1 //===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===//
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 file implements basic block placement transformations using the CFG
11 // structure and branch probability estimates.
13 // The pass strives to preserve the structure of the CFG (that is, retain
14 // a topological ordering of basic blocks) in the absence of a *strong* signal
15 // to the contrary from probabilities. However, within the CFG structure, it
16 // attempts to choose an ordering which favors placing more likely sequences of
17 // blocks adjacent to each other.
19 // The algorithm works from the inner-most loop within a function outward, and
20 // at each stage walks through the basic blocks, trying to coalesce them into
21 // sequential chains where allowed by the CFG (or demanded by heavy
22 // probabilities). Finally, it walks the blocks in topological order, and the
23 // first time it reaches a chain of basic blocks, it schedules them in the
26 //===----------------------------------------------------------------------===//
28 #include "llvm/CodeGen/Passes.h"
29 #include "llvm/ADT/DenseMap.h"
30 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/Statistic.h"
33 #include "llvm/CodeGen/MachineBasicBlock.h"
34 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
35 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
36 #include "llvm/CodeGen/MachineFunction.h"
37 #include "llvm/CodeGen/MachineFunctionPass.h"
38 #include "llvm/CodeGen/MachineLoopInfo.h"
39 #include "llvm/CodeGen/MachineModuleInfo.h"
40 #include "llvm/Support/Allocator.h"
41 #include "llvm/Support/CommandLine.h"
42 #include "llvm/Support/Debug.h"
43 #include "llvm/Target/TargetInstrInfo.h"
44 #include "llvm/Target/TargetLowering.h"
48 #define DEBUG_TYPE "block-placement2"
50 STATISTIC(NumCondBranches, "Number of conditional branches");
51 STATISTIC(NumUncondBranches, "Number of uncondittional branches");
52 STATISTIC(CondBranchTakenFreq,
53 "Potential frequency of taking conditional branches");
54 STATISTIC(UncondBranchTakenFreq,
55 "Potential frequency of taking unconditional branches");
57 static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
58 cl::desc("Force the alignment of all "
59 "blocks in the function."),
60 cl::init(0), cl::Hidden);
62 // FIXME: Find a good default for this flag and remove the flag.
63 static cl::opt<unsigned>
64 ExitBlockBias("block-placement-exit-block-bias",
65 cl::desc("Block frequency percentage a loop exit block needs "
66 "over the original exit to be considered the new exit."),
67 cl::init(0), cl::Hidden);
71 /// \brief Type for our function-wide basic block -> block chain mapping.
72 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
76 /// \brief A chain of blocks which will be laid out contiguously.
78 /// This is the datastructure representing a chain of consecutive blocks that
79 /// are profitable to layout together in order to maximize fallthrough
80 /// probabilities and code locality. We also can use a block chain to represent
81 /// a sequence of basic blocks which have some external (correctness)
82 /// requirement for sequential layout.
84 /// Chains can be built around a single basic block and can be merged to grow
85 /// them. They participate in a block-to-chain mapping, which is updated
86 /// automatically as chains are merged together.
88 /// \brief The sequence of blocks belonging to this chain.
90 /// This is the sequence of blocks for a particular chain. These will be laid
91 /// out in-order within the function.
92 SmallVector<MachineBasicBlock *, 4> Blocks;
94 /// \brief A handle to the function-wide basic block to block chain mapping.
96 /// This is retained in each block chain to simplify the computation of child
97 /// block chains for SCC-formation and iteration. We store the edges to child
98 /// basic blocks, and map them back to their associated chains using this
100 BlockToChainMapType &BlockToChain;
103 /// \brief Construct a new BlockChain.
105 /// This builds a new block chain representing a single basic block in the
106 /// function. It also registers itself as the chain that block participates
107 /// in with the BlockToChain mapping.
108 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
109 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
110 assert(BB && "Cannot create a chain with a null basic block");
111 BlockToChain[BB] = this;
114 /// \brief Iterator over blocks within the chain.
115 typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
117 /// \brief Beginning of blocks within the chain.
118 iterator begin() { return Blocks.begin(); }
120 /// \brief End of blocks within the chain.
121 iterator end() { return Blocks.end(); }
123 /// \brief Merge a block chain into this one.
125 /// This routine merges a block chain into this one. It takes care of forming
126 /// a contiguous sequence of basic blocks, updating the edge list, and
127 /// updating the block -> chain mapping. It does not free or tear down the
128 /// old chain, but the old chain's block list is no longer valid.
