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 raw_string_ostream OS(Result);
269 OS << "BB#" << BB->getNumber()
270 << " (derived from LLVM BB '" << BB->getName() << "')";
275 /// \brief Helper to print the number of a MBB.
277 /// Only used by debug logging.
278 static std::string getBlockNum(MachineBasicBlock *BB) {
280 raw_string_ostream OS(Result);
281 OS << "BB#" << BB->getNumber();
287 /// \brief Mark a chain's successors as having one fewer preds.
289 /// When a chain is being merged into the "placed" chain, this routine will
290 /// quickly walk the successors of each block in the chain and mark them as
291 /// having one fewer active predecessor. It also adds any successors of this
292 /// chain which reach the zero-predecessor state to the worklist passed in.
293 void MachineBlockPlacement::markChainSuccessors(
295 MachineBasicBlock *LoopHeaderBB,
296 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
297 const BlockFilterSet *BlockFilter) {
298 // Walk all the blocks in this chain, marking their successors as having
299 // a predecessor placed.
300 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
302 // Add any successors for which this is the only un-placed in-loop
303 // predecessor to the worklist as a viable candidate for CFG-neutral
304 // placement. No subsequent placement of this block will violate the CFG
305 // shape, so we get to use heuristics to choose a favorable placement.
306 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
307 SE = (*CBI)->succ_end();
309 if (BlockFilter && !BlockFilter->count(*SI))
311 BlockChain &SuccChain = *BlockToChain[*SI];
312 // Disregard edges within a fixed chain, or edges to the loop header.
313 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
316 // This is a cross-chain edge that is within the loop, so decrement the
317 // loop predecessor count of the destination chain.
318 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
319 BlockWorkList.push_back(*SuccChain.begin());
324 /// \brief Select the best successor for a block.
326 /// This looks across all successors of a particular block and attempts to
327 /// select the "best" one to be the layout successor. It only considers direct
328 /// successors which also pass the block filter. It will attempt to avoid
329 /// breaking CFG structure, but cave and break such structures in the case of
330 /// very hot successor edges.
332 /// \returns The best successor block found, or null if none are viable.
333 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
334 MachineBasicBlock *BB, BlockChain &Chain,
335 const BlockFilterSet *BlockFilter) {
336 const BranchProbability HotProb(4, 5); // 80%
338 MachineBasicBlock *BestSucc = nullptr;
339 // FIXME: Due to the performance of the probability and weight routines in
340 // the MBPI analysis, we manually compute probabilities using the edge
341 // weights. This is suboptimal as it means that the somewhat subtle
342 // definition of edge weight semantics is encoded here as well. We should
343 // improve the MBPI interface to efficiently support query patterns such as
345 uint32_t BestWeight = 0;
346 uint32_t WeightScale = 0;
347 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
348 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
349 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
352 if (BlockFilter && !BlockFilter->count(*SI))
354 BlockChain &SuccChain = *BlockToChain[*SI];
355 if (&SuccChain == &Chain) {
356 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
359 if (*SI != *SuccChain.begin()) {
360 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
364 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
365 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
367 // Only consider successors which are either "hot", or wouldn't violate
368 // any CFG constraints.
369 if (SuccChain.LoopPredecessors != 0) {
370 if (SuccProb < HotProb) {
371 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
372 << " (prob) (CFG conflict)\n");
376 // Make sure that a hot successor doesn't have a globally more important
378 BlockFrequency CandidateEdgeFreq
379 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
380 bool BadCFGConflict = false;
381 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
382 PE = (*SI)->pred_end();
384 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
385 BlockToChain[*PI] == &Chain)
387 BlockFrequency PredEdgeFreq
388 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
389 if (PredEdgeFreq >= CandidateEdgeFreq) {
390 BadCFGConflict = true;
394 if (BadCFGConflict) {
395 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
396 << " (prob) (non-cold CFG conflict)\n");
401 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
403 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
405 if (BestSucc && BestWeight >= SuccWeight)
408 BestWeight = SuccWeight;
413 /// \brief Select the best block from a worklist.
