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 instructions are special in that their result value is not available
86 // along their unwind edge. The code below tests to see whether DomBB
88 // the value, so adjust DomBB to the normal destination block, which is
89 // effectively where the value is first usable.
90 BasicBlock *DomBB = Inst.getParent();
91 if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
92 DomBB = Inv->getNormalDest();
94 DomTreeNode *DomNode = DT.getNode(DomBB);
96 SmallVector<PHINode *, 16> AddedPHIs;
97 SmallVector<PHINode *, 8> PostProcessPHIs;
100 SSAUpdate.Initialize(Inst.getType(), Inst.getName());
102 // Insert the LCSSA phi's into all of the exit blocks dominated by the
103 // value, and add them to the Phi's map.
104 for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
105 BBE = ExitBlocks.end();
107 BasicBlock *ExitBB = *BBI;
108 if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
111 // If we already inserted something for this BB, don't reprocess it.
112 if (SSAUpdate.HasValueForBlock(ExitBB))
115 PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
116 Inst.getName() + ".lcssa", ExitBB->begin());
118 // Add inputs from inside the loop for this PHI.
119 for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
120 PN->addIncoming(&Inst, *PI);
122 // If the exit block has a predecessor not within the loop, arrange for
123 // the incoming value use corresponding to that predecessor to be
124 // rewritten in terms of a different LCSSA PHI.
125 if (!L.contains(*PI))
126 UsesToRewrite.push_back(
127 &PN->getOperandUse(PN->getOperandNumForIncomingValue(
128 PN->getNumIncomingValues() - 1)));
131 AddedPHIs.push_back(PN);
133 // Remember that this phi makes the value alive in this block.
134 SSAUpdate.AddAvailableValue(ExitBB, PN);
136 // LoopSimplify might fail to simplify some loops (e.g. when indirect
137 // branches are involved). In such situations, it might happen that an exit
138 // for Loop L1 is the header of a disjoint Loop L2. Thus, when we create
139 // PHIs in such an exit block, we are also inserting PHIs into L2's header.
140 // This could break LCSSA form for L2 because these inserted PHIs can also
141 // have uses outside of L2. Remember all PHIs in such situation as to
142 // revisit than later on. FIXME: Remove this if indirectbr support into
143 // LoopSimplify gets improved.
144 if (auto *OtherLoop = LI->getLoopFor(ExitBB))
145 if (!L.contains(OtherLoop))
146 PostProcessPHIs.push_back(PN);
149 // Rewrite all uses outside the loop in terms of the new PHIs we just
151 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
152 // If this use is in an exit block, rewrite to use the newly inserted PHI.
153 // This is required for correctness because SSAUpdate doesn't handle uses in
154 // the same block. It assumes the PHI we inserted is at the end of the
156 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
157 BasicBlock *UserBB = User->getParent();
158 if (PHINode *PN = dyn_cast<PHINode>(User))
159 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
161 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
162 // Tell the VHs that the uses changed. This updates SCEV's caches.
163 if (UsesToRewrite[i]->get()->hasValueHandle())
164 ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
165 UsesToRewrite[i]->set(UserBB->begin());
169 // Otherwise, do full PHI insertion.
170 SSAUpdate.RewriteUse(*UsesToRewrite[i]);
173 // Post process PHI instructions that were inserted into another disjoint loop
174 // and update their exits properly.
175 for (auto *I : PostProcessPHIs) {
179 BasicBlock *PHIBB = I->getParent();
180 Loop *OtherLoop = LI->getLoopFor(PHIBB);
181 SmallVector<BasicBlock *, 8> EBs;
182 OtherLoop->getExitBlocks(EBs);
186 // Recurse and re-process each PHI instruction. FIXME: we should really
187 // convert this entire thing to a worklist approach where we process a
188 // vector of instructions...
189 processInstruction(*OtherLoop, *I, DT, EBs, PredCache, LI);
192 // Remove PHI nodes that did not have any uses rewritten.