129 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
131 assert(!Blocks.empty());
133 // Fast path in case we don't have a chain already.
135 assert(!BlockToChain[BB]);
136 Blocks.push_back(BB);
137 BlockToChain[BB] = this;
141 assert(BB == *Chain->begin());
142 assert(Chain->begin() != Chain->end());
144 // Update the incoming blocks to point to this chain, and add them to the
146 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
148 Blocks.push_back(*BI);
149 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
150 BlockToChain[*BI] = this;
155 /// \brief Dump the blocks in this chain.
156 LLVM_DUMP_METHOD void dump() {
157 for (iterator I = begin(), E = end(); I != E; ++I)
162 /// \brief Count of predecessors within the loop currently being processed.
164 /// This count is updated at each loop we process to represent the number of
165 /// in-loop predecessors of this chain.
166 unsigned LoopPredecessors;
171 class MachineBlockPlacement : public MachineFunctionPass {
172 /// \brief A typedef for a block filter set.
173 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
175 /// \brief A handle to the branch probability pass.
176 const MachineBranchProbabilityInfo *MBPI;
178 /// \brief A handle to the function-wide block frequency pass.
179 const MachineBlockFrequencyInfo *MBFI;
181 /// \brief A handle to the loop info.
182 const MachineLoopInfo *MLI;
184 /// \brief A handle to the target's instruction info.
185 const TargetInstrInfo *TII;
187 /// \brief A handle to the target's lowering info.
188 const TargetLoweringBase *TLI;
190 /// \brief Allocator and owner of BlockChain structures.
192 /// We build BlockChains lazily while processing the loop structure of
193 /// a function. To reduce malloc traffic, we allocate them using this
194 /// slab-like allocator, and destroy them after the pass completes. An
195 /// important guarantee is that this allocator produces stable pointers to
197 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
199 /// \brief Function wide BasicBlock to BlockChain mapping.
201 /// This mapping allows efficiently moving from any given basic block to the
202 /// BlockChain it participates in, if any. We use it to, among other things,
203 /// allow implicitly defining edges between chains as the existing edges
204 /// between basic blocks.
205 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
207 void markChainSuccessors(BlockChain &Chain,
208 MachineBasicBlock *LoopHeaderBB,
209 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
210 const BlockFilterSet *BlockFilter = nullptr);
211 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
213 const BlockFilterSet *BlockFilter);
214 MachineBasicBlock *selectBestCandidateBlock(
215 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
216 const BlockFilterSet *BlockFilter);
217 MachineBasicBlock *getFirstUnplacedBlock(
219 const BlockChain &PlacedChain,
220 MachineFunction::iterator &PrevUnplacedBlockIt,
221 const BlockFilterSet *BlockFilter);
222 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
223 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
224 const BlockFilterSet *BlockFilter = nullptr);
225 MachineBasicBlock *findBestLoopTop(MachineLoop &L,
226 const BlockFilterSet &LoopBlockSet);
227 MachineBasicBlock *findBestLoopExit(MachineFunction &F,
229 const BlockFilterSet &LoopBlockSet);
230 void buildLoopChains(MachineFunction &F, MachineLoop &L);
231 void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
232 const BlockFilterSet &LoopBlockSet);
233 void buildCFGChains(MachineFunction &F);
236 static char ID; // Pass identification, replacement for typeid
237 MachineBlockPlacement() : MachineFunctionPass(ID) {
238 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
241 bool runOnMachineFunction(MachineFunction &F) override;
243 void getAnalysisUsage(AnalysisUsage &AU) const override {
244 AU.addRequired<MachineBranchProbabilityInfo>();
245 AU.addRequired<MachineBlockFrequencyInfo>();
246 AU.addRequired<MachineLoopInfo>();
247 MachineFunctionPass::getAnalysisUsage(AU);
252 char MachineBlockPlacement::ID = 0;
253 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
254 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
255 "Branch Probability Basic Block Placement", false, false)
256 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
257 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
258 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
259 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
260 "Branch Probability Basic Block Placement", false, false)
263 /// \brief Helper to print the name of a MBB.
265 /// Only used by debug logging.
266 static std::string getBlockName(MachineBasicBlock *BB) {
268 OS << "BB#" << BB->getNumber()
269 << " (derived from LLVM BB '" << BB->getName() << "')";
273 /// \brief Helper to print the number of a MBB.
275 /// Only used by debug logging.
276 static std::string getBlockNum(MachineBasicBlock *BB) {
278 OS << "BB#" << BB->getNumber();
283 /// \brief Mark a chain's successors as having one fewer preds.
