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 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/ADT/STLExtras.h"
32 #include "llvm/ADT/Statistic.h"
33 #include "llvm/Analysis/AliasAnalysis.h"
34 #include "llvm/Analysis/LoopPass.h"
35 #include "llvm/Analysis/ScalarEvolution.h"
36 #include "llvm/IR/Constants.h"
37 #include "llvm/IR/Dominators.h"
38 #include "llvm/IR/Function.h"
39 #include "llvm/IR/Instructions.h"
40 #include "llvm/IR/PredIteratorCache.h"
41 #include "llvm/Pass.h"
42 #include "llvm/Transforms/Utils/LoopUtils.h"
43 #include "llvm/Transforms/Utils/SSAUpdater.h"
46 #define DEBUG_TYPE "lcssa"
48 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
50 /// Return true if the specified block is in the list.
51 static bool isExitBlock(BasicBlock *BB,
52 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
53 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
54 if (ExitBlocks[i] == BB)
59 /// Given an instruction in the loop, check to see if it has any uses that are
60 /// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the
62 static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
63 const SmallVectorImpl<BasicBlock *> &ExitBlocks,
64 PredIteratorCache &PredCache, LoopInfo *LI) {
65 SmallVector<Use *, 16> UsesToRewrite;
67 BasicBlock *InstBB = Inst.getParent();
69 for (Use &U : Inst.uses()) {
70 Instruction *User = cast<Instruction>(U.getUser());
71 BasicBlock *UserBB = User->getParent();
72 if (PHINode *PN = dyn_cast<PHINode>(User))
73 UserBB = PN->getIncomingBlock(U);
75 if (InstBB != UserBB && !L.contains(UserBB))
76 UsesToRewrite.push_back(&U);
79 // If there are no uses outside the loop, exit with no change.
80 if (UsesToRewrite.empty())
83 ++NumLCSSA; // We are applying the transformation
85 // Invoke/CatchPad instructions are special in that their result value is not
86 // available along their unwind edge. The code below tests to see whether
87 // DomBB dominates the value, so adjust DomBB to the normal destination block,
88 // which is effectively where the value is first usable.
89 BasicBlock *DomBB = Inst.getParent();
90 if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
91 DomBB = Inv->getNormalDest();
92 if (auto *CPI = dyn_cast<CatchPadInst>(&Inst))
93 DomBB = CPI->getNormalDest();
95 DomTreeNode *DomNode = DT.getNode(DomBB);
97 SmallVector<PHINode *, 16> AddedPHIs;
98 SmallVector<PHINode *, 8> PostProcessPHIs;
100 SSAUpdater SSAUpdate;
101 SSAUpdate.Initialize(Inst.getType(), Inst.getName());
103 // Insert the LCSSA phi's into all of the exit blocks dominated by the
104 // value, and add them to the Phi's map.
105 for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
106 BBE = ExitBlocks.end();
108 BasicBlock *ExitBB = *BBI;
109 if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
112 // If we already inserted something for this BB, don't reprocess it.
113 if (SSAUpdate.HasValueForBlock(ExitBB))
116 PHINode *PN = PHINode::Create(Inst.getType(), PredCache.size(ExitBB),
117 Inst.getName() + ".lcssa", ExitBB->begin());
119 // Add inputs from inside the loop for this PHI.
120 for (BasicBlock *Pred : PredCache.get(ExitBB)) {
121 PN->addIncoming(&Inst, Pred);
123 // If the exit block has a predecessor not within the loop, arrange for
124 // the incoming value use corresponding to that predecessor to be
125 // rewritten in terms of a different LCSSA PHI.
126 if (!L.contains(Pred))
127 UsesToRewrite.push_back(
128 &PN->getOperandUse(PN->getOperandNumForIncomingValue(
129 PN->getNumIncomingValues() - 1)));
132 AddedPHIs.push_back(PN);
134 // Remember that this phi makes the value alive in this block.
135 SSAUpdate.AddAvailableValue(ExitBB, PN);
137 // LoopSimplify might fail to simplify some loops (e.g. when indirect
138 // branches are involved). In such situations, it might happen that an exit
139 // for Loop L1 is the header of a disjoint Loop L2. Thus, when we create
140 // PHIs in such an exit block, we are also inserting PHIs into L2's header.
141 // This could break LCSSA form for L2 because these inserted PHIs can also
142 // have uses outside of L2. Remember all PHIs in such situation as to
143 // revisit than later on. FIXME: Remove this if indirectbr support into
144 // LoopSimplify gets improved.
145 if (auto *OtherLoop = LI->getLoopFor(ExitBB))
146 if (!L.contains(OtherLoop))
147 PostProcessPHIs.push_back(PN);
150 // Rewrite all uses outside the loop in terms of the new PHIs we just
152 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
153 // If this use is in an exit block, rewrite to use the newly inserted PHI.
