916 lines · cpp
1//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This pass performs several transformations to transform natural loops into a10// simpler form, which makes subsequent analyses and transformations simpler and11// more effective.12//13// Loop pre-header insertion guarantees that there is a single, non-critical14// entry edge from outside of the loop to the loop header. This simplifies a15// number of analyses and transformations, such as LICM.16//17// Loop exit-block insertion guarantees that all exit blocks from the loop18// (blocks which are outside of the loop that have predecessors inside of the19// loop) only have predecessors from inside of the loop (and are thus dominated20// by the loop header). This simplifies transformations such as store-sinking21// that are built into LICM.22//23// This pass also guarantees that loops will have exactly one backedge.24//25// Indirectbr instructions introduce several complications. If the loop26// contains or is entered by an indirectbr instruction, it may not be possible27// to transform the loop and make these guarantees. Client code should check28// that these conditions are true before relying on them.29//30// Similar complications arise from callbr instructions, particularly in31// asm-goto where blockaddress expressions are used.32//33// Note that the simplifycfg pass will clean up blocks which are split out but34// end up being unnecessary, so usage of this pass should not pessimize35// generated code.36//37// This pass obviously modifies the CFG, but updates loop information and38// dominator information.39//40//===----------------------------------------------------------------------===//41 42#include "llvm/Transforms/Utils/LoopSimplify.h"43#include "llvm/ADT/SetVector.h"44#include "llvm/ADT/SmallVector.h"45#include "llvm/ADT/Statistic.h"46#include "llvm/Analysis/AliasAnalysis.h"47#include "llvm/Analysis/AssumptionCache.h"48#include "llvm/Analysis/BasicAliasAnalysis.h"49#include "llvm/Analysis/BranchProbabilityInfo.h"50#include "llvm/Analysis/GlobalsModRef.h"51#include "llvm/Analysis/InstructionSimplify.h"52#include "llvm/Analysis/LoopInfo.h"53#include "llvm/Analysis/MemorySSA.h"54#include "llvm/Analysis/MemorySSAUpdater.h"55#include "llvm/Analysis/ScalarEvolution.h"56#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"57#include "llvm/IR/CFG.h"58#include "llvm/IR/Constants.h"59#include "llvm/IR/Dominators.h"60#include "llvm/IR/Function.h"61#include "llvm/IR/Instructions.h"62#include "llvm/IR/LLVMContext.h"63#include "llvm/IR/Module.h"64#include "llvm/InitializePasses.h"65#include "llvm/Support/Debug.h"66#include "llvm/Support/raw_ostream.h"67#include "llvm/Transforms/Utils.h"68#include "llvm/Transforms/Utils/BasicBlockUtils.h"69#include "llvm/Transforms/Utils/Local.h"70#include "llvm/Transforms/Utils/LoopUtils.h"71using namespace llvm;72 73#define DEBUG_TYPE "loop-simplify"74 75STATISTIC(NumNested , "Number of nested loops split out");76 77// If the block isn't already, move the new block to right after some 'outside78// block' block. This prevents the preheader from being placed inside the loop79// body, e.g. when the loop hasn't been rotated.80static void placeSplitBlockCarefully(BasicBlock *NewBB,81 SmallVectorImpl<BasicBlock *> &SplitPreds,82 Loop *L) {83 // Check to see if NewBB is already well placed.84 Function::iterator BBI = --NewBB->getIterator();85 if (llvm::is_contained(SplitPreds, &*BBI))86 return;87 88 // If it isn't already after an outside block, move it after one. This is89 // always good as it makes the uncond branch from the outside block into a90 // fall-through.91 92 // Figure out *which* outside block to put this after. Prefer an outside93 // block that neighbors a BB actually in the loop.94 BasicBlock *FoundBB = nullptr;95 for (BasicBlock *Pred : SplitPreds) {96 Function::iterator BBI = Pred->getIterator();97 if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {98 FoundBB = Pred;99 break;100 }101 }102 103 // If our heuristic for a *good* bb to place this after doesn't find104 // anything, just pick something. It's likely better than leaving it within105 // the loop.106 if (!FoundBB)107 FoundBB = SplitPreds[0];108 