1//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass transforms loops by placing phi nodes at the end of the loops for
11// all values that are live across the loop boundary.  For example, it turns
12// the left into the right code:
13//
14// for (...)                for (...)
15//   if (c)                   if (c)
16//     X1 = ...                 X1 = ...
17//   else                     else
18//     X2 = ...                 X2 = ...
19//   X3 = phi(X1, X2)         X3 = phi(X1, X2)
20// ... = X3 + 4             X4 = phi(X3)
21//                          ... = X4 + 4
22//
23// This is still valid LLVM; the extra phi nodes are purely redundant, and will
24// be trivially eliminated by InstCombine.  The major benefit of this
25// transformation is that it makes many other loop optimizations, such as
26// LoopUnswitching, simpler.
27//
28//===----------------------------------------------------------------------===//
29
30#define DEBUG_TYPE "lcssa"
31#include "llvm/Transforms/Scalar.h"
32#include "llvm/ADT/STLExtras.h"
33#include "llvm/ADT/Statistic.h"
34#include "llvm/Analysis/Dominators.h"
35#include "llvm/Analysis/AliasAnalysis.h"
36#include "llvm/Analysis/LoopPass.h"
37#include "llvm/Analysis/ScalarEvolution.h"
38#include "llvm/IR/Constants.h"
39#include "llvm/IR/Function.h"
40#include "llvm/IR/Instructions.h"
41#include "llvm/Pass.h"
42#include "llvm/Support/PredIteratorCache.h"
43#include "llvm/Transforms/Utils/SSAUpdater.h"
44using namespace llvm;
45
46STATISTIC(NumLCSSA, "Number of live out of a loop variables");
47
48namespace {
49  struct LCSSA : public LoopPass {
50    static char ID; // Pass identification, replacement for typeid
51    LCSSA() : LoopPass(ID) {
52      initializeLCSSAPass(*PassRegistry::getPassRegistry());
53    }
54
55    // Cached analysis information for the current function.
56    DominatorTree *DT;
57    LoopInfo *LI;
58    ScalarEvolution *SE;
59    PredIteratorCache PredCache;
60    Loop *L;
61
62    virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
63
64    /// This transformation requires natural loop information & requires that
65    /// loop preheaders be inserted into the CFG.  It maintains both of these,
66    /// as well as the CFG.  It also requires dominator information.
67    ///
68    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
69      AU.setPreservesCFG();
70
71      AU.addRequired<DominatorTree>();
72      AU.addRequired<LoopInfo>();
73      AU.addPreservedID(LoopSimplifyID);
74      AU.addPreserved<AliasAnalysis>();
75      AU.addPreserved<ScalarEvolution>();
76    }
77  private:
78    bool ProcessInstruction(Instruction *Inst,
79                            const SmallVectorImpl<BasicBlock*> &ExitBlocks);
80
81    /// verifyAnalysis() - Verify loop nest.
82    virtual void verifyAnalysis() const {
83      // Check the special guarantees that LCSSA makes.
84      assert(L->isLCSSAForm(*DT) && "LCSSA form not preserved!");
85    }
86  };
87}
88
89char LCSSA::ID = 0;
90INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
91INITIALIZE_PASS_DEPENDENCY(DominatorTree)
92INITIALIZE_PASS_DEPENDENCY(LoopInfo)
93INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
94
95Pass *llvm::createLCSSAPass() { return new LCSSA(); }
96char &llvm::LCSSAID = LCSSA::ID;
97
98
99/// BlockDominatesAnExit - Return true if the specified block dominates at least
100/// one of the blocks in the specified list.
101static bool BlockDominatesAnExit(BasicBlock *BB,
102                                 const SmallVectorImpl<BasicBlock*> &ExitBlocks,
103                                 DominatorTree *DT) {
104  DomTreeNode *DomNode = DT->getNode(BB);
105  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
106    if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i])))
107      return true;
108
109  return false;
110}
111
112
113/// runOnFunction - Process all loops in the function, inner-most out.
114bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) {
115  L = TheLoop;
116
117  DT = &getAnalysis<DominatorTree>();
118  LI = &getAnalysis<LoopInfo>();
119  SE = getAnalysisIfAvailable<ScalarEvolution>();
120
121  // Get the set of exiting blocks.
122  SmallVector<BasicBlock*, 8> ExitBlocks;
123  L->getExitBlocks(ExitBlocks);
124
125  if (ExitBlocks.empty())
126    return false;
127
128  // Look at all the instructions in the loop, checking to see if they have uses
129  // outside the loop.  If so, rewrite those uses.
130  bool MadeChange = false;
131
132  for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end();
133       BBI != E; ++BBI) {
134    BasicBlock *BB = *BBI;
135
136    // For large loops, avoid use-scanning by using dominance information:  In
137    // particular, if a block does not dominate any of the loop exits, then none
138    // of the values defined in the block could be used outside the loop.
139    if (!BlockDominatesAnExit(BB, ExitBlocks, DT))
140      continue;
141
142    for (BasicBlock::iterator I = BB->begin(), E = BB->end();
143         I != E; ++I) {
144      // Reject two common cases fast: instructions with no uses (like stores)
145      // and instructions with one use that is in the same block as this.