415 /// This looks through the provided worklist as a list of candidate basic
416 /// blocks and select the most profitable one to place. The definition of
417 /// profitable only really makes sense in the context of a loop. This returns
418 /// the most frequently visited block in the worklist, which in the case of
419 /// a loop, is the one most desirable to be physically close to the rest of the
420 /// loop body in order to improve icache behavior.
422 /// \returns The best block found, or null if none are viable.
423 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
424 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
425 const BlockFilterSet *BlockFilter) {
426 // Once we need to walk the worklist looking for a candidate, cleanup the
427 // worklist of already placed entries.
428 // FIXME: If this shows up on profiles, it could be folded (at the cost of
429 // some code complexity) into the loop below.
430 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
431 [&](MachineBasicBlock *BB) {
432 return BlockToChain.lookup(BB) == &Chain;
436 MachineBasicBlock *BestBlock = nullptr;
437 BlockFrequency BestFreq;
438 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
439 WBE = WorkList.end();
441 BlockChain &SuccChain = *BlockToChain[*WBI];
442 if (&SuccChain == &Chain) {
443 DEBUG(dbgs() << " " << getBlockName(*WBI)
444 << " -> Already merged!\n");
447 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
449 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
450 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> ";
451 MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
452 if (BestBlock && BestFreq >= CandidateFreq)
455 BestFreq = CandidateFreq;
460 /// \brief Retrieve the first unplaced basic block.
462 /// This routine is called when we are unable to use the CFG to walk through
463 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
464 /// We walk through the function's blocks in order, starting from the
465 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
466 /// re-scanning the entire sequence on repeated calls to this routine.
467 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
468 MachineFunction &F, const BlockChain &PlacedChain,
469 MachineFunction::iterator &PrevUnplacedBlockIt,
470 const BlockFilterSet *BlockFilter) {
471 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
473 if (BlockFilter && !BlockFilter->count(I))
475 if (BlockToChain[I] != &PlacedChain) {
476 PrevUnplacedBlockIt = I;
477 // Now select the head of the chain to which the unplaced block belongs
478 // as the block to place. This will force the entire chain to be placed,
479 // and satisfies the requirements of merging chains.
480 return *BlockToChain[I]->begin();
486 void MachineBlockPlacement::buildChain(
487 MachineBasicBlock *BB,
489 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
490 const BlockFilterSet *BlockFilter) {
492 assert(BlockToChain[BB] == &Chain);
493 MachineFunction &F = *BB->getParent();
494 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
496 MachineBasicBlock *LoopHeaderBB = BB;
497 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
498 BB = *std::prev(Chain.end());
501 assert(BlockToChain[BB] == &Chain);
502 assert(*std::prev(Chain.end()) == BB);
504 // Look for the best viable successor if there is one to place immediately
506 MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
508 // If an immediate successor isn't available, look for the best viable
509 // block among those we've identified as not violating the loop's CFG at
510 // this point. This won't be a fallthrough, but it will increase locality.
512 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
515 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
520 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
521 "layout successor until the CFG reduces\n");
524 // Place this block, updating the datastructures to reflect its placement.
525 BlockChain &SuccChain = *BlockToChain[BestSucc];
526 // Zero out LoopPredecessors for the successor we're about to merge in case
527 // we selected a successor that didn't fit naturally into the CFG.
528 SuccChain.LoopPredecessors = 0;
529 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
530 << " to " << getBlockNum(BestSucc) << "\n");
531 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
532 Chain.merge(BestSucc, &SuccChain);
533 BB = *std::prev(Chain.end());
536 DEBUG(dbgs() << "Finished forming chain for header block "
537 << getBlockNum(*Chain.begin()) << "\n");
540 /// \brief Find the best loop top block for layout.
