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"
45 #include "llvm/Target/TargetSubtargetInfo.h"
49 #define DEBUG_TYPE "block-placement2"
51 STATISTIC(NumCondBranches, "Number of conditional branches");
52 STATISTIC(NumUncondBranches, "Number of uncondittional branches");
53 STATISTIC(CondBranchTakenFreq,
54 "Potential frequency of taking conditional branches");
55 STATISTIC(UncondBranchTakenFreq,
56 "Potential frequency of taking unconditional branches");
58 static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
59 cl::desc("Force the alignment of all "
60 "blocks in the function."),
61 cl::init(0), cl::Hidden);
63 // FIXME: Find a good default for this flag and remove the flag.
64 static cl::opt<unsigned>
65 ExitBlockBias("block-placement-exit-block-bias",
66 cl::desc("Block frequency percentage a loop exit block needs "
67 "over the original exit to be considered the new exit."),
68 cl::init(0), cl::Hidden);
72 /// \brief Type for our function-wide basic block -> block chain mapping.
73 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
77 /// \brief A chain of blocks which will be laid out contiguously.
79 /// This is the datastructure representing a chain of consecutive blocks that
80 /// are profitable to layout together in order to maximize fallthrough
81 /// probabilities and code locality. We also can use a block chain to represent
82 /// a sequence of basic blocks which have some external (correctness)
83 /// requirement for sequential layout.
85 /// Chains can be built around a single basic block and can be merged to grow
86 /// them. They participate in a block-to-chain mapping, which is updated
87 /// automatically as chains are merged together.
89 /// \brief The sequence of blocks belonging to this chain.
91 /// This is the sequence of blocks for a particular chain. These will be laid
92 /// out in-order within the function.
93 SmallVector<MachineBasicBlock *, 4> Blocks;
95 /// \brief A handle to the function-wide basic block to block chain mapping.
97 /// This is retained in each block chain to simplify the computation of child
98 /// block chains for SCC-formation and iteration. We store the edges to child
99 /// basic blocks, and map them back to their associated chains using this
101 BlockToChainMapType &BlockToChain;
104 /// \brief Construct a new BlockChain.
106 /// This builds a new block chain representing a single basic block in the
107 /// function. It also registers itself as the chain that block participates
108 /// in with the BlockToChain mapping.
109 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
110 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
111 assert(BB && "Cannot create a chain with a null basic block");
112 BlockToChain[BB] = this;
115 /// \brief Iterator over blocks within the chain.
116 typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
118 /// \brief Beginning of blocks within the chain.
119 iterator begin() { return Blocks.begin(); }
121 /// \brief End of blocks within the chain.
122 iterator end() { return Blocks.end(); }
124 /// \brief Merge a block chain into this one.
126 /// This routine merges a block chain into this one. It takes care of forming
127 /// a contiguous sequence of basic blocks, updating the edge list, and
128 /// updating the block -> chain mapping. It does not free or tear down the
129 /// old chain, but the old chain's block list is no longer valid.
130 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
132 assert(!Blocks.empty());
134 // Fast path in case we don't have a chain already.
136 assert(!BlockToChain[BB]);
137 Blocks.push_back(BB);
138 BlockToChain[BB] = this;
142 assert(BB == *Chain->begin());
143 assert(Chain->begin() != Chain->end());
145 // Update the incoming blocks to point to this chain, and add them to the
147 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
149 Blocks.push_back(*BI);
150 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
151 BlockToChain[*BI] = this;
156 /// \brief Dump the blocks in this chain.
157 LLVM_DUMP_METHOD void dump() {
158 for (iterator I = begin(), E = end(); I != E; ++I)
163 /// \brief Count of predecessors within the loop currently being processed.
165 /// This count is updated at each loop we process to represent the number of
166 /// in-loop predecessors of this chain.
167 unsigned LoopPredecessors;
172 class MachineBlockPlacement : public MachineFunctionPass {
173 /// \brief A typedef for a block filter set.
174 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
176 /// \brief A handle to the branch probability pass.
177 const MachineBranchProbabilityInfo *MBPI;
179 /// \brief A handle to the function-wide block frequency pass.
180 const MachineBlockFrequencyInfo *MBFI;
182 /// \brief A handle to the loop info.
183 const MachineLoopInfo *MLI;
185 /// \brief A handle to the target's instruction info.