NewBB->moveAfter(FoundBB);109}110 111/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a112/// preheader, this method is called to insert one. This method has two phases:113/// preheader insertion and analysis updating.114///115BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,116 LoopInfo *LI, MemorySSAUpdater *MSSAU,117 bool PreserveLCSSA) {118 BasicBlock *Header = L->getHeader();119 120 // Compute the set of predecessors of the loop that are not in the loop.121 SmallVector<BasicBlock*, 8> OutsideBlocks;122 for (BasicBlock *P : predecessors(Header)) {123 if (!L->contains(P)) { // Coming in from outside the loop?124 // If the loop is branched to from an indirect terminator, we won't125 // be able to fully transform the loop, because it prohibits126 // edge splitting.127 if (isa<IndirectBrInst>(P->getTerminator()))128 return nullptr;129 130 // Keep track of it.131 OutsideBlocks.push_back(P);132 }133 }134 135 // Split out the loop pre-header.136 BasicBlock *PreheaderBB;137 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,138 LI, MSSAU, PreserveLCSSA);139 if (!PreheaderBB)140 return nullptr;141 142 LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "143 << PreheaderBB->getName() << "\n");144 145 // Make sure that NewBB is put someplace intelligent, which doesn't mess up146 // code layout too horribly.147 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);148 149 return PreheaderBB;150}151 152/// Add the specified block, and all of its predecessors, to the specified set,153/// if it's not already in there. Stop predecessor traversal when we reach154/// StopBlock.155static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,156 SmallPtrSetImpl<BasicBlock *> &Blocks) {157 SmallVector<BasicBlock *, 8> Worklist;158 Worklist.push_back(InputBB);159 do {160 BasicBlock *BB = Worklist.pop_back_val();161 if (Blocks.insert(BB).second && BB != StopBlock)162 // If BB is not already processed and it is not a stop block then163 // insert its predecessor in the work list164 append_range(Worklist, predecessors(BB));165 } while (!Worklist.empty());166}167 168/// The first part of loop-nestification is to find a PHI node that tells169/// us how to partition the loops.170static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,171 AssumptionCache *AC) {172 const DataLayout &DL = L->getHeader()->getDataLayout();173 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {174 PHINode *PN = cast<PHINode>(I);175 ++I;176 if (Value *V = simplifyInstruction(PN, {DL, nullptr, DT, AC})) {177 // This is a degenerate PHI already, don't modify it!178 PN->replaceAllUsesWith(V);179 PN->eraseFromParent();180 continue;181 }182 183 // Scan this PHI node looking for a use of the PHI node by itself.184 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)185 if (PN->getIncomingValue(i) == PN &&186 L->contains(PN->getIncomingBlock(i)))187 // We found something tasty to remove.188 return PN;189 }190 return nullptr;191}192 193/// If this loop has multiple backedges, try to pull one of them out into194/// a nested loop.195///196/// This is important for code that looks like197/// this:198///199/// Loop:200/// ...201/// br cond, Loop, Next202/// ...203/// br cond2, Loop, Out204///205/// To identify this common case, we look at the PHI nodes in the header of the206/// loop. PHI nodes with unchanging values on one backedge correspond to values207/// that change in the "outer" loop, but not in the "inner" loop.208///209/// If we are able to separate out a loop, return the new outer loop that was210/// created.211///212static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,213 DominatorTree *DT, LoopInfo *LI,214 ScalarEvolution *SE, bool PreserveLCSSA,215 AssumptionCache *AC, MemorySSAUpdater *MSSAU) {216 // Don't try to separate loops without a preheader.217 if (!Preheader)218 return nullptr;219 220 // Treat the presence of convergent functions conservatively. The221 // transformation is invalid if calls to certain convergent222 // functions (like an AMDGPU barrier) get included in the resulting223 // inner loop. But blocks meant for the inner loop will be224 // identified later at a point where it's too late to abort the225 // transformation. Also, the convergent attribute is not really226 // sufficient to express the semantics of functions that are227 // affected by this transformation. So we choose to back off if such228 // a function call is present until a better alternative becomes229 // available. This is similar to the conservative treatment of230 // convergent function calls in GVNHoist and JumpThreading.231 for (auto *BB : L->blocks()) {232 for (auto &II : *BB) {233 if (auto CI = dyn_cast<CallBase>(&II)) {234 if (CI->isConvergent()) {235 return nullptr;236 }237 }238 }239 }240 241 // The header is not a landing pad; preheader insertion should ensure this.242 BasicBlock *Header = L->getHeader();243 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");244 245 PHINode *PN = findPHIToPartitionLoops(L, DT, AC);246 if (!PN) return nullptr; // No known way to partition.247 248 // Pull out all predecessors that have varying values in the loop. This249 // handles the case when a PHI node has multiple instances of itself as250 // arguments.251 SmallVector<BasicBlock*, 8> OuterLoopPreds;252 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {253 if (PN->getIncomingValue(i) != PN ||254 !L->contains(PN->getIncomingBlock(i))) {255 // We can't split indirect control flow edges.256 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))257 return nullptr;258 OuterLoopPreds.push_back(PN->getIncomingBlock(i));259 }260 }261 LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");262 263 // If ScalarEvolution is around and knows anything about values in264 // this loop, tell it to forget them, because we're about to265 // substantially change it.266 if (SE)267 SE->forgetLoop(L);268 269 BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",270 DT, LI, MSSAU, PreserveLCSSA);271 272 // Make sure that NewBB is put someplace intelligent, which doesn't mess up273 // code layout too horribly.274 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);275 276 // Create the new outer loop.277 Loop *NewOuter = LI->AllocateLoop();278 279 // Change the parent loop to use the outer loop as its child now.280 if (Loop *Parent = L->getParentLoop())281 Parent->replaceChildLoopWith(L, NewOuter);282 else283 LI->changeTopLevelLoop(L, NewOuter);284 285 // L is now a subloop of our outer loop.286 NewOuter->addChildLoop(L);287 288 for (BasicBlock *BB : L->blocks())289 NewOuter->addBlockEntry(BB);290 291 // Now reset the header in L, which had been moved by292 // SplitBlockPredecessors for the outer loop.293 L->moveToHeader(Header);294 295 // Determine which blocks should stay in L and which should be moved out to296 // the Outer loop now.297 SmallPtrSet<BasicBlock *, 4> BlocksInL;298 for (BasicBlock *P : predecessors(Header)) {299 if (DT->dominates(Header, P))300 addBlockAndPredsToSet(P, Header, BlocksInL);301 }302 303 // Scan all of the loop children of L, moving them to OuterLoop if they are304 // not part of the inner loop.305 const std::vector<Loop*> &SubLoops = L->getSubLoops();306 for (size_t I = 0; I != SubLoops.size(); )307 if (BlocksInL.count(SubLoops[I]->getHeader()))308 ++I; // Loop remains in L309 else310 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));311 312 SmallVector<BasicBlock *, 8> OuterLoopBlocks;313 OuterLoopBlocks.push_back(NewBB);314 // Now that we know which blocks are in L and which need to be moved to315 // OuterLoop, move any blocks that need it.316 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {317 BasicBlock *BB = L->getBlocks()[i];318 if (!BlocksInL.count(BB)) {319 // Move this block to the parent, updating the exit blocks sets320 L->removeBlockFromLoop(BB);321 if ((*LI)[BB] == L) {322 LI->changeLoopFor(BB, NewOuter);323 OuterLoopBlocks.push_back(BB);324 }325 --i;326 }327 }328 329 // Split edges to exit blocks from the inner loop, if they emerged in the330 // process of separating the outer one.331 formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA);332 333 if (PreserveLCSSA) {334 // Fix LCSSA form for L. Some values, which previously were only used inside335 // L, can now be used in NewOuter loop. We need to insert phi-nodes for them336 // in corresponding exit blocks.337 // We don't need to form LCSSA