542 /// Look for a block which is strictly better than the loop header for laying
543 /// out at the top of the loop. This looks for one and only one pattern:
544 /// a latch block with no conditional exit. This block will cause a conditional
545 /// jump around it or will be the bottom of the loop if we lay it out in place,
546 /// but if it it doesn't end up at the bottom of the loop for any reason,
547 /// rotation alone won't fix it. Because such a block will always result in an
548 /// unconditional jump (for the backedge) rotating it in front of the loop
549 /// header is always profitable.
551 MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
552 const BlockFilterSet &LoopBlockSet) {
553 // Check that the header hasn't been fused with a preheader block due to
554 // crazy branches. If it has, we need to start with the header at the top to
555 // prevent pulling the preheader into the loop body.
556 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
557 if (!LoopBlockSet.count(*HeaderChain.begin()))
558 return L.getHeader();
560 DEBUG(dbgs() << "Finding best loop top for: "
561 << getBlockName(L.getHeader()) << "\n");
563 BlockFrequency BestPredFreq;
564 MachineBasicBlock *BestPred = nullptr;
565 for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
566 PE = L.getHeader()->pred_end();
568 MachineBasicBlock *Pred = *PI;
569 if (!LoopBlockSet.count(Pred))
571 DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
572 << Pred->succ_size() << " successors, ";
573 MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
574 if (Pred->succ_size() > 1)
577 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
578 if (!BestPred || PredFreq > BestPredFreq ||
579 (!(PredFreq < BestPredFreq) &&
580 Pred->isLayoutSuccessor(L.getHeader()))) {
582 BestPredFreq = PredFreq;
586 // If no direct predecessor is fine, just use the loop header.
588 return L.getHeader();
590 // Walk backwards through any straight line of predecessors.
591 while (BestPred->pred_size() == 1 &&
592 (*BestPred->pred_begin())->succ_size() == 1 &&
593 *BestPred->pred_begin() != L.getHeader())
594 BestPred = *BestPred->pred_begin();
596 DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
601 /// \brief Find the best loop exiting block for layout.
603 /// This routine implements the logic to analyze the loop looking for the best
604 /// block to layout at the top of the loop. Typically this is done to maximize
605 /// fallthrough opportunities.
607 MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
609 const BlockFilterSet &LoopBlockSet) {
610 // We don't want to layout the loop linearly in all cases. If the loop header
611 // is just a normal basic block in the loop, we want to look for what block
612 // within the loop is the best one to layout at the top. However, if the loop
613 // header has be pre-merged into a chain due to predecessors not having
614 // analyzable branches, *and* the predecessor it is merged with is *not* part
615 // of the loop, rotating the header into the middle of the loop will create
616 // a non-contiguous range of blocks which is Very Bad. So start with the
617 // header and only rotate if safe.
618 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
619 if (!LoopBlockSet.count(*HeaderChain.begin()))
622 BlockFrequency BestExitEdgeFreq;
623 unsigned BestExitLoopDepth = 0;
624 MachineBasicBlock *ExitingBB = nullptr;
625 // If there are exits to outer loops, loop rotation can severely limit
626 // fallthrough opportunites unless it selects such an exit. Keep a set of
627 // blocks where rotating to exit with that block will reach an outer loop.
628 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
630 DEBUG(dbgs() << "Finding best loop exit for: "
631 << getBlockName(L.getHeader()) << "\n");
632 for (MachineLoop::block_iterator I = L.block_begin(),
635 BlockChain &Chain = *BlockToChain[*I];
636 // Ensure that this block is at the end of a chain; otherwise it could be
637 // mid-way through an inner loop or a successor of an analyzable branch.
638 if (*I != *std::prev(Chain.end()))
641 // Now walk the successors. We need to establish whether this has a viable
642 // exiting successor and whether it has a viable non-exiting successor.
643 // We store the old exiting state and restore it if a viable looping
644 // successor isn't found.