285 /// When a chain is being merged into the "placed" chain, this routine will
286 /// quickly walk the successors of each block in the chain and mark them as
287 /// having one fewer active predecessor. It also adds any successors of this
288 /// chain which reach the zero-predecessor state to the worklist passed in.
289 void MachineBlockPlacement::markChainSuccessors(
291 MachineBasicBlock *LoopHeaderBB,
292 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
293 const BlockFilterSet *BlockFilter) {
294 // Walk all the blocks in this chain, marking their successors as having
295 // a predecessor placed.
296 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
298 // Add any successors for which this is the only un-placed in-loop
299 // predecessor to the worklist as a viable candidate for CFG-neutral
300 // placement. No subsequent placement of this block will violate the CFG
301 // shape, so we get to use heuristics to choose a favorable placement.
302 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
303 SE = (*CBI)->succ_end();
305 if (BlockFilter && !BlockFilter->count(*SI))
307 BlockChain &SuccChain = *BlockToChain[*SI];
308 // Disregard edges within a fixed chain, or edges to the loop header.
309 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
312 // This is a cross-chain edge that is within the loop, so decrement the
313 // loop predecessor count of the destination chain.
314 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
315 BlockWorkList.push_back(*SuccChain.begin());
320 /// \brief Select the best successor for a block.
322 /// This looks across all successors of a particular block and attempts to
323 /// select the "best" one to be the layout successor. It only considers direct
324 /// successors which also pass the block filter. It will attempt to avoid
325 /// breaking CFG structure, but cave and break such structures in the case of
326 /// very hot successor edges.
328 /// \returns The best successor block found, or null if none are viable.
329 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
330 MachineBasicBlock *BB, BlockChain &Chain,
331 const BlockFilterSet *BlockFilter) {
332 const BranchProbability HotProb(4, 5); // 80%
334 MachineBasicBlock *BestSucc = nullptr;
335 // FIXME: Due to the performance of the probability and weight routines in
336 // the MBPI analysis, we manually compute probabilities using the edge
337 // weights. This is suboptimal as it means that the somewhat subtle
338 // definition of edge weight semantics is encoded here as well. We should
339 // improve the MBPI interface to efficiently support query patterns such as
341 uint32_t BestWeight = 0;
342 uint32_t WeightScale = 0;
343 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
344 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
345 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
348 if (BlockFilter && !BlockFilter->count(*SI))
350 BlockChain &SuccChain = *BlockToChain[*SI];
351 if (&SuccChain == &Chain) {
352 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
355 if (*SI != *SuccChain.begin()) {
356 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
360 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
361 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
363 // Only consider successors which are either "hot", or wouldn't violate
364 // any CFG constraints.
365 if (SuccChain.LoopPredecessors != 0) {
366 if (SuccProb < HotProb) {
367 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
368 << " (prob) (CFG conflict)\n");
372 // Make sure that a hot successor doesn't have a globally more important
374 BlockFrequency CandidateEdgeFreq
375 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
376 bool BadCFGConflict = false;
377 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
378 PE = (*SI)->pred_end();
380 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
381 BlockToChain[*PI] == &Chain)
383 BlockFrequency PredEdgeFreq
384 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
385 if (PredEdgeFreq >= CandidateEdgeFreq) {
386 BadCFGConflict = true;
390 if (BadCFGConflict) {
391 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
392 << " (prob) (non-cold CFG conflict)\n");
397 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
399 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
401 if (BestSucc && BestWeight >= SuccWeight)
404 BestWeight = SuccWeight;
409 /// \brief Select the best block from a worklist.
411 /// This looks through the provided worklist as a list of candidate basic
412 /// blocks and select the most profitable one to place. The definition of
413 /// profitable only really makes sense in the context of a loop. This returns
414 /// the most frequently visited block in the worklist, which in the case of
415 /// a loop, is the one most desirable to be physically close to the rest of the
416 /// loop body in order to improve icache behavior.
418 /// \returns The best block found, or null if none are viable.
419 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
420 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
421 const BlockFilterSet *BlockFilter) {
422 // Once we need to walk the worklist looking for a candidate, cleanup the
423 // worklist of already placed entries.
424 // FIXME: If this shows up on profiles, it could be folded (at the cost of
425 // some code complexity) into the loop below.