recursively, because there cannot be uses338 // inside a newly created loop of defs from inner loops as those would339 // already be a use of an LCSSA phi node.340 formLCSSA(*L, *DT, LI, SE);341 342 assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&343 "LCSSA is broken after separating nested loops!");344 }345 346 return NewOuter;347}348 349/// This method is called when the specified loop has more than one350/// backedge in it.351///352/// If this occurs, revector all of these backedges to target a new basic block353/// and have that block branch to the loop header. This ensures that loops354/// have exactly one backedge.355static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,356 DominatorTree *DT, LoopInfo *LI,357 MemorySSAUpdater *MSSAU) {358 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");359 360 // Get information about the loop361 BasicBlock *Header = L->getHeader();362 Function *F = Header->getParent();363 364 // Unique backedge insertion currently depends on having a preheader.365 if (!Preheader)366 return nullptr;367 368 // The header is not an EH pad; preheader insertion should ensure this.369 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");370 371 // Figure out which basic blocks contain back-edges to the loop header.372 std::vector<BasicBlock*> BackedgeBlocks;373 for (BasicBlock *P : predecessors(Header)) {374 // Indirect edges cannot be split, so we must fail if we find one.375 if (isa<IndirectBrInst>(P->getTerminator()))376 return nullptr;377 378 if (P != Preheader) BackedgeBlocks.push_back(P);379 }380 381 // Create and insert the new backedge block.382 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),383 Header->getName() + ".backedge", F);384 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);385 BETerminator->setDebugLoc(Header->getFirstNonPHIIt()->getDebugLoc());386 387 LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "388 << BEBlock->getName() << "\n");389 390 // Move the new backedge block to right after the last backedge block.391 Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();392 F->splice(InsertPos, F, BEBlock->getIterator());393 394 // Now that the block has been inserted into the function, create PHI nodes in395 // the backedge block which correspond to any PHI nodes in the header block.396 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {397 PHINode *PN = cast<PHINode>(I);398 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),399 PN->getName()+".be", BETerminator->getIterator());400 401 // Loop over the PHI node, moving all entries except the one for the402 // preheader over to the new PHI node.403 unsigned PreheaderIdx = ~0U;404 bool HasUniqueIncomingValue = true;405 Value *UniqueValue = nullptr;406 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {407 BasicBlock *IBB = PN->getIncomingBlock(i);408 Value *IV = PN->getIncomingValue(i);409 if (IBB == Preheader) {410 PreheaderIdx = i;411 } else {412 NewPN->addIncoming(IV, IBB);413 if (HasUniqueIncomingValue) {414 if (!UniqueValue)415 UniqueValue = IV;416 else if (UniqueValue != IV)417 HasUniqueIncomingValue = false;418 }419 }420 }421 422 // Delete all of the incoming values from the old PN except the preheader's423 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");424 if (PreheaderIdx != 0) {425 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));426 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));427 }428 // Nuke all entries except the zero'th.429 PN->removeIncomingValueIf([](unsigned Idx) { return Idx != 0; },430 /* DeletePHIIfEmpty */ false);431 432 // Finally, add the newly constructed PHI node as the entry for the BEBlock.433 PN->addIncoming(NewPN, BEBlock);434 435 // As an optimization, if all incoming values in the new PhiNode (which is a436 // subset of the incoming values of the old PHI node) have the same value,437 // eliminate the PHI Node.438 if (HasUniqueIncomingValue) {439 NewPN->replaceAllUsesWith(UniqueValue);440 NewPN->eraseFromParent();441 }442 }443 444 // Now that all of the PHI nodes have been inserted and adjusted, modify the445 // backedge blocks to jump to the BEBlock instead of the header.446 // If one of the backedges has llvm.loop metadata