186 const TargetInstrInfo *TII;
188 /// \brief A handle to the target's lowering info.
189 const TargetLoweringBase *TLI;
191 /// \brief Allocator and owner of BlockChain structures.
193 /// We build BlockChains lazily while processing the loop structure of
194 /// a function. To reduce malloc traffic, we allocate them using this
195 /// slab-like allocator, and destroy them after the pass completes. An
196 /// important guarantee is that this allocator produces stable pointers to
198 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
200 /// \brief Function wide BasicBlock to BlockChain mapping.
202 /// This mapping allows efficiently moving from any given basic block to the
203 /// BlockChain it participates in, if any. We use it to, among other things,
204 /// allow implicitly defining edges between chains as the existing edges
205 /// between basic blocks.
206 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
208 void markChainSuccessors(BlockChain &Chain,
209 MachineBasicBlock *LoopHeaderBB,
210 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
211 const BlockFilterSet *BlockFilter = nullptr);
212 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
214 const BlockFilterSet *BlockFilter);
215 MachineBasicBlock *selectBestCandidateBlock(
216 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
217 const BlockFilterSet *BlockFilter);
218 MachineBasicBlock *getFirstUnplacedBlock(
220 const BlockChain &PlacedChain,
221 MachineFunction::iterator &PrevUnplacedBlockIt,
222 const BlockFilterSet *BlockFilter);
223 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
224 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
225 const BlockFilterSet *BlockFilter = nullptr);
226 MachineBasicBlock *findBestLoopTop(MachineLoop &L,
227 const BlockFilterSet &LoopBlockSet);
228 MachineBasicBlock *findBestLoopExit(MachineFunction &F,
230 const BlockFilterSet &LoopBlockSet);
231 void buildLoopChains(MachineFunction &F, MachineLoop &L);
232 void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
233 const BlockFilterSet &LoopBlockSet);
234 void buildCFGChains(MachineFunction &F);
237 static char ID; // Pass identification, replacement for typeid
238 MachineBlockPlacement() : MachineFunctionPass(ID) {
239 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
242 bool runOnMachineFunction(MachineFunction &F) override;
244 void getAnalysisUsage(AnalysisUsage &AU) const override {
245 AU.addRequired<MachineBranchProbabilityInfo>();
246 AU.addRequired<MachineBlockFrequencyInfo>();
247 AU.addRequired<MachineLoopInfo>();
248 MachineFunctionPass::getAnalysisUsage(AU);
253 char MachineBlockPlacement::ID = 0;
254 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
255 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
256 "Branch Probability Basic Block Placement", false, false)
257 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
258 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
259 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
260 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
261 "Branch Probability Basic Block Placement", false, false)
264 /// \brief Helper to print the name of a MBB.
266 /// Only used by debug logging.
267 static std::string getBlockName(MachineBasicBlock *BB) {
269 raw_string_ostream OS(Result);
270 OS << "BB#" << BB->getNumber()
271 << " (derived from LLVM BB '" << BB->getName() << "')";
276 /// \brief Helper to print the number of a MBB.
278 /// Only used by debug logging.
279 static std::string getBlockNum(MachineBasicBlock *BB) {
281 raw_string_ostream OS(Result);
282 OS << "BB#" << BB->getNumber();
288 /// \brief Mark a chain's successors as having one fewer preds.
290 /// When a chain is being merged into the "placed" chain, this routine will
291 /// quickly walk the successors of each block in the chain and mark them as
292 /// having one fewer active predecessor. It also adds any successors of this
293 /// chain which reach the zero-predecessor state to the worklist passed in.
294 void MachineBlockPlacement::markChainSuccessors(
296 MachineBasicBlock *LoopHeaderBB,
297 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
298 const BlockFilterSet *BlockFilter) {
299 // Walk all the blocks in this chain, marking their successors as having
300 // a predecessor placed.
301 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
303 // Add any successors for which this is the only un-placed in-loop
304 // predecessor to the worklist as a viable candidate for CFG-neutral
305 // placement. No subsequent placement of this block will violate the CFG
306 // shape, so we get to use heuristics to choose a favorable placement.
307 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
308 SE = (*CBI)->succ_end();
310 if (BlockFilter && !BlockFilter->count(*SI))
312 BlockChain &SuccChain = *BlockToChain[*SI];
313 // Disregard edges within a fixed chain, or edges to the loop header.