426 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
427 [&](MachineBasicBlock *BB) {
428 return BlockToChain.lookup(BB) == &Chain;
432 MachineBasicBlock *BestBlock = nullptr;
433 BlockFrequency BestFreq;
434 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
435 WBE = WorkList.end();
437 BlockChain &SuccChain = *BlockToChain[*WBI];
438 if (&SuccChain == &Chain) {
439 DEBUG(dbgs() << " " << getBlockName(*WBI)
440 << " -> Already merged!\n");
443 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
445 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
446 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> ";
447 MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
448 if (BestBlock && BestFreq >= CandidateFreq)
451 BestFreq = CandidateFreq;
456 /// \brief Retrieve the first unplaced basic block.
458 /// This routine is called when we are unable to use the CFG to walk through
459 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
460 /// We walk through the function's blocks in order, starting from the
461 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
462 /// re-scanning the entire sequence on repeated calls to this routine.
463 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
464 MachineFunction &F, const BlockChain &PlacedChain,
465 MachineFunction::iterator &PrevUnplacedBlockIt,
466 const BlockFilterSet *BlockFilter) {
467 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
469 if (BlockFilter && !BlockFilter->count(I))
471 if (BlockToChain[I] != &PlacedChain) {
472 PrevUnplacedBlockIt = I;
473 // Now select the head of the chain to which the unplaced block belongs
474 // as the block to place. This will force the entire chain to be placed,
475 // and satisfies the requirements of merging chains.
476 return *BlockToChain[I]->begin();
482 void MachineBlockPlacement::buildChain(
483 MachineBasicBlock *BB,
485 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
486 const BlockFilterSet *BlockFilter) {
488 assert(BlockToChain[BB] == &Chain);
489 MachineFunction &F = *BB->getParent();
490 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
492 MachineBasicBlock *LoopHeaderBB = BB;
493 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
494 BB = *std::prev(Chain.end());
497 assert(BlockToChain[BB] == &Chain);
498 assert(*std::prev(Chain.end()) == BB);
500 // Look for the best viable successor if there is one to place immediately
502 MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
504 // If an immediate successor isn't available, look for the best viable
505 // block among those we've identified as not violating the loop's CFG at
506 // this point. This won't be a fallthrough, but it will increase locality.
508 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
511 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
516 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
517 "layout successor until the CFG reduces\n");
520 // Place this block, updating the datastructures to reflect its placement.
521 BlockChain &SuccChain = *BlockToChain[BestSucc];
522 // Zero out LoopPredecessors for the successor we're about to merge in case
523 // we selected a successor that didn't fit naturally into the CFG.
524 SuccChain.LoopPredecessors = 0;
525 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
526 << " to " << getBlockNum(BestSucc) << "\n");
527 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
528 Chain.merge(BestSucc, &SuccChain);
529 BB = *std::prev(Chain.end());
532 DEBUG(dbgs() << "Finished forming chain for header block "
533 << getBlockNum(*Chain.begin()) << "\n");
536 /// \brief Find the best loop top block for layout.
538 /// Look for a block which is strictly better than the loop header for laying
539 /// out at the top of the loop. This looks for one and only one pattern:
540 /// a latch block with no conditional exit. This block will cause a conditional
541 /// jump around it or will be the bottom of the loop if we lay it out in place,
542 /// but if it it doesn't end up at the bottom of the loop for any reason,
543 /// rotation alone won't fix it. Because such a block will always result in an
544 /// unconditional jump (for the backedge) rotating it in front of the loop
545 /// header is always profitable.
547 MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
548 const BlockFilterSet &LoopBlockSet) {
549 // Check that the header hasn't been fused with a preheader block due to
550 // crazy branches. If it has, we need to start with the header at the top to
551 // prevent pulling the preheader into the loop body.
552 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
553 if (!LoopBlockSet.count(*HeaderChain.begin()))
554 return L.getHeader();
556 DEBUG(dbgs() << "Finding best loop top for: "
557 << getBlockName(L.getHeader()) << "\n");
559 BlockFrequency BestPredFreq;
560 MachineBasicBlock *BestPred = nullptr;
561 for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
562 PE = L.getHeader()->pred_end();
564 MachineBasicBlock *Pred = *PI;
565 if (!LoopBlockSet.count(Pred))
567 DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
568 << Pred->succ_size() << " successors, ";
569 MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
570 if (Pred->succ_size() > 1)
573 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
574 if (!BestPred || PredFreq > BestPredFreq ||
575 (!(PredFreq < BestPredFreq) &&
576 Pred->isLayoutSuccessor(L.getHeader()))) {
578 BestPredFreq = PredFreq;
582 // If no direct predecessor is fine, just use the loop header.
