aboutsummaryrefslogtreecommitdiffstats
path: root/lib/Transforms/Utils/LCSSA.cpp
blob: 51a3d9c1fcedda5b664571e5f4e6d3469e85d79d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass transforms loops by placing phi nodes at the end of the loops for
// all values that are live across the loop boundary.  For example, it turns
// the left into the right code:
// 
// for (...)                for (...)
//   if (c)                   if (c)
//     X1 = ...                 X1 = ...
//   else                     else
//     X2 = ...                 X2 = ...
//   X3 = phi(X1, X2)         X3 = phi(X1, X2)
// ... = X3 + 4             X4 = phi(X3)
//                          ... = X4 + 4
//
// This is still valid LLVM; the extra phi nodes are purely redundant, and will
// be trivially eliminated by InstCombine.  The major benefit of this 
// transformation is that it makes many other loop optimizations, such as 
// LoopUnswitching, simpler.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;

#define DEBUG_TYPE "lcssa"

STATISTIC(NumLCSSA, "Number of live out of a loop variables");

/// Return true if the specified block is in the list.
static bool isExitBlock(BasicBlock *BB,
                        const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
    if (ExitBlocks[i] == BB)
      return true;
  return false;
}

/// Given an instruction in the loop, check to see if it has any uses that are
/// outside the current loop.  If so, insert LCSSA PHI nodes and rewrite the
/// uses.
static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
                               const SmallVectorImpl<BasicBlock *> &ExitBlocks,
                               PredIteratorCache &PredCache) {
  SmallVector<Use *, 16> UsesToRewrite;

  BasicBlock *InstBB = Inst.getParent();

  for (Use &U : Inst.uses()) {
    Instruction *User = cast<Instruction>(U.getUser());
    BasicBlock *UserBB = User->getParent();
    if (PHINode *PN = dyn_cast<PHINode>(User))
      UserBB = PN->getIncomingBlock(U);

    if (InstBB != UserBB && !L.contains(UserBB))
      UsesToRewrite.push_back(&U);
  }

  // If there are no uses outside the loop, exit with no change.
  if (UsesToRewrite.empty())
    return false;

  ++NumLCSSA; // We are applying the transformation

  // Invoke instructions are special in that their result value is not available
  // along their unwind edge. The code below tests to see whether DomBB
  // dominates
  // the value, so adjust DomBB to the normal destination block, which is
  // effectively where the value is first usable.
  BasicBlock *DomBB = Inst.getParent();
  if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
    DomBB = Inv->getNormalDest();

  DomTreeNode *DomNode = DT.getNode(DomBB);

  SmallVector<PHINode *, 16> AddedPHIs;

  SSAUpdater SSAUpdate;
  SSAUpdate.Initialize(Inst.getType(), Inst.getName());

  // Insert the LCSSA phi's into all of the exit blocks dominated by the
  // value, and add them to the Phi's map.
  for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
                                                     BBE = ExitBlocks.end();
       BBI != BBE; ++BBI) {
    BasicBlock *ExitBB = *BBI;
    if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
      continue;

    // If we already inserted something for this BB, don't reprocess it.
    if (SSAUpdate.HasValueForBlock(ExitBB))
      continue;

    PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
                                  Inst.getName() + ".lcssa", ExitBB->begin());

    // Add inputs from inside the loop for this PHI.
    for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
      PN->addIncoming(&Inst, *PI);

      // If the exit block has a predecessor not within the loop, arrange for
      // the incoming value use corresponding to that predecessor to be
      // rewritten in terms of a different LCSSA PHI.
      if (!L.contains(*PI))
        UsesToRewrite.push_back(
            &PN->getOperandUse(PN->getOperandNumForIncomingValue(
                 PN->getNumIncomingValues() - 1)));
    }

    AddedPHIs.push_back(PN);

    // Remember that this phi makes the value alive in this block.
    SSAUpdate.AddAvailableValue(ExitBB, PN);
  }

  // Rewrite all uses outside the loop in terms of the new PHIs we just
  // inserted.
  for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
    // If this use is in an exit block, rewrite to use the newly inserted PHI.
    // This is required for correctness because SSAUpdate doesn't handle uses in
    // the same block.  It assumes the PHI we inserted is at the end of the
    // block.
    Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
    BasicBlock *UserBB = User->getParent();
    if (PHINode *PN = dyn_cast<PHINode>(User))
      UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);

    if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
      // Tell the VHs that the uses changed. This updates SCEV's caches.
      if (UsesToRewrite[i]->get()->hasValueHandle())
        ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
      UsesToRewrite[i]->set(UserBB->begin());
      continue;
    }

