's an interference, we need to insert copies
427 if (interferes(FirstInfo, SecondInfo, scan, mode)) {
428 // Insert copies for First
429 for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
430 if (P->getOperand(i-1).getReg() == p.first) {
431 unsigned SrcReg = p.first;
432 MachineBasicBlock* From = P->getOperand(i).getMBB();
434 Waiting[From].push_back(std::make_pair(SrcReg,
435 P->getOperand(0).getReg()));
436 PHIUnion.erase(SrcReg);
442 // FIXME: Cache renaming information
444 ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
449 /// processPHIUnion - Take a set of candidate registers to be coallesced when
450 /// decomposing the PHI instruction. Use the DominanceForest to remove the ones
451 /// that are known to interfere, and flag others that need to be checked for
452 /// local interferences.
453 void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
454 std::set<unsigned>& PHIUnion,
455 std::vector<StrongPHIElimination::DomForestNode*>& DF,
456 std::vector<std::pair<unsigned, unsigned> >& locals) {
458 std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
459 SmallPtrSet<DomForestNode*, 4> visited;
461 LiveVariables& LV = getAnalysis<LiveVariables>();
462 unsigned DestReg = Inst->getOperand(0).getReg();
464 // DF walk on the DomForest
465 while (!worklist.empty()) {
466 DomForestNode* DFNode = worklist.back();
468 LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
469 visited.insert(DFNode);
471 bool inserted = false;
472 for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
474 DomForestNode* child = *CI;
475 LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
477 if (isLiveOut(Info, CInfo.DefInst->getParent())) {
478 // Insert copies for parent
479 for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
480 if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
481 unsigned SrcReg = DFNode->getReg();
482 MachineBasicBlock* From = Inst->getOperand(i).getMBB();
484 Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
485 PHIUnion.erase(SrcReg);
488 } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
489 Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
490 // Add (p, c) to possible local interferences
491 locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
494 if (!visited.count(child)) {
495 worklist.push_back(child);
500 if (!inserted) worklist.pop_back();
504 bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
507 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
509 I->begin()->getOpcode() == TargetInstrInfo::PHI)
512 // FIXME: Insert copies
513 // FIXME: Perform renaming