1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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:
14 // for (...) for (...)
19 // X3 = phi(X1, X2) X3 = phi(X1, X2)
20 // ... = X3 + 4 X4 = phi(X3)
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.
28 //===----------------------------------------------------------------------===//
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/AliasAnalysis.h"
35 #include "llvm/Analysis/LoopPass.h"
36 #include "llvm/Analysis/ScalarEvolution.h"
37 #include "llvm/IR/Constants.h"
38 #include "llvm/IR/Dominators.h"
39 #include "llvm/IR/Function.h"
40 #include "llvm/IR/Instructions.h"
41 #include "llvm/IR/PredIteratorCache.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Transforms/Utils/LoopUtils.h"
44 #include "llvm/Transforms/Utils/SSAUpdater.h"
47 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
49 /// Return true if the specified block is in the list.
50 static bool isExitBlock(BasicBlock *BB,
51 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
52 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
53 if (ExitBlocks[i] == BB)
58 /// Given an instruction in the loop, check to see if it has any uses that are
59 /// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the
61 static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
62 const SmallVectorImpl<BasicBlock *> &ExitBlocks,
63 PredIteratorCache &PredCache) {
64 SmallVector<Use *, 16> UsesToRewrite;
66 BasicBlock *InstBB = Inst.getParent();
68 for (Use &U : Inst.uses()) {
69 Instruction *User = cast<Instruction>(U.getUser());
70 BasicBlock *UserBB = User->getParent();
71 if (PHINode *PN = dyn_cast<PHINode>(User))
72 UserBB = PN->getIncomingBlock(U);
74 if (InstBB != UserBB && !L.contains(UserBB))
75 UsesToRewrite.push_back(&U);
78 // If there are no uses outside the loop, exit with no change.
79 if (UsesToRewrite.empty())
82 ++NumLCSSA; // We are applying the transformation
84 // Invoke instructions are special in that their result value is not available
85 // along their unwind edge. The code below tests to see whether DomBB
87 // the value, so adjust DomBB to the normal destination block, which is
88 // effectively where the value is first usable.
89 BasicBlock *DomBB = Inst.getParent();
90 if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
91 DomBB = Inv->getNormalDest();
93 DomTreeNode *DomNode = DT.getNode(DomBB);
95 SmallVector<PHINode *, 16> AddedPHIs;
98 SSAUpdate.Initialize(Inst.getType(), Inst.getName());
100 // Insert the LCSSA phi's into all of the exit blocks dominated by the
101 // value, and add them to the Phi's map.
102 for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
103 BBE = ExitBlocks.end();
105 BasicBlock *ExitBB = *BBI;
106 if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
109 // If we already inserted something for this BB, don't reprocess it.
110 if (SSAUpdate.HasValueForBlock(ExitBB))
113 PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
114 Inst.getName() + ".lcssa", ExitBB->begin());
116 // Add inputs from inside the loop for this PHI.
117 for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
118 PN->addIncoming(&Inst, *PI);
120 // If the exit block has a predecessor not within the loop, arrange for
121 // the incoming value use corresponding to that predecessor to be
122 // rewritten in terms of a different LCSSA PHI.
123 if (!L.contains(*PI))
124 UsesToRewrite.push_back(
125 &PN->getOperandUse(PN->getOperandNumForIncomingValue(
126 PN->getNumIncomingValues() - 1)));
129 AddedPHIs.push_back(PN);
131 // Remember that this phi makes the value alive in this block.
132 SSAUpdate.AddAvailableValue(ExitBB, PN);
135 // Rewrite all uses outside the loop in terms of the new PHIs we just
137 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
138 // If this use is in an exit block, rewrite to use the newly inserted PHI.
139 // This is required for correctness because SSAUpdate doesn't handle uses in
140 // the same block. It assumes the PHI we inserted is at the end of the
142 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
143 BasicBlock *UserBB = User->getParent();
144 if (PHINode *PN = dyn_cast<PHINode>(User))
145 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
147 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
148 // Tell the VHs that the uses changed. This updates SCEV's caches.