584 return L.getHeader();
586 // Walk backwards through any straight line of predecessors.
587 while (BestPred->pred_size() == 1 &&
588 (*BestPred->pred_begin())->succ_size() == 1 &&
589 *BestPred->pred_begin() != L.getHeader())
590 BestPred = *BestPred->pred_begin();
592 DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
597 /// \brief Find the best loop exiting block for layout.
599 /// This routine implements the logic to analyze the loop looking for the best
600 /// block to layout at the top of the loop. Typically this is done to maximize
601 /// fallthrough opportunities.
603 MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
605 const BlockFilterSet &LoopBlockSet) {
606 // We don't want to layout the loop linearly in all cases. If the loop header
607 // is just a normal basic block in the loop, we want to look for what block
608 // within the loop is the best one to layout at the top. However, if the loop
609 // header has be pre-merged into a chain due to predecessors not having
610 // analyzable branches, *and* the predecessor it is merged with is *not* part
611 // of the loop, rotating the header into the middle of the loop will create
612 // a non-contiguous range of blocks which is Very Bad. So start with the
613 // header and only rotate if safe.
614 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
615 if (!LoopBlockSet.count(*HeaderChain.begin()))
618 BlockFrequency BestExitEdgeFreq;
619 unsigned BestExitLoopDepth = 0;
620 MachineBasicBlock *ExitingBB = nullptr;
621 // If there are exits to outer loops, loop rotation can severely limit
622 // fallthrough opportunites unless it selects such an exit. Keep a set of
623 // blocks where rotating to exit with that block will reach an outer loop.
624 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
626 DEBUG(dbgs() << "Finding best loop exit for: "
627 << getBlockName(L.getHeader()) << "\n");
628 for (MachineLoop::block_iterator I = L.block_begin(),
631 BlockChain &Chain = *BlockToChain[*I];
632 // Ensure that this block is at the end of a chain; otherwise it could be
633 // mid-way through an inner loop or a successor of an analyzable branch.
634 if (*I != *std::prev(Chain.end()))
637 // Now walk the successors. We need to establish whether this has a viable
638 // exiting successor and whether it has a viable non-exiting successor.
639 // We store the old exiting state and restore it if a viable looping
640 // successor isn't found.
641 MachineBasicBlock *OldExitingBB = ExitingBB;
642 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
643 bool HasLoopingSucc = false;
644 // FIXME: Due to the performance of the probability and weight routines in
645 // the MBPI analysis, we use the internal weights and manually compute the
646 // probabilities to avoid quadratic behavior.
647 uint32_t WeightScale = 0;
648 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
649 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
650 SE = (*I)->succ_end();
652 if ((*SI)->isLandingPad())
656 BlockChain &SuccChain = *BlockToChain[*SI];
657 // Don't split chains, either this chain or the successor's chain.
658 if (&Chain == &SuccChain) {
659 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
660 << getBlockName(*SI) << " (chain conflict)\n");
664 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
665 if (LoopBlockSet.count(*SI)) {
666 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
667 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
668 HasLoopingSucc = true;
672 unsigned SuccLoopDepth = 0;
673 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
674 SuccLoopDepth = ExitLoop->getLoopDepth();
675 if (ExitLoop->contains(&L))
676 BlocksExitingToOuterLoop.insert(*I);
679 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
680 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
681 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
682 << getBlockName(*SI) << " [L:" << SuccLoopDepth
684 MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
685 // Note that we bias this toward an existing layout successor to retain
686 // incoming order in the absence of better information. The exit must have
687 // a frequency higher than the current exit before we consider breaking
689 BranchProbability Bias(100 - ExitBlockBias, 100);
690 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
691 ExitEdgeFreq > BestExitEdgeFreq ||
692 ((*I)->isLayoutSuccessor(*SI) &&
693 !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
694 BestExitEdgeFreq = ExitEdgeFreq;
699 // Restore the old exiting state, no viable looping successor was found.
700 if (!HasLoopingSucc) {
701 ExitingBB = OldExitingBB;
702 BestExitEdgeFreq = OldBestExitEdgeFreq;
706 // Without a candidate exiting block or with only a single block in the
707 // loop, just use the loop header to layout the loop.
708 if (!ExitingBB || L.getNumBlocks() == 1)
711 // Also, if we have exit blocks which lead to outer loops but didn't select
712 // one of them as the exiting block we are rotating toward, disable loop
713 // rotation altogether.
714 if (!BlocksExitingToOuterLoop.empty() &&
715 !BlocksExitingToOuterLoop.count(ExitingBB))
718 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
722 /// \brief Attempt to rotate an exiting block to the bottom of the loop.