314 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
317 // This is a cross-chain edge that is within the loop, so decrement the
318 // loop predecessor count of the destination chain.
319 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
320 BlockWorkList.push_back(*SuccChain.begin());
325 /// \brief Select the best successor for a block.
327 /// This looks across all successors of a particular block and attempts to
328 /// select the "best" one to be the layout successor. It only considers direct
329 /// successors which also pass the block filter. It will attempt to avoid
330 /// breaking CFG structure, but cave and break such structures in the case of
331 /// very hot successor edges.
333 /// \returns The best successor block found, or null if none are viable.
334 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
335 MachineBasicBlock *BB, BlockChain &Chain,
336 const BlockFilterSet *BlockFilter) {
337 const BranchProbability HotProb(4, 5); // 80%
339 MachineBasicBlock *BestSucc = nullptr;
340 // FIXME: Due to the performance of the probability and weight routines in
341 // the MBPI analysis, we manually compute probabilities using the edge
342 // weights. This is suboptimal as it means that the somewhat subtle
343 // definition of edge weight semantics is encoded here as well. We should
344 // improve the MBPI interface to efficiently support query patterns such as
346 uint32_t BestWeight = 0;
347 uint32_t WeightScale = 0;
348 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
349 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
350 for (MachineBasicBlock *Succ : BB->successors()) {
351 if (BlockFilter && !BlockFilter->count(Succ))
353 BlockChain &SuccChain = *BlockToChain[Succ];
354 if (&SuccChain == &Chain) {
355 DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Already merged!\n");
358 if (Succ != *SuccChain.begin()) {
359 DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
363 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, Succ);
364 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
366 // Only consider successors which are either "hot", or wouldn't violate
367 // any CFG constraints.
368 if (SuccChain.LoopPredecessors != 0) {
369 if (SuccProb < HotProb) {
370 DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
371 << " (prob) (CFG conflict)\n");
375 // Make sure that a hot successor doesn't have a globally more
376 // important predecessor.
377 BlockFrequency CandidateEdgeFreq =
378 MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
379 bool BadCFGConflict = false;
380 for (MachineBasicBlock *Pred : Succ->predecessors()) {
381 if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
382 BlockToChain[Pred] == &Chain)
384 BlockFrequency PredEdgeFreq =
385 MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
386 if (PredEdgeFreq >= CandidateEdgeFreq) {
387 BadCFGConflict = true;
391 if (BadCFGConflict) {
392 DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
393 << " (prob) (non-cold CFG conflict)\n");
398 DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
400 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
402 if (BestSucc && BestWeight >= SuccWeight)
405 BestWeight = SuccWeight;
410 /// \brief Select the best block from a worklist.
412 /// This looks through the provided worklist as a list of candidate basic
413 /// blocks and select the most profitable one to place. The definition of
414 /// profitable only really makes sense in the context of a loop. This returns
415 /// the most frequently visited block in the worklist, which in the case of
416 /// a loop, is the one most desirable to be physically close to the rest of the
417 /// loop body in order to improve icache behavior.
419 /// \returns The best block found, or null if none are viable.
420 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
421 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
422 const BlockFilterSet *BlockFilter) {
423 // Once we need to walk the worklist looking for a candidate, cleanup the
424 // worklist of already placed entries.
425 // FIXME: If this shows up on profiles, it could be folded (at the cost of
426 // some code complexity) into the loop below.
427 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
428 [&](MachineBasicBlock *BB) {
429 return BlockToChain.lookup(BB) == &Chain;
433 MachineBasicBlock *BestBlock = nullptr;
434 BlockFrequency BestFreq;
435 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
436 WBE = WorkList.end();
438 BlockChain &SuccChain = *BlockToChain[*WBI];
439 if (&SuccChain == &Chain) {
440 DEBUG(dbgs() << " " << getBlockName(*WBI)
441 << " -> Already merged!\n");
444 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
446 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
447 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> ";
448 MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
449 if (BestBlock && BestFreq >= CandidateFreq)
452 BestFreq = CandidateFreq;
457 /// \brief Retrieve the first unplaced basic block.
459 /// This routine is called when we are unable to use the CFG to walk through
460 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
461 /// We walk through the function's blocks in order, starting from the
462 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
463 /// re-scanning the entire sequence on repeated calls to this routine.