645 MachineBasicBlock *OldExitingBB = ExitingBB;
646 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
647 bool HasLoopingSucc = false;
648 // FIXME: Due to the performance of the probability and weight routines in
649 // the MBPI analysis, we use the internal weights and manually compute the
650 // probabilities to avoid quadratic behavior.
651 uint32_t WeightScale = 0;
652 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
653 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
654 SE = (*I)->succ_end();
656 if ((*SI)->isLandingPad())
660 BlockChain &SuccChain = *BlockToChain[*SI];
661 // Don't split chains, either this chain or the successor's chain.
662 if (&Chain == &SuccChain) {
663 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
664 << getBlockName(*SI) << " (chain conflict)\n");
668 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
669 if (LoopBlockSet.count(*SI)) {
670 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
671 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
672 HasLoopingSucc = true;
676 unsigned SuccLoopDepth = 0;
677 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
678 SuccLoopDepth = ExitLoop->getLoopDepth();
679 if (ExitLoop->contains(&L))
680 BlocksExitingToOuterLoop.insert(*I);
683 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
684 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
685 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
686 << getBlockName(*SI) << " [L:" << SuccLoopDepth
688 MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
689 // Note that we bias this toward an existing layout successor to retain
690 // incoming order in the absence of better information. The exit must have
691 // a frequency higher than the current exit before we consider breaking
693 BranchProbability Bias(100 - ExitBlockBias, 100);
694 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
695 ExitEdgeFreq > BestExitEdgeFreq ||
696 ((*I)->isLayoutSuccessor(*SI) &&
697 !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
698 BestExitEdgeFreq = ExitEdgeFreq;
703 // Restore the old exiting state, no viable looping successor was found.
704 if (!HasLoopingSucc) {
705 ExitingBB = OldExitingBB;
706 BestExitEdgeFreq = OldBestExitEdgeFreq;
710 // Without a candidate exiting block or with only a single block in the
711 // loop, just use the loop header to layout the loop.
712 if (!ExitingBB || L.getNumBlocks() == 1)
715 // Also, if we have exit blocks which lead to outer loops but didn't select
716 // one of them as the exiting block we are rotating toward, disable loop
717 // rotation altogether.
718 if (!BlocksExitingToOuterLoop.empty() &&
719 !BlocksExitingToOuterLoop.count(ExitingBB))
722 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
726 /// \brief Attempt to rotate an exiting block to the bottom of the loop.
728 /// Once we have built a chain, try to rotate it to line up the hot exit block
729 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary
730 /// branches. For example, if the loop has fallthrough into its header and out
731 /// of its bottom already, don't rotate it.
732 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
733 MachineBasicBlock *ExitingBB,
734 const BlockFilterSet &LoopBlockSet) {
738 MachineBasicBlock *Top = *LoopChain.begin();
739 bool ViableTopFallthrough = false;
740 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
741 PE = Top->pred_end();
743 BlockChain *PredChain = BlockToChain[*PI];
744 if (!LoopBlockSet.count(*PI) &&
745 (!PredChain || *PI == *std::prev(PredChain->end()))) {
746 ViableTopFallthrough = true;
751 // If the header has viable fallthrough, check whether the current loop
752 // bottom is a viable exiting block. If so, bail out as rotating will
753 // introduce an unnecessary branch.
754 if (ViableTopFallthrough) {
755 MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
756 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
757 SE = Bottom->succ_end();
759 BlockChain *SuccChain = BlockToChain[*SI];
760 if (!LoopBlockSet.count(*SI) &&
761 (!SuccChain || *SI == *SuccChain->begin()))
766 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
768 if (ExitIt == LoopChain.end())
771 std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
774 /// \brief Forms basic block chains from the natural loop structures.
776 /// These chains are designed to preserve the existing *structure* of the code
777 /// as much as possible. We can then stitch the chains together in a way which
778 /// both preserves the topological structure and minimizes taken conditional
780 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
782 // First recurse through any nested loops, building chains for those inner
784 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
785 buildLoopChains(F, **LI);
787 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
788 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
790 // First check to see if there is an obviously preferable top block for the
791 // loop. This will default to the header, but may end up as one of the
792 // predecessors to the header if there is one which will result in strictly
793 // fewer branches in the loop body.