724 /// Once we have built a chain, try to rotate it to line up the hot exit block
725 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary
726 /// branches. For example, if the loop has fallthrough into its header and out
727 /// of its bottom already, don't rotate it.
728 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
729 MachineBasicBlock *ExitingBB,
730 const BlockFilterSet &LoopBlockSet) {
734 MachineBasicBlock *Top = *LoopChain.begin();
735 bool ViableTopFallthrough = false;
736 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
737 PE = Top->pred_end();
739 BlockChain *PredChain = BlockToChain[*PI];
740 if (!LoopBlockSet.count(*PI) &&
741 (!PredChain || *PI == *std::prev(PredChain->end()))) {
742 ViableTopFallthrough = true;
747 // If the header has viable fallthrough, check whether the current loop
748 // bottom is a viable exiting block. If so, bail out as rotating will
749 // introduce an unnecessary branch.
750 if (ViableTopFallthrough) {
751 MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
752 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
753 SE = Bottom->succ_end();
755 BlockChain *SuccChain = BlockToChain[*SI];
756 if (!LoopBlockSet.count(*SI) &&
757 (!SuccChain || *SI == *SuccChain->begin()))
762 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
764 if (ExitIt == LoopChain.end())
767 std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
770 /// \brief Forms basic block chains from the natural loop structures.
772 /// These chains are designed to preserve the existing *structure* of the code
773 /// as much as possible. We can then stitch the chains together in a way which
774 /// both preserves the topological structure and minimizes taken conditional
776 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
778 // First recurse through any nested loops, building chains for those inner
780 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
781 buildLoopChains(F, **LI);
783 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
784 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
786 // First check to see if there is an obviously preferable top block for the
787 // loop. This will default to the header, but may end up as one of the
788 // predecessors to the header if there is one which will result in strictly
789 // fewer branches in the loop body.
790 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
792 // If we selected just the header for the loop top, look for a potentially
793 // profitable exit block in the event that rotating the loop can eliminate
794 // branches by placing an exit edge at the bottom.
795 MachineBasicBlock *ExitingBB = nullptr;
796 if (LoopTop == L.getHeader())
797 ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
799 BlockChain &LoopChain = *BlockToChain[LoopTop];
801 // FIXME: This is a really lame way of walking the chains in the loop: we
802 // walk the blocks, and use a set to prevent visiting a particular chain
804 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
805 assert(LoopChain.LoopPredecessors == 0);
806 UpdatedPreds.insert(&LoopChain);
807 for (MachineLoop::block_iterator BI = L.block_begin(),
810 BlockChain &Chain = *BlockToChain[*BI];
811 if (!UpdatedPreds.insert(&Chain))
814 assert(Chain.LoopPredecessors == 0);
815 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
817 assert(BlockToChain[*BCI] == &Chain);
818 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
819 PE = (*BCI)->pred_end();
821 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
823 ++Chain.LoopPredecessors;
827 if (Chain.LoopPredecessors == 0)
828 BlockWorkList.push_back(*Chain.begin());
831 buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
832 rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
835 // Crash at the end so we get all of the debugging output first.
836 bool BadLoop = false;
837 if (LoopChain.LoopPredecessors) {
839 dbgs() << "Loop chain contains a block without its preds placed!\n"
840 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
841 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
843 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
845 dbgs() << " ... " << getBlockName(*BCI) << "\n";
846 if (!LoopBlockSet.erase(*BCI)) {
847 // We don't mark the loop as bad here because there are real situations
848 // where this can occur. For example, with an unanalyzable fallthrough
849 // from a loop block to a non-loop block or vice versa.
850 dbgs() << "Loop chain contains a block not contained by the loop!\n"
851 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
852 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
853 << " Bad block: " << getBlockName(*BCI) << "\n";
857 if (!LoopBlockSet.empty()) {
859 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
860 LBE = LoopBlockSet.end();
862 dbgs() << "Loop contains blocks never placed into a chain!\n"
863 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
864 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
865 << " Bad block: " << getBlockName(*LBI) << "\n";
867 assert(!BadLoop && "Detected problems with the placement of this loop.");
871 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
872 // Ensure that every BB in the function has an associated chain to simplify
873 // the assumptions of the remaining algorithm.
874 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
875 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
876 MachineBasicBlock *BB = FI;
878 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
879 // Also, merge any blocks which we cannot reason about and must preserve
880 // the exact fallthrough behavior for.