464 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
465 MachineFunction &F, const BlockChain &PlacedChain,
466 MachineFunction::iterator &PrevUnplacedBlockIt,
467 const BlockFilterSet *BlockFilter) {
468 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
470 if (BlockFilter && !BlockFilter->count(I))
472 if (BlockToChain[I] != &PlacedChain) {
473 PrevUnplacedBlockIt = I;
474 // Now select the head of the chain to which the unplaced block belongs
475 // as the block to place. This will force the entire chain to be placed,
476 // and satisfies the requirements of merging chains.
477 return *BlockToChain[I]->begin();
483 void MachineBlockPlacement::buildChain(
484 MachineBasicBlock *BB,
486 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
487 const BlockFilterSet *BlockFilter) {
489 assert(BlockToChain[BB] == &Chain);
490 MachineFunction &F = *BB->getParent();
491 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
493 MachineBasicBlock *LoopHeaderBB = BB;
494 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
495 BB = *std::prev(Chain.end());
498 assert(BlockToChain[BB] == &Chain);
499 assert(*std::prev(Chain.end()) == BB);
501 // Look for the best viable successor if there is one to place immediately
503 MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
505 // If an immediate successor isn't available, look for the best viable
506 // block among those we've identified as not violating the loop's CFG at
507 // this point. This won't be a fallthrough, but it will increase locality.
509 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
512 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
517 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
518 "layout successor until the CFG reduces\n");
521 // Place this block, updating the datastructures to reflect its placement.
522 BlockChain &SuccChain = *BlockToChain[BestSucc];
523 // Zero out LoopPredecessors for the successor we're about to merge in case
524 // we selected a successor that didn't fit naturally into the CFG.
525 SuccChain.LoopPredecessors = 0;
526 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
527 << " to " << getBlockNum(BestSucc) << "\n");
528 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
529 Chain.merge(BestSucc, &SuccChain);
530 BB = *std::prev(Chain.end());
533 DEBUG(dbgs() << "Finished forming chain for header block "
534 << getBlockNum(*Chain.begin()) << "\n");
537 /// \brief Find the best loop top block for layout.
539 /// Look for a block which is strictly better than the loop header for laying
540 /// out at the top of the loop. This looks for one and only one pattern:
541 /// a latch block with no conditional exit. This block will cause a conditional
542 /// jump around it or will be the bottom of the loop if we lay it out in place,
543 /// but if it it doesn't end up at the bottom of the loop for any reason,
544 /// rotation alone won't fix it. Because such a block will always result in an
545 /// unconditional jump (for the backedge) rotating it in front of the loop
546 /// header is always profitable.
548 MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
549 const BlockFilterSet &LoopBlockSet) {
550 // Check that the header hasn't been fused with a preheader block due to
551 // crazy branches. If it has, we need to start with the header at the top to
552 // prevent pulling the preheader into the loop body.
553 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
554 if (!LoopBlockSet.count(*HeaderChain.begin()))
555 return L.getHeader();
557 DEBUG(dbgs() << "Finding best loop top for: "
558 << getBlockName(L.getHeader()) << "\n");
560 BlockFrequency BestPredFreq;
561 MachineBasicBlock *BestPred = nullptr;
562 for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
563 PE = L.getHeader()->pred_end();
565 MachineBasicBlock *Pred = *PI;
566 if (!LoopBlockSet.count(Pred))
568 DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
569 << Pred->succ_size() << " successors, ";
570 MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
571 if (Pred->succ_size() > 1)
574 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
575 if (!BestPred || PredFreq > BestPredFreq ||
576 (!(PredFreq < BestPredFreq) &&
577 Pred->isLayoutSuccessor(L.getHeader()))) {
579 BestPredFreq = PredFreq;
583 // If no direct predecessor is fine, just use the loop header.
585 return L.getHeader();
587 // Walk backwards through any straight line of predecessors.
588 while (BestPred->pred_size() == 1 &&
589 (*BestPred->pred_begin())->succ_size() == 1 &&
590 *BestPred->pred_begin() != L.getHeader())
591 BestPred = *BestPred->pred_begin();
593 DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
598 /// \brief Find the best loop exiting block for layout.
600 /// This routine implements the logic to analyze the loop looking for the best
601 /// block to layout at the top of the loop. Typically this is done to maximize
602 /// fallthrough opportunities.