794 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
796 // If we selected just the header for the loop top, look for a potentially
797 // profitable exit block in the event that rotating the loop can eliminate
798 // branches by placing an exit edge at the bottom.
799 MachineBasicBlock *ExitingBB = nullptr;
800 if (LoopTop == L.getHeader())
801 ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
803 BlockChain &LoopChain = *BlockToChain[LoopTop];
805 // FIXME: This is a really lame way of walking the chains in the loop: we
806 // walk the blocks, and use a set to prevent visiting a particular chain
808 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
809 assert(LoopChain.LoopPredecessors == 0);
810 UpdatedPreds.insert(&LoopChain);
811 for (MachineLoop::block_iterator BI = L.block_begin(),
814 BlockChain &Chain = *BlockToChain[*BI];
815 if (!UpdatedPreds.insert(&Chain))
818 assert(Chain.LoopPredecessors == 0);
819 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
821 assert(BlockToChain[*BCI] == &Chain);
822 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
823 PE = (*BCI)->pred_end();
825 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
827 ++Chain.LoopPredecessors;
831 if (Chain.LoopPredecessors == 0)
832 BlockWorkList.push_back(*Chain.begin());
835 buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
836 rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
839 // Crash at the end so we get all of the debugging output first.
840 bool BadLoop = false;
841 if (LoopChain.LoopPredecessors) {
843 dbgs() << "Loop chain contains a block without its preds placed!\n"
844 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
845 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
847 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
849 dbgs() << " ... " << getBlockName(*BCI) << "\n";
850 if (!LoopBlockSet.erase(*BCI)) {
851 // We don't mark the loop as bad here because there are real situations
852 // where this can occur. For example, with an unanalyzable fallthrough
853 // from a loop block to a non-loop block or vice versa.
854 dbgs() << "Loop chain contains a block not contained by the loop!\n"
855 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
856 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
857 << " Bad block: " << getBlockName(*BCI) << "\n";
861 if (!LoopBlockSet.empty()) {
863 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
864 LBE = LoopBlockSet.end();
866 dbgs() << "Loop contains blocks never placed into a chain!\n"
867 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
868 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
869 << " Bad block: " << getBlockName(*LBI) << "\n";
871 assert(!BadLoop && "Detected problems with the placement of this loop.");
875 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
876 // Ensure that every BB in the function has an associated chain to simplify
877 // the assumptions of the remaining algorithm.
878 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
879 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
880 MachineBasicBlock *BB = FI;
882 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
883 // Also, merge any blocks which we cannot reason about and must preserve
884 // the exact fallthrough behavior for.
887 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
888 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
891 MachineFunction::iterator NextFI(std::next(FI));
892 MachineBasicBlock *NextBB = NextFI;
893 // Ensure that the layout successor is a viable block, as we know that
894 // fallthrough is a possibility.
895 assert(NextFI != FE && "Can't fallthrough past the last block.");
896 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
897 << getBlockName(BB) << " -> " << getBlockName(NextBB)
899 Chain->merge(NextBB, nullptr);
905 // Build any loop-based chains.
906 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
908 buildLoopChains(F, **LI);
910 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
912 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
913 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
914 MachineBasicBlock *BB = &*FI;
915 BlockChain &Chain = *BlockToChain[BB];
916 if (!UpdatedPreds.insert(&Chain))
919 assert(Chain.LoopPredecessors == 0);
920 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
922 assert(BlockToChain[*BCI] == &Chain);
923 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
924 PE = (*BCI)->pred_end();
926 if (BlockToChain[*PI] == &Chain)
928 ++Chain.LoopPredecessors;
932 if (Chain.LoopPredecessors == 0)
933 BlockWorkList.push_back(*Chain.begin());
936 BlockChain &FunctionChain = *BlockToChain[&F.front()];
937 buildChain(&F.front(), FunctionChain, BlockWorkList);
940 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
943 // Crash at the end so we get all of the debugging output first.