149 if (UsesToRewrite[i]->get()->hasValueHandle())
150 ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
151 UsesToRewrite[i]->set(UserBB->begin());
155 // Otherwise, do full PHI insertion.
156 SSAUpdate.RewriteUse(*UsesToRewrite[i]);
159 // Remove PHI nodes that did not have any uses rewritten.
160 for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
161 if (AddedPHIs[i]->use_empty())
162 AddedPHIs[i]->eraseFromParent();
168 /// Return true if the specified block dominates at least
169 /// one of the blocks in the specified list.
171 blockDominatesAnExit(BasicBlock *BB,
173 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
174 DomTreeNode *DomNode = DT.getNode(BB);
175 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
176 if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
182 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
183 bool Changed = false;
185 // Get the set of exiting blocks.
186 SmallVector<BasicBlock *, 8> ExitBlocks;
187 L.getExitBlocks(ExitBlocks);
189 if (ExitBlocks.empty())
192 PredIteratorCache PredCache;
194 // Look at all the instructions in the loop, checking to see if they have uses
195 // outside the loop. If so, rewrite those uses.
196 for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
198 BasicBlock *BB = *BBI;
200 // For large loops, avoid use-scanning by using dominance information: In
201 // particular, if a block does not dominate any of the loop exits, then none
202 // of the values defined in the block could be used outside the loop.
203 if (!blockDominatesAnExit(BB, DT, ExitBlocks))
206 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
207 // Reject two common cases fast: instructions with no uses (like stores)
208 // and instructions with one use that is in the same block as this.
209 if (I->use_empty() ||
210 (I->hasOneUse() && I->user_back()->getParent() == BB &&
211 !isa<PHINode>(I->user_back())))
214 Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache);
218 // If we modified the code, remove any caches about the loop from SCEV to
219 // avoid dangling entries.
220 // FIXME: This is a big hammer, can we clear the cache more selectively?
224 assert(L.isLCSSAForm(DT));
229 /// Process a loop nest depth first.
230 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
231 ScalarEvolution *SE) {
232 bool Changed = false;
234 // Recurse depth-first through inner loops.
235 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
236 Changed |= formLCSSARecursively(**LI, DT, SE);
238 Changed |= formLCSSA(L, DT, SE);
243 struct LCSSA : public FunctionPass {
244 static char ID; // Pass identification, replacement for typeid
245 LCSSA() : FunctionPass(ID) {
246 initializeLCSSAPass(*PassRegistry::getPassRegistry());
249 // Cached analysis information for the current function.
254 bool runOnFunction(Function &F) override;
256 /// This transformation requires natural loop information & requires that
257 /// loop preheaders be inserted into the CFG. It maintains both of these,
258 /// as well as the CFG. It also requires dominator information.
259 void getAnalysisUsage(AnalysisUsage &AU) const override {
260 AU.setPreservesCFG();
262 AU.addRequired<DominatorTreeWrapperPass>();
263 AU.addRequired<LoopInfo>();
264 AU.addPreservedID(LoopSimplifyID);
265 AU.addPreserved<AliasAnalysis>();
266 AU.addPreserved<ScalarEvolution>();
270 bool processLoop(Loop &L);
272 void verifyAnalysis() const override;
277 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
278 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
279 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
280 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
282 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
283 char &llvm::LCSSAID = LCSSA::ID;
286 /// Process all loops in the function, inner-most out.
287 bool LCSSA::runOnFunction(Function &F) {
288 bool Changed = false;
289 LI = &getAnalysis<LoopInfo>();
290 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
291 SE = getAnalysisIfAvailable<ScalarEvolution>();
293 // Simplify each loop nest in the function.
294 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
295 Changed |= formLCSSARecursively(**I, *DT, SE);
300 static void verifyLoop(Loop &L, DominatorTree &DT) {
301 // Recurse depth-first through inner loops.
302 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
303 verifyLoop(**LI, DT);
305 // Check the special guarantees that LCSSA makes.
306 //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
309 void LCSSA::verifyAnalysis() const {
310 // Verify each loop nest in the function, assuming LI still points at that
311 // function's loop info.
312 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
313 verifyLoop(**I, *DT);