604 MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
606 const BlockFilterSet &LoopBlockSet) {
607 // We don't want to layout the loop linearly in all cases. If the loop header
608 // is just a normal basic block in the loop, we want to look for what block
609 // within the loop is the best one to layout at the top. However, if the loop
610 // header has be pre-merged into a chain due to predecessors not having
611 // analyzable branches, *and* the predecessor it is merged with is *not* part
612 // of the loop, rotating the header into the middle of the loop will create
613 // a non-contiguous range of blocks which is Very Bad. So start with the
614 // header and only rotate if safe.
615 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
616 if (!LoopBlockSet.count(*HeaderChain.begin()))
619 BlockFrequency BestExitEdgeFreq;
620 unsigned BestExitLoopDepth = 0;
621 MachineBasicBlock *ExitingBB = nullptr;
622 // If there are exits to outer loops, loop rotation can severely limit
623 // fallthrough opportunites unless it selects such an exit. Keep a set of
624 // blocks where rotating to exit with that block will reach an outer loop.
625 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
627 DEBUG(dbgs() << "Finding best loop exit for: "
628 << getBlockName(L.getHeader()) << "\n");
629 for (MachineLoop::block_iterator I = L.block_begin(),
632 BlockChain &Chain = *BlockToChain[*I];
633 // Ensure that this block is at the end of a chain; otherwise it could be
634 // mid-way through an inner loop or a successor of an analyzable branch.
635 if (*I != *std::prev(Chain.end()))
638 // Now walk the successors. We need to establish whether this has a viable
639 // exiting successor and whether it has a viable non-exiting successor.
640 // We store the old exiting state and restore it if a viable looping
641 // successor isn't found.
642 MachineBasicBlock *OldExitingBB = ExitingBB;
643 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
644 bool HasLoopingSucc = false;
645 // FIXME: Due to the performance of the probability and weight routines in
646 // the MBPI analysis, we use the internal weights and manually compute the
647 // probabilities to avoid quadratic behavior.
648 uint32_t WeightScale = 0;
649 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
650 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
651 SE = (*I)->succ_end();
653 if ((*SI)->isLandingPad())
657 BlockChain &SuccChain = *BlockToChain[*SI];
658 // Don't split chains, either this chain or the successor's chain.
659 if (&Chain == &SuccChain) {
660 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
661 << getBlockName(*SI) << " (chain conflict)\n");
665 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
666 if (LoopBlockSet.count(*SI)) {
667 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
668 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
669 HasLoopingSucc = true;
673 unsigned SuccLoopDepth = 0;
674 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
675 SuccLoopDepth = ExitLoop->getLoopDepth();
676 if (ExitLoop->contains(&L))
677 BlocksExitingToOuterLoop.insert(*I);
680 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
681 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
682 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
683 << getBlockName(*SI) << " [L:" << SuccLoopDepth
685 MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
686 // Note that we bias this toward an existing layout successor to retain
687 // incoming order in the absence of better information. The exit must have
688 // a frequency higher than the current exit before we consider breaking
690 BranchProbability Bias(100 - ExitBlockBias, 100);
691 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
692 ExitEdgeFreq > BestExitEdgeFreq ||
693 ((*I)->isLayoutSuccessor(*SI) &&
694 !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
695 BestExitEdgeFreq = ExitEdgeFreq;
700 // Restore the old exiting state, no viable looping successor was found.
701 if (!HasLoopingSucc) {
702 ExitingBB = OldExitingBB;
703 BestExitEdgeFreq = OldBestExitEdgeFreq;
707 // Without a candidate exiting block or with only a single block in the
708 // loop, just use the loop header to layout the loop.
709 if (!ExitingBB || L.getNumBlocks() == 1)
712 // Also, if we have exit blocks which lead to outer loops but didn't select
713 // one of them as the exiting block we are rotating toward, disable loop
714 // rotation altogether.
715 if (!BlocksExitingToOuterLoop.empty() &&
716 !BlocksExitingToOuterLoop.count(ExitingBB))
719 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
723 /// \brief Attempt to rotate an exiting block to the bottom of the loop.
725 /// Once we have built a chain, try to rotate it to line up the hot exit block
726 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary
727 /// branches. For example, if the loop has fallthrough into its header and out
728 /// of its bottom already, don't rotate it.