attached, we remove447 // it from the backedge and add it to BEBlock.448 MDNode *LoopMD = nullptr;449 for (BasicBlock *BB : BackedgeBlocks) {450 Instruction *TI = BB->getTerminator();451 if (!LoopMD)452 LoopMD = TI->getMetadata(LLVMContext::MD_loop);453 TI->setMetadata(LLVMContext::MD_loop, nullptr);454 TI->replaceSuccessorWith(Header, BEBlock);455 }456 BEBlock->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopMD);457 458 //===--- Update all analyses which we must preserve now -----------------===//459 460 // Update Loop Information - we know that this block is now in the current461 // loop and all parent loops.462 L->addBasicBlockToLoop(BEBlock, *LI);463 464 // Update dominator information465 DT->splitBlock(BEBlock);466 467 if (MSSAU)468 MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(Header, Preheader,469 BEBlock);470 471 return BEBlock;472}473 474/// Simplify one loop and queue further loops for simplification.475static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,476 DominatorTree *DT, LoopInfo *LI,477 ScalarEvolution *SE, AssumptionCache *AC,478 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {479 bool Changed = false;480 if (MSSAU && VerifyMemorySSA)481 MSSAU->getMemorySSA()->verifyMemorySSA();482 483ReprocessLoop:484 485 // Check to see that no blocks (other than the header) in this loop have486 // predecessors that are not in the loop. This is not valid for natural487 // loops, but can occur if the blocks are unreachable. Since they are488 // unreachable we can just shamelessly delete those CFG edges!489 for (BasicBlock *BB : L->blocks()) {490 if (BB == L->getHeader())491 continue;492 493 SmallPtrSet<BasicBlock*, 4> BadPreds;494 for (BasicBlock *P : predecessors(BB))495 if (!L->contains(P))496 BadPreds.insert(P);497 498 // Delete each unique out-of-loop (and thus dead) predecessor.499 for (BasicBlock *P : BadPreds) {500 501 LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "502 << P->getName() << "\n");503 504 // Zap the dead pred's terminator and replace it with unreachable.505 Instruction *TI = P->getTerminator();506 changeToUnreachable(TI, PreserveLCSSA,507 /*DTU=*/nullptr, MSSAU);508 Changed = true;509 }510 }511 512 if (MSSAU && VerifyMemorySSA)513 MSSAU->getMemorySSA()->verifyMemorySSA();514 515 // If there are exiting blocks with branches on undef, resolve the undef in516 // the direction which will exit the loop. This will help simplify loop517 // trip count computations.518 SmallVector<BasicBlock*, 8> ExitingBlocks;519 L->getExitingBlocks(ExitingBlocks);520 for (BasicBlock *ExitingBlock : ExitingBlocks)521 if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()))522 if (BI->isConditional()) {523 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {524 525 LLVM_DEBUG(dbgs()526 << "LoopSimplify: Resolving \"br i1 undef\" to exit in "527 << ExitingBlock->getName() << "\n");528 529 BI->setCondition(ConstantInt::get(Cond->getType(),530 !L->contains(BI->getSuccessor(0))));531 532 Changed = true;533 }534 }535 536 // Does the loop already have a preheader? If so, don't insert one.537 BasicBlock *Preheader = L->getLoopPreheader();538 if (!Preheader) {539 Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA);540 if (Preheader)541 Changed = true;542 }543 544 // Next, check to make sure that all exit nodes of the loop only have545 // predecessors that are inside of the loop. This check guarantees that the546 // loop preheader/header will dominate the exit blocks. If the exit block has547 // predecessors from outside of the loop, split the edge now.548 if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA))549 Changed = true;550 551 if (MSSAU && VerifyMemorySSA)552 MSSAU->getMemorySSA()->verifyMemorySSA();553 554 // If the header has more than two predecessors at this point (from the555 // preheader and from multiple backedges), we must adjust the loop.556 BasicBlock *LoopLatch = L->getLoopLatch();557 if (!LoopLatch) {558 // If this is really a nested loop, rip it out into a child loop. Don't do559 // this for loops with a giant number of backedges, just factor them into a560 // common backedge instead.561 if (L->getNumBackEdges() < 8) {562 