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/Utils/LCSSA.h"
31 #include "llvm/ADT/STLExtras.h"
32 #include "llvm/ADT/Statistic.h"
33 #include "llvm/Analysis/AliasAnalysis.h"
34 #include "llvm/Analysis/BasicAliasAnalysis.h"
35 #include "llvm/Analysis/GlobalsModRef.h"
36 #include "llvm/Analysis/LoopPass.h"
37 #include "llvm/Analysis/ScalarEvolution.h"
38 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
39 #include "llvm/IR/Constants.h"
40 #include "llvm/IR/Dominators.h"
41 #include "llvm/IR/Function.h"
42 #include "llvm/IR/Instructions.h"
43 #include "llvm/IR/PredIteratorCache.h"
44 #include "llvm/Pass.h"
45 #include "llvm/Transforms/Scalar.h"
46 #include "llvm/Transforms/Utils/LoopUtils.h"
47 #include "llvm/Transforms/Utils/SSAUpdater.h"
50 #define DEBUG_TYPE "lcssa"
52 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
54 #ifdef EXPENSIVE_CHECKS
55 static bool VerifyLoopLCSSA = true;
57 static bool VerifyLoopLCSSA = false;
59 static cl::opt<bool, true>
60 VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),
62 cl::desc("Verify loop lcssa form (time consuming)"));
64 /// Return true if the specified block is in the list.
65 static bool isExitBlock(BasicBlock *BB,
66 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
67 return is_contained(ExitBlocks, BB);
70 /// For every instruction from the worklist, check to see if it has any uses
71 /// that are outside the current loop. If so, insert LCSSA PHI nodes and
73 bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
74 DominatorTree &DT, LoopInfo &LI) {
75 SmallVector<Use *, 16> UsesToRewrite;
76 SmallSetVector<PHINode *, 16> PHIsToRemove;
77 PredIteratorCache PredCache;
80 // Cache the Loop ExitBlocks across this loop. We expect to get a lot of
81 // instructions within the same loops, computing the exit blocks is
82 // expensive, and we're not mutating the loop structure.
83 SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
85 while (!Worklist.empty()) {
86 UsesToRewrite.clear();
88 Instruction *I = Worklist.pop_back_val();
89 assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");
90 BasicBlock *InstBB = I->getParent();
91 Loop *L = LI.getLoopFor(InstBB);
92 assert(L && "Instruction belongs to a BB that's not part of a loop");
93 if (!LoopExitBlocks.count(L))
94 L->getExitBlocks(LoopExitBlocks[L]);
95 assert(LoopExitBlocks.count(L));
96 const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
98 if (ExitBlocks.empty())
101 for (Use &U : I->uses()) {
102 Instruction *User = cast<Instruction>(U.getUser());
103 BasicBlock *UserBB = User->getParent();
104 if (auto *PN = dyn_cast<PHINode>(User))
105 UserBB = PN->getIncomingBlock(U);
107 if (InstBB != UserBB && !L->contains(UserBB))
108 UsesToRewrite.push_back(&U);
111 // If there are no uses outside the loop, exit with no change.
112 if (UsesToRewrite.empty())
115 ++NumLCSSA; // We are applying the transformation
117 // Invoke instructions are special in that their result value is not
118 // available along their unwind edge. The code below tests to see whether
119 // DomBB dominates the value, so adjust DomBB to the normal destination
120 // block, which is effectively where the value is first usable.
121 BasicBlock *DomBB = InstBB;
122 if (auto *Inv = dyn_cast<InvokeInst>(I))
123 DomBB = Inv->getNormalDest();
125 DomTreeNode *DomNode = DT.getNode(DomBB);
127 SmallVector<PHINode *, 16> AddedPHIs;
128 SmallVector<PHINode *, 8> PostProcessPHIs;
130 SmallVector<PHINode *, 4> InsertedPHIs;
131 SSAUpdater SSAUpdate(&InsertedPHIs);
132 SSAUpdate.Initialize(I->getType(), I->getName());
134 // Insert the LCSSA phi's into all of the exit blocks dominated by the
135 // value, and add them to the Phi's map.
136 for (BasicBlock *ExitBB : ExitBlocks) {
137 if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
140 // If we already inserted something for this BB, don't reprocess it.
141 if (SSAUpdate.HasValueForBlock(ExitBB))
144 PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),
145 I->getName() + ".lcssa", &ExitBB->front());
147 // Add inputs from inside the loop for this PHI.
148 for (BasicBlock *Pred : PredCache.get(ExitBB)) {
149 PN->addIncoming(I, Pred);
151 // If the exit block has a predecessor not within the loop, arrange for
152 // the incoming value use corresponding to that predecessor to be
153 // rewritten in terms of a different LCSSA PHI.