729 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
730 MachineBasicBlock *ExitingBB,
731 const BlockFilterSet &LoopBlockSet) {
735 MachineBasicBlock *Top = *LoopChain.begin();
736 bool ViableTopFallthrough = false;
737 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
738 PE = Top->pred_end();
740 BlockChain *PredChain = BlockToChain[*PI];
741 if (!LoopBlockSet.count(*PI) &&
742 (!PredChain || *PI == *std::prev(PredChain->end()))) {
743 ViableTopFallthrough = true;
748 // If the header has viable fallthrough, check whether the current loop
749 // bottom is a viable exiting block. If so, bail out as rotating will
750 // introduce an unnecessary branch.
751 if (ViableTopFallthrough) {
752 MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
753 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
754 SE = Bottom->succ_end();
756 BlockChain *SuccChain = BlockToChain[*SI];
757 if (!LoopBlockSet.count(*SI) &&
758 (!SuccChain || *SI == *SuccChain->begin()))
763 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
765 if (ExitIt == LoopChain.end())
768 std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
771 /// \brief Forms basic block chains from the natural loop structures.
773 /// These chains are designed to preserve the existing *structure* of the code
774 /// as much as possible. We can then stitch the chains together in a way which
775 /// both preserves the topological structure and minimizes taken conditional
777 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
779 // First recurse through any nested loops, building chains for those inner
781 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
782 buildLoopChains(F, **LI);
784 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
785 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
787 // First check to see if there is an obviously preferable top block for the
788 // loop. This will default to the header, but may end up as one of the
789 // predecessors to the header if there is one which will result in strictly
790 // fewer branches in the loop body.
791 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
793 // If we selected just the header for the loop top, look for a potentially
794 // profitable exit block in the event that rotating the loop can eliminate
795 // branches by placing an exit edge at the bottom.
796 MachineBasicBlock *ExitingBB = nullptr;
797 if (LoopTop == L.getHeader())
798 ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
800 BlockChain &LoopChain = *BlockToChain[LoopTop];
802 // FIXME: This is a really lame way of walking the chains in the loop: we
803 // walk the blocks, and use a set to prevent visiting a particular chain
805 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
806 assert(LoopChain.LoopPredecessors == 0);
807 UpdatedPreds.insert(&LoopChain);
808 for (MachineLoop::block_iterator BI = L.block_begin(),
811 BlockChain &Chain = *BlockToChain[*BI];
812 if (!UpdatedPreds.insert(&Chain).second)
815 assert(Chain.LoopPredecessors == 0);
816 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
818 assert(BlockToChain[*BCI] == &Chain);
819 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
820 PE = (*BCI)->pred_end();
822 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
824 ++Chain.LoopPredecessors;
828 if (Chain.LoopPredecessors == 0)
829 BlockWorkList.push_back(*Chain.begin());
832 buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
833 rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
836 // Crash at the end so we get all of the debugging output first.
837 bool BadLoop = false;
838 if (LoopChain.LoopPredecessors) {
840 dbgs() << "Loop chain contains a block without its preds placed!\n"
841 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
842 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
844 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
846 dbgs() << " ... " << getBlockName(*BCI) << "\n";
847 if (!LoopBlockSet.erase(*BCI)) {
848 // We don't mark the loop as bad here because there are real situations
849 // where this can occur. For example, with an unanalyzable fallthrough
850 // from a loop block to a non-loop block or vice versa.
851 dbgs() << "Loop chain contains a block not contained by the loop!\n"
852 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
853 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
854 << " Bad block: " << getBlockName(*BCI) << "\n";
858 if (!LoopBlockSet.empty()) {
860 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
861 LBE = LoopBlockSet.end();
863 dbgs() << "Loop contains blocks never placed into a chain!\n"
864 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
865 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
866 << " Bad block: " << getBlockName(*LBI) << "\n";
868 assert(!BadLoop && "Detected problems with the placement of this loop.");
872 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
873 // Ensure that every BB in the function has an associated chain to simplify
874 // the assumptions of the remaining algorithm.
875 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
876 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
877 MachineBasicBlock *BB = FI;
879 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
880 // Also, merge any blocks which we cannot reason about and must preserve
881 // the exact fallthrough behavior for.
884 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
885 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
888 MachineFunction::iterator NextFI(std::next(FI));
889 MachineBasicBlock *NextBB = NextFI;
890 // Ensure that the layout successor is a viable block, as we know that
891 // fallthrough is a possibility.