if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE,563 PreserveLCSSA, AC, MSSAU)) {564 ++NumNested;565 // Enqueue the outer loop as it should be processed next in our566 // depth-first nest walk.567 Worklist.push_back(OuterL);568 569 // This is a big restructuring change, reprocess the whole loop.570 Changed = true;571 // GCC doesn't tail recursion eliminate this.572 // FIXME: It isn't clear we can't rely on LLVM to TRE this.573 goto ReprocessLoop;574 }575 }576 577 // If we either couldn't, or didn't want to, identify nesting of the loops,578 // insert a new block that all backedges target, then make it jump to the579 // loop header.580 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU);581 if (LoopLatch)582 Changed = true;583 }584 585 if (MSSAU && VerifyMemorySSA)586 MSSAU->getMemorySSA()->verifyMemorySSA();587 588 const DataLayout &DL = L->getHeader()->getDataLayout();589 590 // Scan over the PHI nodes in the loop header. Since they now have only two591 // incoming values (the loop is canonicalized), we may have simplified the PHI592 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.593 PHINode *PN;594 for (BasicBlock::iterator I = L->getHeader()->begin();595 (PN = dyn_cast<PHINode>(I++)); )596 if (Value *V = simplifyInstruction(PN, {DL, nullptr, DT, AC})) {597 if (SE) SE->forgetValue(PN);598 if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) {599 PN->replaceAllUsesWith(V);600 PN->eraseFromParent();601 Changed = true;602 }603 }604 605 // If this loop has multiple exits and the exits all go to the same606 // block, attempt to merge the exits. This helps several passes, such607 // as LoopRotation, which do not support loops with multiple exits.608 // SimplifyCFG also does this (and this code uses the same utility609 // function), however this code is loop-aware, where SimplifyCFG is610 // not. That gives it the advantage of being able to hoist611 // loop-invariant instructions out of the way to open up more612 // opportunities, and the disadvantage of having the responsibility613 // to preserve dominator information.614 auto HasUniqueExitBlock = [&]() {615 BasicBlock *UniqueExit = nullptr;616 for (auto *ExitingBB : ExitingBlocks)617 for (auto *SuccBB : successors(ExitingBB)) {618 if (L->contains(SuccBB))619 continue;620 621 if (!UniqueExit)622 UniqueExit = SuccBB;623 else if (UniqueExit != SuccBB)624 return false;625 }626 627 return true;628 };629 if (HasUniqueExitBlock()) {630 for (BasicBlock *ExitingBlock : ExitingBlocks) {631 if (!ExitingBlock->getSinglePredecessor()) continue;632 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());633 if (!BI || !BI->isConditional()) continue;634 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());635 if (!CI || CI->getParent() != ExitingBlock) continue;636 637 // Attempt to hoist out all instructions except for the638 // comparison and the branch.639 bool AllInvariant = true;640 bool AnyInvariant = false;641 for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {642 Instruction *Inst = &*I++;643 if (Inst == CI)644 continue;645 if (!L->makeLoopInvariant(646 Inst, AnyInvariant,647 Preheader ? Preheader->getTerminator() : nullptr, MSSAU, SE)) {648 AllInvariant = false;649 break;650 }651 }652 if (AnyInvariant)653 Changed = true;654 if (!AllInvariant) continue;655 656 // The block has now been cleared of all instructions except for657 // a comparison and a conditional branch. SimplifyCFG may be able658 // to fold it now.659 if (!foldBranchToCommonDest(BI, /*DTU=*/nullptr, MSSAU))660 continue;661 662 // Success. The block is now dead, so remove it from the loop,663 // update the dominator tree and delete it.664 LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "665 << ExitingBlock->getName() << "\n");666 667 assert(pred_empty(ExitingBlock));668 Changed = true;669 LI->removeBlock(ExitingBlock);670 671 DomTreeNode *Node = DT->getNode(ExitingBlock);672 while (!Node->isLeaf()) {673 DomTreeNode *Child = Node->back();674 DT->changeImmediateDominator(Child, Node->getIDom());675 }676 DT->eraseNode(ExitingBlock);677 if (MSSAU) {678 SmallSetVector<BasicBlock *, 8> ExitBlockSet;679 ExitBlockSet.insert(ExitingBlock);680 MSSAU->removeBlocks(ExitBlockSet);681 }682 683 