154 // This is required for correctness because SSAUpdate doesn't handle uses in
155 // the same block. It assumes the PHI we inserted is at the end of the
157 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
158 BasicBlock *UserBB = User->getParent();
159 if (PHINode *PN = dyn_cast<PHINode>(User))
160 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
162 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
163 // Tell the VHs that the uses changed. This updates SCEV's caches.
164 if (UsesToRewrite[i]->get()->hasValueHandle())
165 ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
166 UsesToRewrite[i]->set(UserBB->begin());
170 // Otherwise, do full PHI insertion.
171 SSAUpdate.RewriteUse(*UsesToRewrite[i]);
174 // Post process PHI instructions that were inserted into another disjoint loop
175 // and update their exits properly.
176 for (auto *I : PostProcessPHIs) {
180 BasicBlock *PHIBB = I->getParent();
181 Loop *OtherLoop = LI->getLoopFor(PHIBB);
182 SmallVector<BasicBlock *, 8> EBs;
183 OtherLoop->getExitBlocks(EBs);
187 // Recurse and re-process each PHI instruction. FIXME: we should really
188 // convert this entire thing to a worklist approach where we process a
189 // vector of instructions...
190 processInstruction(*OtherLoop, *I, DT, EBs, PredCache, LI);
193 // Remove PHI nodes that did not have any uses rewritten.
194 for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
195 if (AddedPHIs[i]->use_empty())
196 AddedPHIs[i]->eraseFromParent();
202 /// Return true if the specified block dominates at least
203 /// one of the blocks in the specified list.
205 blockDominatesAnExit(BasicBlock *BB,
207 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
208 DomTreeNode *DomNode = DT.getNode(BB);
209 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
210 if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
216 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
217 ScalarEvolution *SE) {
218 bool Changed = false;
220 // Get the set of exiting blocks.
221 SmallVector<BasicBlock *, 8> ExitBlocks;
222 L.getExitBlocks(ExitBlocks);
224 if (ExitBlocks.empty())
227 PredIteratorCache PredCache;
229 // Look at all the instructions in the loop, checking to see if they have uses
230 // outside the loop. If so, rewrite those uses.
231 for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
233 BasicBlock *BB = *BBI;
235 // For large loops, avoid use-scanning by using dominance information: In
236 // particular, if a block does not dominate any of the loop exits, then none
237 // of the values defined in the block could be used outside the loop.
238 if (!blockDominatesAnExit(BB, DT, ExitBlocks))
241 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
242 // Reject two common cases fast: instructions with no uses (like stores)
243 // and instructions with one use that is in the same block as this.
244 if (I->use_empty() ||
245 (I->hasOneUse() && I->user_back()->getParent() == BB &&
246 !isa<PHINode>(I->user_back())))
249 Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache, LI);
253 // If we modified the code, remove any caches about the loop from SCEV to
254 // avoid dangling entries.
255 // FIXME: This is a big hammer, can we clear the cache more selectively?
259 assert(L.isLCSSAForm(DT));
264 /// Process a loop nest depth first.
265 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
266 ScalarEvolution *SE) {
267 bool Changed = false;
269 // Recurse depth-first through inner loops.
270 for (Loop::iterator I = L.begin(), E = L.end(); I != E; ++I)
271 Changed |= formLCSSARecursively(**I, DT, LI, SE);
273 Changed |= formLCSSA(L, DT, LI, SE);
278 struct LCSSA : public FunctionPass {
279 static char ID; // Pass identification, replacement for typeid
280 LCSSA() : FunctionPass(ID) {
281 initializeLCSSAPass(*PassRegistry::getPassRegistry());
284 // Cached analysis information for the current function.
289 bool runOnFunction(Function &F) override;
291 /// This transformation requires natural loop information & requires that
292 /// loop preheaders be inserted into the CFG. It maintains both of these,
293 /// as well as the CFG. It also requires dominator information.
294 void getAnalysisUsage(AnalysisUsage &AU) const override {
295 AU.setPreservesCFG();
297 AU.addRequired<DominatorTreeWrapperPass>();
298 AU.addRequired<LoopInfoWrapperPass>();
299 AU.addPreservedID(LoopSimplifyID);
300 AU.addPreserved<AliasAnalysis>();
301 AU.addPreserved<ScalarEvolution>();
307 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
308 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
309 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
310 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
312 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
313 char &llvm::LCSSAID = LCSSA::ID;
316 /// Process all loops in the function, inner-most out.
317 bool LCSSA::runOnFunction(Function &F) {
318 bool Changed = false;
319 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
320 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
321 SE = getAnalysisIfAvailable<ScalarEvolution>();
323 // Simplify each loop nest in the function.
324 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
325 Changed |= formLCSSARecursively(**I, *DT, LI, SE);