944 bool BadFunc = false;
945 FunctionBlockSetType FunctionBlockSet;
946 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
947 FunctionBlockSet.insert(FI);
949 for (BlockChain::iterator BCI = FunctionChain.begin(),
950 BCE = FunctionChain.end();
952 if (!FunctionBlockSet.erase(*BCI)) {
954 dbgs() << "Function chain contains a block not in the function!\n"
955 << " Bad block: " << getBlockName(*BCI) << "\n";
958 if (!FunctionBlockSet.empty()) {
960 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
961 FBE = FunctionBlockSet.end();
963 dbgs() << "Function contains blocks never placed into a chain!\n"
964 << " Bad block: " << getBlockName(*FBI) << "\n";
966 assert(!BadFunc && "Detected problems with the block placement.");
969 // Splice the blocks into place.
970 MachineFunction::iterator InsertPos = F.begin();
971 for (BlockChain::iterator BI = FunctionChain.begin(),
972 BE = FunctionChain.end();
974 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
976 << getBlockName(*BI) << "\n");
977 if (InsertPos != MachineFunction::iterator(*BI))
978 F.splice(InsertPos, *BI);
982 // Update the terminator of the previous block.
983 if (BI == FunctionChain.begin())
985 MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(*BI));
987 // FIXME: It would be awesome of updateTerminator would just return rather
988 // than assert when the branch cannot be analyzed in order to remove this
991 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
992 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
993 // The "PrevBB" is not yet updated to reflect current code layout, so,
994 // o. it may fall-through to a block without explict "goto" instruction
995 // before layout, and no longer fall-through it after layout; or
998 // AnalyzeBranch() may return erroneous value for FBB when these two
999 // situations take place. For the first scenario FBB is mistakenly set
1000 // NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
1001 // is mistakenly pointing to "*BI".
1003 bool needUpdateBr = true;
1004 if (!Cond.empty() && (!FBB || FBB == *BI)) {
1005 PrevBB->updateTerminator();
1006 needUpdateBr = false;
1008 TBB = FBB = nullptr;
1009 if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
1010 // FIXME: This should never take place.
1011 TBB = FBB = nullptr;
1015 // If PrevBB has a two-way branch, try to re-order the branches
1016 // such that we branch to the successor with higher weight first.
1017 if (TBB && !Cond.empty() && FBB &&
1018 MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
1019 !TII->ReverseBranchCondition(Cond)) {
1020 DEBUG(dbgs() << "Reverse order of the two branches: "
1021 << getBlockName(PrevBB) << "\n");
1022 DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
1023 << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
1024 DebugLoc dl; // FIXME: this is nowhere
1025 TII->RemoveBranch(*PrevBB);
1026 TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
1027 needUpdateBr = true;
1030 PrevBB->updateTerminator();
1034 // Fixup the last block.
1036 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
1037 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
1038 F.back().updateTerminator();
1040 // Walk through the backedges of the function now that we have fully laid out
1041 // the basic blocks and align the destination of each backedge. We don't rely
1042 // exclusively on the loop info here so that we can align backedges in
1043 // unnatural CFGs and backedges that were introduced purely because of the
1044 // loop rotations done during this layout pass.
1045 if (F.getFunction()->getAttributes().
1046 hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize))
1048 unsigned Align = TLI->getPrefLoopAlignment();
1050 return; // Don't care about loop alignment.
1051 if (FunctionChain.begin() == FunctionChain.end())
1052 return; // Empty chain.