146      if (I->use_empty() ||
147          (I->hasOneUse() && I->use_back()->getParent() == BB &&
148           !isa<PHINode>(I->use_back())))
149        continue;
150
151      MadeChange |= ProcessInstruction(I, ExitBlocks);
152    }
153  }
154
155  // If we modified the code, remove any caches about the loop from SCEV to
156  // avoid dangling entries.
157  // FIXME: This is a big hammer, can we clear the cache more selectively?
158  if (SE && MadeChange)
159    SE->forgetLoop(L);
160
161  assert(L->isLCSSAForm(*DT));
162  PredCache.clear();
163
164  return MadeChange;
165}
166
167/// isExitBlock - Return true if the specified block is in the list.
168static bool isExitBlock(BasicBlock *BB,
169                        const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
170  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
171    if (ExitBlocks[i] == BB)
172      return true;
173  return false;
174}
175
176/// ProcessInstruction - Given an instruction in the loop, check to see if it
177/// has any uses that are outside the current loop.  If so, insert LCSSA PHI
178/// nodes and rewrite the uses.
179bool LCSSA::ProcessInstruction(Instruction *Inst,
180                               const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
181  SmallVector<Use*, 16> UsesToRewrite;
182
183  BasicBlock *InstBB = Inst->getParent();
184
185  for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
186       UI != E; ++UI) {
187    User *U = *UI;
188    BasicBlock *UserBB = cast<Instruction>(U)->getParent();
189    if (PHINode *PN = dyn_cast<PHINode>(U))
190      UserBB = PN->getIncomingBlock(UI);
191
192    if (InstBB != UserBB && !L->contains(UserBB))
193      UsesToRewrite.push_back(&UI.getUse());
194  }
195
196  // If there are no uses outside the loop, exit with no change.
197  if (UsesToRewrite.empty()) return false;
198
199  ++NumLCSSA; // We are applying the transformation
200
201  // Invoke instructions are special in that their result value is not available
202  // along their unwind edge. The code below tests to see whether DomBB dominates
203  // the value, so adjust DomBB to the normal destination block, which is
204  // effectively where the value is first usable.
205  BasicBlock *DomBB = Inst->getParent();
206  if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
207    DomBB = Inv->getNormalDest();
208
209  DomTreeNode *DomNode = DT->getNode(DomBB);
210
211  SmallVector<PHINode*, 16> AddedPHIs;
212
213  SSAUpdater SSAUpdate;
214  SSAUpdate.Initialize(Inst->getType(), Inst->getName());
215
216  // Insert the LCSSA phi's into all of the exit blocks dominated by the
217  // value, and add them to the Phi's map.
218  for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
219      BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
220    BasicBlock *ExitBB = *BBI;
221    if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue;
222
223    // If we already inserted something for this BB, don't reprocess it.
224    if (SSAUpdate.HasValueForBlock(ExitBB)) continue;
225
226    PHINode *PN = PHINode::Create(Inst->getType(),
227                                  PredCache.GetNumPreds(ExitBB),
228                                  Inst->getName()+".lcssa",
229                                  ExitBB->begin());
230
231    // Add inputs from inside the loop for this PHI.
232    for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
233      PN->addIncoming(Inst, *PI);
234
235      // If the exit block has a predecessor not within the loop, arrange for
236      // the incoming value use corresponding to that predecessor to be
237      // rewritten in terms of a different LCSSA PHI.
238      if (!L->contains(*PI))
239        UsesToRewrite.push_back(
240          &PN->getOperandUse(
241            PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1)));
242    }
243
244    AddedPHIs.push_back(PN);
245
246    // Remember that this phi makes the value alive in this block.
247    SSAUpdate.AddAvailableValue(ExitBB, PN);
248  }
249
250  // Rewrite all uses outside the loop in terms of the new PHIs we just
251  // inserted.
252  for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
253    // If this use is in an exit block, rewrite to use the newly inserted PHI.
254    // This is required for correctness because SSAUpdate doesn't handle uses in
255    // the same block.  It assumes the PHI we inserted is at the end of the
256    // block.
257    Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
258    BasicBlock *UserBB = User->getParent();
259    if (PHINode *PN = dyn_cast<PHINode>(User))
260      UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
261
262    if (isa<PHINode>(UserBB->begin()) &&
263        isExitBlock(UserBB, ExitBlocks)) {
264      // Tell the VHs that the uses changed. This updates SCEV's caches.
265      if (UsesToRewrite[i]->get()->hasValueHandle())
266        ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
267      UsesToRewrite[i]->set(UserBB->begin());
268      continue;
269    }
270
271    // Otherwise, do full PHI insertion.
272    SSAUpdate.RewriteUse(*UsesToRewrite[i]);
273  }
274
275  // Remove PHI nodes that did not have any uses rewritten.
276  for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
277    if (AddedPHIs[i]->use_empty())
278      AddedPHIs[i]->eraseFromParent();
279  }
280
281  return true;
282}
283
284