883 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
884 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
887 MachineFunction::iterator NextFI(std::next(FI));
888 MachineBasicBlock *NextBB = NextFI;
889 // Ensure that the layout successor is a viable block, as we know that
890 // fallthrough is a possibility.
891 assert(NextFI != FE && "Can't fallthrough past the last block.");
892 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
893 << getBlockName(BB) << " -> " << getBlockName(NextBB)
895 Chain->merge(NextBB, nullptr);
901 // Build any loop-based chains.
902 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
904 buildLoopChains(F, **LI);
906 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
908 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
909 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
910 MachineBasicBlock *BB = &*FI;
911 BlockChain &Chain = *BlockToChain[BB];
912 if (!UpdatedPreds.insert(&Chain))
915 assert(Chain.LoopPredecessors == 0);
916 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
918 assert(BlockToChain[*BCI] == &Chain);
919 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
920 PE = (*BCI)->pred_end();
922 if (BlockToChain[*PI] == &Chain)
924 ++Chain.LoopPredecessors;
928 if (Chain.LoopPredecessors == 0)
929 BlockWorkList.push_back(*Chain.begin());
932 BlockChain &FunctionChain = *BlockToChain[&F.front()];
933 buildChain(&F.front(), FunctionChain, BlockWorkList);
936 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
939 // Crash at the end so we get all of the debugging output first.
940 bool BadFunc = false;
941 FunctionBlockSetType FunctionBlockSet;
942 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
943 FunctionBlockSet.insert(FI);
945 for (BlockChain::iterator BCI = FunctionChain.begin(),
946 BCE = FunctionChain.end();
948 if (!FunctionBlockSet.erase(*BCI)) {
950 dbgs() << "Function chain contains a block not in the function!\n"
951 << " Bad block: " << getBlockName(*BCI) << "\n";
954 if (!FunctionBlockSet.empty()) {
956 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
957 FBE = FunctionBlockSet.end();
959 dbgs() << "Function contains blocks never placed into a chain!\n"
960 << " Bad block: " << getBlockName(*FBI) << "\n";
962 assert(!BadFunc && "Detected problems with the block placement.");
965 // Splice the blocks into place.
966 MachineFunction::iterator InsertPos = F.begin();
967 for (BlockChain::iterator BI = FunctionChain.begin(),
968 BE = FunctionChain.end();
970 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
972 << getBlockName(*BI) << "\n");
973 if (InsertPos != MachineFunction::iterator(*BI))
974 F.splice(InsertPos, *BI);
978 // Update the terminator of the previous block.
979 if (BI == FunctionChain.begin())
981 MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(*BI));
983 // FIXME: It would be awesome of updateTerminator would just return rather
984 // than assert when the branch cannot be analyzed in order to remove this
987 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
988 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
989 // The "PrevBB" is not yet updated to reflect current code layout, so,
990 // o. it may fall-through to a block without explict "goto" instruction
991 // before layout, and no longer fall-through it after layout; or
994 // AnalyzeBranch() may return erroneous value for FBB when these two
995 // situations take place. For the first scenario FBB is mistakenly set
996 // NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
997 // is mistakenly pointing to "*BI".
999 bool needUpdateBr = true;
1000 if (!Cond.empty() && (!FBB || FBB == *BI)) {
1001 PrevBB->updateTerminator();
1002 needUpdateBr = false;
1004 TBB = FBB = nullptr;
1005 if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
1006 // FIXME: This should never take place.
1007 TBB = FBB = nullptr;
1011 // If PrevBB has a two-way branch, try to re-order the branches
1012 // such that we branch to the successor with higher weight first.
1013 if (TBB && !Cond.empty() && FBB &&
1014 MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
1015 !TII->ReverseBranchCondition(Cond)) {
1016 DEBUG(dbgs() << "Reverse order of the two branches: "
1017 << getBlockName(PrevBB) << "\n");
1018 DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
1019 << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
1020 DebugLoc dl; // FIXME: this is nowhere
1021 TII->RemoveBranch(*PrevBB);
1022 TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
1023 needUpdateBr = true;
1026 PrevBB->updateTerminator();
1030 // Fixup the last block.
1032 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
1033 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
1034 F.back().updateTerminator();
1036 // Walk through the backedges of the function now that we have fully laid out
1037 // the basic blocks and align the destination of each backedge. We don't rely
1038 // exclusively on the loop info here so that we can align backedges in
1039 // unnatural CFGs and backedges that were introduced purely because of the
1040 // loop rotations done during this layout pass.