193 for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
194 if (AddedPHIs[i]->use_empty())
195 AddedPHIs[i]->eraseFromParent();
201 /// Return true if the specified block dominates at least
202 /// one of the blocks in the specified list.
204 blockDominatesAnExit(BasicBlock *BB,
206 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
207 DomTreeNode *DomNode = DT.getNode(BB);
208 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
209 if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
215 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
216 ScalarEvolution *SE) {
217 bool Changed = false;
219 // Get the set of exiting blocks.
220 SmallVector<BasicBlock *, 8> ExitBlocks;
221 L.getExitBlocks(ExitBlocks);
223 if (ExitBlocks.empty())
226 PredIteratorCache PredCache;
228 // Look at all the instructions in the loop, checking to see if they have uses
229 // outside the loop. If so, rewrite those uses.
230 for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
232 BasicBlock *BB = *BBI;
234 // For large loops, avoid use-scanning by using dominance information: In
235 // particular, if a block does not dominate any of the loop exits, then none
236 // of the values defined in the block could be used outside the loop.
237 if (!blockDominatesAnExit(BB, DT, ExitBlocks))
240 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
241 // Reject two common cases fast: instructions with no uses (like stores)
242 // and instructions with one use that is in the same block as this.
243 if (I->use_empty() ||
244 (I->hasOneUse() && I->user_back()->getParent() == BB &&
245 !isa<PHINode>(I->user_back())))
248 Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache, LI);
252 // If we modified the code, remove any caches about the loop from SCEV to
253 // avoid dangling entries.
254 // FIXME: This is a big hammer, can we clear the cache more selectively?
258 assert(L.isLCSSAForm(DT));
263 /// Process a loop nest depth first.
264 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
265 ScalarEvolution *SE) {
266 bool Changed = false;
268 // Recurse depth-first through inner loops.
269 for (Loop::iterator I = L.begin(), E = L.end(); I != E; ++I)
270 Changed |= formLCSSARecursively(**I, DT, LI, SE);
272 Changed |= formLCSSA(L, DT, LI, SE);
277 struct LCSSA : public FunctionPass {
278 static char ID; // Pass identification, replacement for typeid
279 LCSSA() : FunctionPass(ID) {
280 initializeLCSSAPass(*PassRegistry::getPassRegistry());
283 // Cached analysis information for the current function.
288 bool runOnFunction(Function &F) override;
290 /// This transformation requires natural loop information & requires that
291 /// loop preheaders be inserted into the CFG. It maintains both of these,
292 /// as well as the CFG. It also requires dominator information.
293 void getAnalysisUsage(AnalysisUsage &AU) const override {
294 AU.setPreservesCFG();
296 AU.addRequired<DominatorTreeWrapperPass>();
297 AU.addRequired<LoopInfo>();
298 AU.addPreservedID(LoopSimplifyID);
299 AU.addPreserved<AliasAnalysis>();
300 AU.addPreserved<ScalarEvolution>();
304 void verifyAnalysis() const override;
309 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
310 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
311 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
312 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
314 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
315 char &llvm::LCSSAID = LCSSA::ID;
318 /// Process all loops in the function, inner-most out.
319 bool LCSSA::runOnFunction(Function &F) {
320 bool Changed = false;
321 LI = &getAnalysis<LoopInfo>();
322 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
323 SE = getAnalysisIfAvailable<ScalarEvolution>();
325 // Simplify each loop nest in the function.
326 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
327 Changed |= formLCSSARecursively(**I, *DT, LI, SE);
332 static void verifyLoop(Loop &L, DominatorTree &DT) {
333 // Recurse depth-first through inner loops.
334 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
335 verifyLoop(**LI, DT);
337 // Check the special guarantees that LCSSA makes.
338 //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
341 void LCSSA::verifyAnalysis() const {
342 // Verify each loop nest in the function, assuming LI still points at that
343 // function's loop info.
344 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
345 verifyLoop(**I, *DT);