892 assert(NextFI != FE && "Can't fallthrough past the last block.");
893 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
894 << getBlockName(BB) << " -> " << getBlockName(NextBB)
896 Chain->merge(NextBB, nullptr);
902 // Build any loop-based chains.
903 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
905 buildLoopChains(F, **LI);
907 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
909 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
910 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
911 MachineBasicBlock *BB = &*FI;
912 BlockChain &Chain = *BlockToChain[BB];
913 if (!UpdatedPreds.insert(&Chain).second)
916 assert(Chain.LoopPredecessors == 0);
917 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
919 assert(BlockToChain[*BCI] == &Chain);
920 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
921 PE = (*BCI)->pred_end();
923 if (BlockToChain[*PI] == &Chain)
925 ++Chain.LoopPredecessors;
929 if (Chain.LoopPredecessors == 0)
930 BlockWorkList.push_back(*Chain.begin());
933 BlockChain &FunctionChain = *BlockToChain[&F.front()];
934 buildChain(&F.front(), FunctionChain, BlockWorkList);
937 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
940 // Crash at the end so we get all of the debugging output first.
941 bool BadFunc = false;
942 FunctionBlockSetType FunctionBlockSet;
943 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
944 FunctionBlockSet.insert(FI);
946 for (BlockChain::iterator BCI = FunctionChain.begin(),
947 BCE = FunctionChain.end();
949 if (!FunctionBlockSet.erase(*BCI)) {
951 dbgs() << "Function chain contains a block not in the function!\n"
952 << " Bad block: " << getBlockName(*BCI) << "\n";
955 if (!FunctionBlockSet.empty()) {
957 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
958 FBE = FunctionBlockSet.end();
960 dbgs() << "Function contains blocks never placed into a chain!\n"
961 << " Bad block: " << getBlockName(*FBI) << "\n";
963 assert(!BadFunc && "Detected problems with the block placement.");
966 // Splice the blocks into place.
967 MachineFunction::iterator InsertPos = F.begin();
968 for (BlockChain::iterator BI = FunctionChain.begin(),
969 BE = FunctionChain.end();
971 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
973 << getBlockName(*BI) << "\n");
974 if (InsertPos != MachineFunction::iterator(*BI))
975 F.splice(InsertPos, *BI);
979 // Update the terminator of the previous block.
980 if (BI == FunctionChain.begin())
982 MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(*BI));
984 // FIXME: It would be awesome of updateTerminator would just return rather
985 // than assert when the branch cannot be analyzed in order to remove this
988 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
989 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
990 // The "PrevBB" is not yet updated to reflect current code layout, so,
991 // o. it may fall-through to a block without explict "goto" instruction
992 // before layout, and no longer fall-through it after layout; or
995 // AnalyzeBranch() may return erroneous value for FBB when these two
996 // situations take place. For the first scenario FBB is mistakenly set
997 // NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
998 // is mistakenly pointing to "*BI".
1000 bool needUpdateBr = true;
1001 if (!Cond.empty() && (!FBB || FBB == *BI)) {
1002 PrevBB->updateTerminator();
1003 needUpdateBr = false;
1005 TBB = FBB = nullptr;
1006 if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
1007 // FIXME: This should never take place.
1008 TBB = FBB = nullptr;
1012 // If PrevBB has a two-way branch, try to re-order the branches
1013 // such that we branch to the successor with higher weight first.
1014 if (TBB && !Cond.empty() && FBB &&
1015 MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
1016 !TII->ReverseBranchCondition(Cond)) {
1017 DEBUG(dbgs() << "Reverse order of the two branches: "
1018 << getBlockName(PrevBB) << "\n");
1019 DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
1020 << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
1021 DebugLoc dl; // FIXME: this is nowhere
1022 TII->RemoveBranch(*PrevBB);
1023 TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
1024 needUpdateBr = true;
1027 PrevBB->updateTerminator();
1031 // Fixup the last block.
1033 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
1034 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
1035 F.back().updateTerminator();
1037 // Walk through the backedges of the function now that we have fully laid out
1038 // the basic blocks and align the destination of each backedge. We don't rely
1039 // exclusively on the loop info here so that we can align backedges in
1040 // unnatural CFGs and backedges that were introduced purely because of the
1041 // loop rotations done during this layout pass.
1042 if (F.getFunction()->hasFnAttribute(Attribute::OptimizeForSize))
1044 if (FunctionChain.begin() == FunctionChain.end())
1045 return; // Empty chain.