154 if (!L->contains(Pred))
155 UsesToRewrite.push_back(
156 &PN->getOperandUse(PN->getOperandNumForIncomingValue(
157 PN->getNumIncomingValues() - 1)));
160 AddedPHIs.push_back(PN);
162 // Remember that this phi makes the value alive in this block.
163 SSAUpdate.AddAvailableValue(ExitBB, PN);
165 // LoopSimplify might fail to simplify some loops (e.g. when indirect
166 // branches are involved). In such situations, it might happen that an
167 // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
168 // create PHIs in such an exit block, we are also inserting PHIs into L2's
169 // header. This could break LCSSA form for L2 because these inserted PHIs
170 // can also have uses outside of L2. Remember all PHIs in such situation
171 // as to revisit than later on. FIXME: Remove this if indirectbr support
172 // into LoopSimplify gets improved.
173 if (auto *OtherLoop = LI.getLoopFor(ExitBB))
174 if (!L->contains(OtherLoop))
175 PostProcessPHIs.push_back(PN);
178 // Rewrite all uses outside the loop in terms of the new PHIs we just
180 for (Use *UseToRewrite : UsesToRewrite) {
181 // If this use is in an exit block, rewrite to use the newly inserted PHI.
182 // This is required for correctness because SSAUpdate doesn't handle uses
183 // in the same block. It assumes the PHI we inserted is at the end of the
185 Instruction *User = cast<Instruction>(UseToRewrite->getUser());
186 BasicBlock *UserBB = User->getParent();
187 if (auto *PN = dyn_cast<PHINode>(User))
188 UserBB = PN->getIncomingBlock(*UseToRewrite);
190 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
191 // Tell the VHs that the uses changed. This updates SCEV's caches.
192 if (UseToRewrite->get()->hasValueHandle())
193 ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
194 UseToRewrite->set(&UserBB->front());
198 // Otherwise, do full PHI insertion.
199 SSAUpdate.RewriteUse(*UseToRewrite);
202 // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
203 // to post-process them to keep LCSSA form.
204 for (PHINode *InsertedPN : InsertedPHIs) {
205 if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
206 if (!L->contains(OtherLoop))
207 PostProcessPHIs.push_back(InsertedPN);
210 // Post process PHI instructions that were inserted into another disjoint
211 // loop and update their exits properly.
212 for (auto *PostProcessPN : PostProcessPHIs)
213 if (!PostProcessPN->use_empty())
214 Worklist.push_back(PostProcessPN);
216 // Keep track of PHI nodes that we want to remove because they did not have
217 // any uses rewritten.
218 for (PHINode *PN : AddedPHIs)
220 PHIsToRemove.insert(PN);
224 // Remove PHI nodes that did not have any uses rewritten.
225 for (PHINode *PN : PHIsToRemove) {
226 assert (PN->use_empty() && "Trying to remove a phi with uses.");
227 PN->eraseFromParent();
232 // Compute the set of BasicBlocks in the loop `L` dominating at least one exit.
233 static void computeBlocksDominatingExits(
234 Loop &L, DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks,
235 SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {
236 SmallVector<BasicBlock *, 8> BBWorklist;
238 // We start from the exit blocks, as every block trivially dominates itself
240 for (BasicBlock *BB : ExitBlocks)
241 BBWorklist.push_back(BB);
243 while (!BBWorklist.empty()) {
244 BasicBlock *BB = BBWorklist.pop_back_val();
246 // Check if this is a loop header. If this is the case, we're done.
247 if (L.getHeader() == BB)
250 // Otherwise, add its immediate predecessor in the dominator tree to the
251 // worklist, unless we visited it already.
252 BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();
254 // Exit blocks can have an immediate dominator not beloinging to the
255 // loop. For an exit block to be immediately dominated by another block
256 // outside the loop, it implies not all paths from that dominator, to the
257 // exit block, go through the loop.
268 // C is the exit block of the loop and it's immediately dominated by A,
269 // which doesn't belong to the loop.
270 if (!L.contains(IDomBB))
273 if (BlocksDominatingExits.insert(IDomBB))
274 BBWorklist.push_back(IDomBB);
278 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
279 ScalarEvolution *SE) {
280 bool Changed = false;
282 SmallVector<BasicBlock *, 8> ExitBlocks;
283 L.getExitBlocks(ExitBlocks);
284 if (ExitBlocks.empty())
287 SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
289 // We want to avoid use-scanning leveraging dominance informations.
290 // If a block doesn't dominate any of the loop exits, the none of the values
291 // defined in the loop can be used outside.