1054 const BranchProbability ColdProb(1, 5); // 20%
1055 BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
1056 BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
1057 for (BlockChain::iterator BI = std::next(FunctionChain.begin()),
1058 BE = FunctionChain.end();
1060 // Don't align non-looping basic blocks. These are unlikely to execute
1061 // enough times to matter in practice. Note that we'll still handle
1062 // unnatural CFGs inside of a natural outer loop (the common case) and
1064 MachineLoop *L = MLI->getLoopFor(*BI);
1068 // If the block is cold relative to the function entry don't waste space
1070 BlockFrequency Freq = MBFI->getBlockFreq(*BI);
1071 if (Freq < WeightedEntryFreq)
1074 // If the block is cold relative to its loop header, don't align it
1075 // regardless of what edges into the block exist.
1076 MachineBasicBlock *LoopHeader = L->getHeader();
1077 BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
1078 if (Freq < (LoopHeaderFreq * ColdProb))
1081 // Check for the existence of a non-layout predecessor which would benefit
1082 // from aligning this block.
1083 MachineBasicBlock *LayoutPred = *std::prev(BI);
1085 // Force alignment if all the predecessors are jumps. We already checked
1086 // that the block isn't cold above.
1087 if (!LayoutPred->isSuccessor(*BI)) {
1088 (*BI)->setAlignment(Align);
1092 // Align this block if the layout predecessor's edge into this block is
1093 // cold relative to the block. When this is true, other predecessors make up
1094 // all of the hot entries into the block and thus alignment is likely to be
1096 BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
1097 BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
1098 if (LayoutEdgeFreq <= (Freq * ColdProb))
1099 (*BI)->setAlignment(Align);
1103 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
1104 // Check for single-block functions and skip them.
1105 if (std::next(F.begin()) == F.end())
1108 if (skipOptnoneFunction(*F.getFunction()))
1111 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1112 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1113 MLI = &getAnalysis<MachineLoopInfo>();
1114 TII = F.getTarget().getInstrInfo();
1115 TLI = F.getTarget().getTargetLowering();
1116 assert(BlockToChain.empty());
1120 BlockToChain.clear();
1121 ChainAllocator.DestroyAll();
1124 // Align all of the blocks in the function to a specific alignment.
1125 for (MachineFunction::iterator FI = F.begin(), FE = F.end();
1127 FI->setAlignment(AlignAllBlock);
1129 // We always return true as we have no way to track whether the final order
1130 // differs from the original order.
1135 /// \brief A pass to compute block placement statistics.
1137 /// A separate pass to compute interesting statistics for evaluating block
1138 /// placement. This is separate from the actual placement pass so that they can
1139 /// be computed in the absence of any placement transformations or when using
1140 /// alternative placement strategies.
1141 class MachineBlockPlacementStats : public MachineFunctionPass {
1142 /// \brief A handle to the branch probability pass.
1143 const MachineBranchProbabilityInfo *MBPI;
1145 /// \brief A handle to the function-wide block frequency pass.
1146 const MachineBlockFrequencyInfo *MBFI;
1149 static char ID; // Pass identification, replacement for typeid
1150 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
1151 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
1154 bool runOnMachineFunction(MachineFunction &F) override;
1156 void getAnalysisUsage(AnalysisUsage &AU) const override {
1157 AU.addRequired<MachineBranchProbabilityInfo>();
1158 AU.addRequired<MachineBlockFrequencyInfo>();
1159 AU.setPreservesAll();
1160 MachineFunctionPass::getAnalysisUsage(AU);
1165 char MachineBlockPlacementStats::ID = 0;
1166 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
1167 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
1168 "Basic Block Placement Stats", false, false)
1169 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
1170 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
1171 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
1172 "Basic Block Placement Stats", false, false)
1174 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
1175 // Check for single-block functions and skip them.
1176 if (std::next(F.begin()) == F.end())
1179 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1180 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1182 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
1183 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
1184 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
1185 : NumUncondBranches;
1186 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
1187 : UncondBranchTakenFreq;
1188 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
1191 // Skip if this successor is a fallthrough.
1192 if (I->isLayoutSuccessor(*SI))
1195 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
1197 BranchTakenFreq += EdgeFreq.getFrequency();