1047 const BranchProbability ColdProb(1, 5); // 20%
1048 BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
1049 BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
1050 for (BlockChain::iterator BI = std::next(FunctionChain.begin()),
1051 BE = FunctionChain.end();
1053 // Don't align non-looping basic blocks. These are unlikely to execute
1054 // enough times to matter in practice. Note that we'll still handle
1055 // unnatural CFGs inside of a natural outer loop (the common case) and
1057 MachineLoop *L = MLI->getLoopFor(*BI);
1061 unsigned Align = TLI->getPrefLoopAlignment(L);
1063 continue; // Don't care about loop alignment.
1065 // If the block is cold relative to the function entry don't waste space
1067 BlockFrequency Freq = MBFI->getBlockFreq(*BI);
1068 if (Freq < WeightedEntryFreq)
1071 // If the block is cold relative to its loop header, don't align it
1072 // regardless of what edges into the block exist.
1073 MachineBasicBlock *LoopHeader = L->getHeader();
1074 BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
1075 if (Freq < (LoopHeaderFreq * ColdProb))
1078 // Check for the existence of a non-layout predecessor which would benefit
1079 // from aligning this block.
1080 MachineBasicBlock *LayoutPred = *std::prev(BI);
1082 // Force alignment if all the predecessors are jumps. We already checked
1083 // that the block isn't cold above.
1084 if (!LayoutPred->isSuccessor(*BI)) {
1085 (*BI)->setAlignment(Align);
1089 // Align this block if the layout predecessor's edge into this block is
1090 // cold relative to the block. When this is true, other predecessors make up
1091 // all of the hot entries into the block and thus alignment is likely to be
1093 BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
1094 BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
1095 if (LayoutEdgeFreq <= (Freq * ColdProb))
1096 (*BI)->setAlignment(Align);
1100 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
1101 // Check for single-block functions and skip them.
1102 if (std::next(F.begin()) == F.end())
1105 if (skipOptnoneFunction(*F.getFunction()))
1108 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1109 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1110 MLI = &getAnalysis<MachineLoopInfo>();
1111 TII = F.getSubtarget().getInstrInfo();
1112 TLI = F.getSubtarget().getTargetLowering();
1113 assert(BlockToChain.empty());
1117 BlockToChain.clear();
1118 ChainAllocator.DestroyAll();
1121 // Align all of the blocks in the function to a specific alignment.
1122 for (MachineFunction::iterator FI = F.begin(), FE = F.end();
1124 FI->setAlignment(AlignAllBlock);
1126 // We always return true as we have no way to track whether the final order
1127 // differs from the original order.
1132 /// \brief A pass to compute block placement statistics.
1134 /// A separate pass to compute interesting statistics for evaluating block
1135 /// placement. This is separate from the actual placement pass so that they can
1136 /// be computed in the absence of any placement transformations or when using
1137 /// alternative placement strategies.
1138 class MachineBlockPlacementStats : public MachineFunctionPass {
1139 /// \brief A handle to the branch probability pass.
1140 const MachineBranchProbabilityInfo *MBPI;
1142 /// \brief A handle to the function-wide block frequency pass.
1143 const MachineBlockFrequencyInfo *MBFI;
1146 static char ID; // Pass identification, replacement for typeid
1147 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
1148 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
1151 bool runOnMachineFunction(MachineFunction &F) override;
1153 void getAnalysisUsage(AnalysisUsage &AU) const override {
1154 AU.addRequired<MachineBranchProbabilityInfo>();
1155 AU.addRequired<MachineBlockFrequencyInfo>();
1156 AU.setPreservesAll();
1157 MachineFunctionPass::getAnalysisUsage(AU);
1162 char MachineBlockPlacementStats::ID = 0;
1163 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
1164 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
1165 "Basic Block Placement Stats", false, false)
1166 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
1167 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
1168 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
1169 "Basic Block Placement Stats", false, false)
1171 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
1172 // Check for single-block functions and skip them.
1173 if (std::next(F.begin()) == F.end())
1176 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1177 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1179 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
1180 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
1181 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
1182 : NumUncondBranches;
1183 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
1184 : UncondBranchTakenFreq;
1185 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
1188 // Skip if this successor is a fallthrough.
1189 if (I->isLayoutSuccessor(*SI))
1192 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
1194 BranchTakenFreq += EdgeFreq.getFrequency();