292 // We compute the set of blocks fullfilling the conditions in advance
293 // walking the dominator tree upwards until we hit a loop header.
294 computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
296 SmallVector<Instruction *, 8> Worklist;
298 // Look at all the instructions in the loop, checking to see if they have uses
299 // outside the loop. If so, put them into the worklist to rewrite those uses.
300 for (BasicBlock *BB : BlocksDominatingExits) {
301 for (Instruction &I : *BB) {
302 // Reject two common cases fast: instructions with no uses (like stores)
303 // and instructions with one use that is in the same block as this.
305 (I.hasOneUse() && I.user_back()->getParent() == BB &&
306 !isa<PHINode>(I.user_back())))
309 // Tokens cannot be used in PHI nodes, so we skip over them.
310 // We can run into tokens which are live out of a loop with catchswitch
311 // instructions in Windows EH if the catchswitch has one catchpad which
312 // is inside the loop and another which is not.
313 if (I.getType()->isTokenTy())
316 Worklist.push_back(&I);
319 Changed = formLCSSAForInstructions(Worklist, DT, *LI);
321 // If we modified the code, remove any caches about the loop from SCEV to
322 // avoid dangling entries.
323 // FIXME: This is a big hammer, can we clear the cache more selectively?
327 assert(L.isLCSSAForm(DT));
332 /// Process a loop nest depth first.
333 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
334 ScalarEvolution *SE) {
335 bool Changed = false;
337 // Recurse depth-first through inner loops.
338 for (Loop *SubLoop : L.getSubLoops())
339 Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
341 Changed |= formLCSSA(L, DT, LI, SE);
345 /// Process all loops in the function, inner-most out.
346 static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT,
347 ScalarEvolution *SE) {
348 bool Changed = false;
350 Changed |= formLCSSARecursively(*L, DT, LI, SE);
355 struct LCSSAWrapperPass : public FunctionPass {
356 static char ID; // Pass identification, replacement for typeid
357 LCSSAWrapperPass() : FunctionPass(ID) {
358 initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
361 // Cached analysis information for the current function.
366 bool runOnFunction(Function &F) override;
367 void verifyAnalysis() const override {
368 // This check is very expensive. On the loop intensive compiles it may cause
369 // up to 10x slowdown. Currently it's disabled by default. LPPassManager
370 // always does limited form of the LCSSA verification. Similar reasoning
371 // was used for the LoopInfo verifier.
372 if (VerifyLoopLCSSA) {
375 return L->isRecursivelyLCSSAForm(*DT, *LI);
377 "LCSSA form is broken!");
381 /// This transformation requires natural loop information & requires that
382 /// loop preheaders be inserted into the CFG. It maintains both of these,
383 /// as well as the CFG. It also requires dominator information.
384 void getAnalysisUsage(AnalysisUsage &AU) const override {
385 AU.setPreservesCFG();
387 AU.addRequired<DominatorTreeWrapperPass>();
388 AU.addRequired<LoopInfoWrapperPass>();
389 AU.addPreservedID(LoopSimplifyID);
390 AU.addPreserved<AAResultsWrapperPass>();
391 AU.addPreserved<BasicAAWrapperPass>();
392 AU.addPreserved<GlobalsAAWrapperPass>();
393 AU.addPreserved<ScalarEvolutionWrapperPass>();
394 AU.addPreserved<SCEVAAWrapperPass>();
396 // This is needed to perform LCSSA verification inside LPPassManager
397 AU.addRequired<LCSSAVerificationPass>();
398 AU.addPreserved<LCSSAVerificationPass>();
403 char LCSSAWrapperPass::ID = 0;
404 INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
406 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
407 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
408 INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)
409 INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
412 Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
413 char &llvm::LCSSAID = LCSSAWrapperPass::ID;
415 /// Transform \p F into loop-closed SSA form.
416 bool LCSSAWrapperPass::runOnFunction(Function &F) {
417 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
418 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
419 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
420 SE = SEWP ? &SEWP->getSE() : nullptr;
422 return formLCSSAOnAllLoops(LI, *DT, SE);
425 PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
426 auto &LI = AM.getResult<LoopAnalysis>(F);
427 auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
428 auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
429 if (!formLCSSAOnAllLoops(&LI, DT, SE))
430 return PreservedAnalyses::all();
432 PreservedAnalyses PA;
433 PA.preserveSet<CFGAnalyses>();
434 PA.preserve<BasicAA>();
435 PA.preserve<GlobalsAA>();
436 PA.preserve<SCEVAA>();
437 PA.preserve<ScalarEvolutionAnalysis>();