BI->getSuccessor(0)->removePredecessor(684 ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);685 BI->getSuccessor(1)->removePredecessor(686 ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);687 ExitingBlock->eraseFromParent();688 }689 }690 691 if (MSSAU && VerifyMemorySSA)692 MSSAU->getMemorySSA()->verifyMemorySSA();693 694 return Changed;695}696 697bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,698 ScalarEvolution *SE, AssumptionCache *AC,699 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {700 bool Changed = false;701 702#ifndef NDEBUG703 // If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA704 // form.705 if (PreserveLCSSA) {706 assert(DT && "DT not available.");707 assert(LI && "LI not available.");708 assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&709 "Requested to preserve LCSSA, but it's already broken.");710 }711#endif712 713 // Worklist maintains our depth-first queue of loops in this nest to process.714 SmallVector<Loop *, 4> Worklist;715 Worklist.push_back(L);716 717 // Walk the worklist from front to back, pushing newly found sub loops onto718 // the back. This will let us process loops from back to front in depth-first719 // order. We can use this simple process because loops form a tree.720 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {721 Loop *L2 = Worklist[Idx];722 Worklist.append(L2->begin(), L2->end());723 }724 725 while (!Worklist.empty())726 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,727 AC, MSSAU, PreserveLCSSA);728 729 // Changing exit conditions for blocks may affect exit counts of this loop and730 // any of its parents, so we must invalidate the entire subtree if we've made731 // any changes. Do this here rather than in simplifyOneLoop() as the top-most732 // loop is going to be the same for all child loops.733 if (Changed && SE)734 SE->forgetTopmostLoop(L);735 736 return Changed;737}738 739namespace {740struct LoopSimplify : public FunctionPass {741 static char ID; // Pass identification, replacement for typeid742 LoopSimplify() : FunctionPass(ID) {743 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());744 }745 746 bool runOnFunction(Function &F) override;747 748 void getAnalysisUsage(AnalysisUsage &AU) const override {749 AU.addRequired<AssumptionCacheTracker>();750 751 // We need loop information to identify the loops.752 AU.addRequired<DominatorTreeWrapperPass>();753 AU.addPreserved<DominatorTreeWrapperPass>();754 755 AU.addRequired<LoopInfoWrapperPass>();756 AU.addPreserved<LoopInfoWrapperPass>();757 758 AU.addPreserved<BasicAAWrapperPass>();759 AU.addPreserved<AAResultsWrapperPass>();760 AU.addPreserved<GlobalsAAWrapperPass>();761 AU.addPreserved<ScalarEvolutionWrapperPass>();762 AU.addPreserved<SCEVAAWrapperPass>();763 AU.addPreservedID(LCSSAID);764 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.765 AU.addPreserved<BranchProbabilityInfoWrapperPass>();766 AU.addPreserved<MemorySSAWrapperPass>();767 }768 769 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.770 void verifyAnalysis() const override;771};772} // namespace773 774char LoopSimplify::ID = 0;775INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",776 "Canonicalize natural loops", false, false)777INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)778INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)779INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)780INITIALIZE_PASS_END(LoopSimplify, "loop-simplify", "Canonicalize natural loops",781 false, false)782 783// Publicly exposed interface to pass.784char &llvm::LoopSimplifyID = LoopSimplify::ID;785Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }786 787/// runOnFunction - Run down all loops in the CFG (recursively, but we could do788/// it in any convenient order) inserting preheaders.789///790bool LoopSimplify::runOnFunction(Function &F) {791 bool Changed = false;792 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();793 DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();794 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();795 ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;796 AssumptionCache *AC =797 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);798 MemorySSA *MSSA = nullptr;799 std::unique_ptr<MemorySSAUpdater> MSSAU;800 auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();801 if (MSSAAnalysis) {802 MSSA = &MSSAAnalysis->getMSSA();803 MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);804 }805 806 bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);807 808 // Simplify each loop nest in the function.809 for (auto *L : *LI)810 Changed |= simplifyLoop(L, DT, LI, SE, AC, MSSAU.get(), PreserveLCSSA);811 812#ifndef NDEBUG813 if (PreserveLCSSA) {814 bool InLCSSA = all_of(815 *LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); });816 assert(InLCSSA && "LCSSA is broken after loop-simplify.");817 }818#endif819 return Changed;820}821 822PreservedAnalyses LoopSimplifyPass::run(Function &F,823 FunctionAnalysisManager &AM) {824 bool Changed = false;825 LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);826 DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);827 ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);828 AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);829 auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(F);830 std::unique_ptr<MemorySSAUpdater> MSSAU;831 if (MSSAAnalysis) {832 auto *MSSA = &MSSAAnalysis->getMSSA();833 MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);834 }835 836 837 // Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA838 // after simplifying the loops. MemorySSA is preserved if it exists.839 for (auto *L : *LI)840 Changed |=841 simplifyLoop(L, DT, LI, SE, AC, MSSAU.get(), /*PreserveLCSSA*/ false);842 843 if (!Changed)844 return PreservedAnalyses::all();845 846 PreservedAnalyses PA;847 PA.preserve<DominatorTreeAnalysis>();848 PA.preserve<LoopAnalysis>();849 PA.preserve<ScalarEvolutionAnalysis>();850 if (MSSAAnalysis)851 PA.preserve<MemorySSAAnalysis>();852 // BPI maps conditional terminators to probabilities, LoopSimplify can insert853 // blocks, but it does so only by splitting existing blocks and edges. This854 // results in the interesting property that all new terminators inserted are855 // unconditional branches which do not appear in BPI. All deletions are856 // handled via ValueHandle callbacks w/in BPI.857 PA.preserve<BranchProbabilityAnalysis>();858 return PA;859}860 861// FIXME: Restore this code when we re-enable verification in verifyAnalysis862// below.863#if 0864static void verifyLoop(Loop *L) {865 // Verify subloops.866 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)867 verifyLoop(*I);868 869 // It used to be possible to just assert L->isLoopSimplifyForm(), however870 // with the introduction of indirectbr, there are now cases where it's871 // not possible to transform a loop as necessary. We can at least check872 // that there is an indirectbr near any time there's trouble.873 874 // Indirectbr can interfere with preheader and unique backedge insertion.875 if (!L->getLoopPreheader() || !L->getLoopLatch()) {876 bool HasIndBrPred = false;877 for (BasicBlock *Pred : predecessors(L->getHeader()))878 if (isa<IndirectBrInst>(Pred->getTerminator())) {879 HasIndBrPred = true;880 break;881 }882 assert(HasIndBrPred &&883 "LoopSimplify has no excuse for missing loop header info!");884 (void)HasIndBrPred;885 }886 887 // Indirectbr can interfere with exit block canonicalization.888 if (!L->hasDedicatedExits()) {889 bool HasIndBrExiting = false;890 SmallVector<BasicBlock*, 8> ExitingBlocks;891 L->getExitingBlocks(ExitingBlocks);892 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {893 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {894 HasIndBrExiting = true;895 break;896 }897 }898 899 assert(HasIndBrExiting &&900 "LoopSimplify has no excuse for missing exit block info!");901 (void)HasIndBrExiting;902 }903}904#endif905 906void LoopSimplify::verifyAnalysis() const {907 // FIXME: This routine is being called mid-way through the loop pass manager908 // as loop passes destroy this analysis. That's actually fine, but we have no909 // way of expressing that here. Once all of the passes that destroy this are910 // hoisted out of the loop pass manager we can add back verification here.911#if 0912 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)913 verifyLoop(*I);914#endif915}916