1041 if (F.getFunction()->getAttributes().
1042 hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize))
1044 unsigned Align = TLI->getPrefLoopAlignment();
1046 return; // Don't care about loop alignment.
1047 if (FunctionChain.begin() == FunctionChain.end())
1048 return; // Empty chain.
1050 const BranchProbability ColdProb(1, 5); // 20%
1051 BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
1052 BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
1053 for (BlockChain::iterator BI = std::next(FunctionChain.begin()),
1054 BE = FunctionChain.end();
1056 // Don't align non-looping basic blocks. These are unlikely to execute
1057 // enough times to matter in practice. Note that we'll still handle
1058 // unnatural CFGs inside of a natural outer loop (the common case) and
1060 MachineLoop *L = MLI->getLoopFor(*BI);
1064 // If the block is cold relative to the function entry don't waste space
1066 BlockFrequency Freq = MBFI->getBlockFreq(*BI);
1067 if (Freq < WeightedEntryFreq)
1070 // If the block is cold relative to its loop header, don't align it
1071 // regardless of what edges into the block exist.
1072 MachineBasicBlock *LoopHeader = L->getHeader();
1073 BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
1074 if (Freq < (LoopHeaderFreq * ColdProb))
1077 // Check for the existence of a non-layout predecessor which would benefit
1078 // from aligning this block.
1079 MachineBasicBlock *LayoutPred = *std::prev(BI);
1081 // Force alignment if all the predecessors are jumps. We already checked
1082 // that the block isn't cold above.
1083 if (!LayoutPred->isSuccessor(*BI)) {
1084 (*BI)->setAlignment(Align);
1088 // Align this block if the layout predecessor's edge into this block is
1089 // cold relative to the block. When this is true, other predecessors make up
1090 // all of the hot entries into the block and thus alignment is likely to be
1092 BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
1093 BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
1094 if (LayoutEdgeFreq <= (Freq * ColdProb))
1095 (*BI)->setAlignment(Align);
1099 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
1100 // Check for single-block functions and skip them.
1101 if (std::next(F.begin()) == F.end())
1104 if (skipOptnoneFunction(*F.getFunction()))
1107 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1108 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1109 MLI = &getAnalysis<MachineLoopInfo>();
1110 TII = F.getTarget().getInstrInfo();
1111 TLI = F.getTarget().getTargetLowering();
1112 assert(BlockToChain.empty());
1116 BlockToChain.clear();
1117 ChainAllocator.DestroyAll();
1120 // Align all of the blocks in the function to a specific alignment.
1121 for (MachineFunction::iterator FI = F.begin(), FE = F.end();
1123 FI->setAlignment(AlignAllBlock);
1125 // We always return true as we have no way to track whether the final order
1126 // differs from the original order.
1131 /// \brief A pass to compute block placement statistics.
1133 /// A separate pass to compute interesting statistics for evaluating block
1134 /// placement. This is separate from the actual placement pass so that they can
1135 /// be computed in the absence of any placement transformations or when using
1136 /// alternative placement strategies.
1137 class MachineBlockPlacementStats : public MachineFunctionPass {
1138 /// \brief A handle to the branch probability pass.
1139 const MachineBranchProbabilityInfo *MBPI;
1141 /// \brief A handle to the function-wide block frequency pass.
1142 const MachineBlockFrequencyInfo *MBFI;
1145 static char ID; // Pass identification, replacement for typeid
1146 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
1147 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
1150 bool runOnMachineFunction(MachineFunction &F) override;
1152 void getAnalysisUsage(AnalysisUsage &AU) const override {
1153 AU.addRequired<MachineBranchProbabilityInfo>();
1154 AU.addRequired<MachineBlockFrequencyInfo>();
1155 AU.setPreservesAll();
1156 MachineFunctionPass::getAnalysisUsage(AU);
1161 char MachineBlockPlacementStats::ID = 0;
1162 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
1163 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
1164 "Basic Block Placement Stats", false, false)
1165 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
1166 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
1167 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
1168 "Basic Block Placement Stats", false, false)
1170 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
1171 // Check for single-block functions and skip them.
1172 if (std::next(F.begin()) == F.end())
1175 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1176 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1178 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
1179 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
1180 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
1181 : NumUncondBranches;
1182 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
1183 : UncondBranchTakenFreq;
1184 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
1187 // Skip if this successor is a fallthrough.
1188 if (I->isLayoutSuccessor(*SI))
1191 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
1193 BranchTakenFreq += EdgeFreq.getFrequency();