    // Otherwise, do full PHI insertion.
    SSAUpdate.RewriteUse(*UsesToRewrite[i]);
  }

  // Remove PHI nodes that did not have any uses rewritten.
  for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
    if (AddedPHIs[i]->use_empty())
      AddedPHIs[i]->eraseFromParent();
  }

  return true;
}

/// Return true if the specified block dominates at least
/// one of the blocks in the specified list.
static bool
blockDominatesAnExit(BasicBlock *BB,
                     DominatorTree &DT,
                     const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
  DomTreeNode *DomNode = DT.getNode(BB);
  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
    if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
      return true;

  return false;
}

bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
  bool Changed = false;

  // Get the set of exiting blocks.
  SmallVector<BasicBlock *, 8> ExitBlocks;
  L.getExitBlocks(ExitBlocks);

  if (ExitBlocks.empty())
    return false;

  PredIteratorCache PredCache;

  // Look at all the instructions in the loop, checking to see if they have uses
  // outside the loop.  If so, rewrite those uses.
  for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
       BBI != BBE; ++BBI) {
    BasicBlock *BB = *BBI;

    // For large loops, avoid use-scanning by using dominance information:  In
    // particular, if a block does not dominate any of the loop exits, then none
    // of the values defined in the block could be used outside the loop.
    if (!blockDominatesAnExit(BB, DT, ExitBlocks))
      continue;

    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      // Reject two common cases fast: instructions with no uses (like stores)
      // and instructions with one use that is in the same block as this.
      if (I->use_empty() ||
          (I->hasOneUse() && I->user_back()->getParent() == BB &&
           !isa<PHINode>(I->user_back())))
        continue;

      Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache);
    }
  }

  // If we modified the code, remove any caches about the loop from SCEV to
  // avoid dangling entries.
  // FIXME: This is a big hammer, can we clear the cache more selectively?
  if (SE && Changed)
    SE->forgetLoop(&L);

  assert(L.isLCSSAForm(DT));

  return Changed;
}

/// Process a loop nest depth first.
bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
                                ScalarEvolution *SE) {
  bool Changed = false;

  // Recurse depth-first through inner loops.
  for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
    Changed |= formLCSSARecursively(**LI, DT, SE);

  Changed |= formLCSSA(L, DT, SE);
  return Changed;
}

namespace {
struct LCSSA : public FunctionPass {
  static char ID; // Pass identification, replacement for typeid
  LCSSA() : FunctionPass(ID) {
    initializeLCSSAPass(*PassRegistry::getPassRegistry());
  }

  // Cached analysis information for the current function.
  DominatorTree *DT;
  LoopInfo *LI;
  ScalarEvolution *SE;

  bool runOnFunction(Function &F) override;

  /// This transformation requires natural loop information & requires that
  /// loop preheaders be inserted into the CFG.  It maintains both of these,
  /// as well as the CFG.  It also requires dominator information.
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();

    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addRequired<LoopInfo>();
    AU.addPreservedID(LoopSimplifyID);
    AU.addPreserved<AliasAnalysis>();
    AU.addPreserved<ScalarEvolution>();
  }

private:
  void verifyAnalysis() const override;
};
}

char LCSSA::ID = 0;
INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)

Pass *llvm::createLCSSAPass() { return new LCSSA(); }
char &llvm::LCSSAID = LCSSA::ID;


/// Process all loops in the function, inner-most out.
bool LCSSA::runOnFunction(Function &F) {
  bool Changed = false;
  LI = &getAnalysis<LoopInfo>();
  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  SE = getAnalysisIfAvailable<ScalarEvolution>();

  // Simplify each loop nest in the function.
  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
    Changed |= formLCSSARecursively(**I, *DT, SE);

  return Changed;
}

static void verifyLoop(Loop &L, DominatorTree &DT) {
  // Recurse depth-first through inner loops.
  for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
    verifyLoop(**LI, DT);

  // Check the special guarantees that LCSSA makes.
  //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
}

void LCSSA::verifyAnalysis() const {
  // Verify each loop nest in the function, assuming LI still points at that
  // function's loop info.
  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
    verifyLoop(**I, *DT);
}