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 absense 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 #define DEBUG_TYPE "block-placement2"
29 #include "llvm/CodeGen/MachineBasicBlock.h"
30 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
31 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
32 #include "llvm/CodeGen/MachineFunction.h"
33 #include "llvm/CodeGen/MachineFunctionPass.h"
34 #include "llvm/CodeGen/MachineLoopInfo.h"
35 #include "llvm/CodeGen/MachineModuleInfo.h"
36 #include "llvm/CodeGen/Passes.h"
37 #include "llvm/Support/Allocator.h"
38 #include "llvm/Support/Debug.h"
39 #include "llvm/ADT/DenseMap.h"
40 #include "llvm/ADT/SmallPtrSet.h"
41 #include "llvm/ADT/SmallVector.h"
42 #include "llvm/ADT/Statistic.h"
43 #include "llvm/Target/TargetInstrInfo.h"
44 #include "llvm/Target/TargetLowering.h"
48 STATISTIC(NumCondBranches, "Number of conditional branches");
49 STATISTIC(NumUncondBranches, "Number of uncondittional branches");
50 STATISTIC(CondBranchTakenFreq,
51 "Potential frequency of taking conditional branches");
52 STATISTIC(UncondBranchTakenFreq,
53 "Potential frequency of taking unconditional branches");
57 /// \brief Type for our function-wide basic block -> block chain mapping.
58 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
62 /// \brief A chain of blocks which will be laid out contiguously.
64 /// This is the datastructure representing a chain of consecutive blocks that
65 /// are profitable to layout together in order to maximize fallthrough
66 /// probabilities. We also can use a block chain to represent a sequence of
67 /// basic blocks which have some external (correctness) requirement for
68 /// sequential layout.
70 /// Eventually, the block chains will form a directed graph over the function.
71 /// We provide an SCC-supporting-iterator in order to quicky build and walk the
72 /// SCCs of block chains within a function.
74 /// The block chains also have support for calculating and caching probability
75 /// information related to the chain itself versus other chains. This is used
76 /// for ranking during the final layout of block chains.
78 /// \brief The sequence of blocks belonging to this chain.
80 /// This is the sequence of blocks for a particular chain. These will be laid
81 /// out in-order within the function.
82 SmallVector<MachineBasicBlock *, 4> Blocks;
84 /// \brief A handle to the function-wide basic block to block chain mapping.
86 /// This is retained in each block chain to simplify the computation of child
87 /// block chains for SCC-formation and iteration. We store the edges to child
88 /// basic blocks, and map them back to their associated chains using this
90 BlockToChainMapType &BlockToChain;
93 /// \brief Construct a new BlockChain.
95 /// This builds a new block chain representing a single basic block in the
96 /// function. It also registers itself as the chain that block participates
97 /// in with the BlockToChain mapping.
98 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
99 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
100 assert(BB && "Cannot create a chain with a null basic block");
101 BlockToChain[BB] = this;
104 /// \brief Iterator over blocks within the chain.
105 typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
107 /// \brief Beginning of blocks within the chain.
108 iterator begin() { return Blocks.begin(); }
110 /// \brief End of blocks within the chain.
111 iterator end() { return Blocks.end(); }
113 /// \brief Merge a block chain into this one.
115 /// This routine merges a block chain into this one. It takes care of forming
116 /// a contiguous sequence of basic blocks, updating the edge list, and
117 /// updating the block -> chain mapping. It does not free or tear down the
118 /// old chain, but the old chain's block list is no longer valid.
119 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
121 assert(!Blocks.empty());
123 // Fast path in case we don't have a chain already.
125 assert(!BlockToChain[BB]);
126 Blocks.push_back(BB);
127 BlockToChain[BB] = this;
131 assert(BB == *Chain->begin());
132 assert(Chain->begin() != Chain->end());
134 // Update the incoming blocks to point to this chain, and add them to the
136 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
138 Blocks.push_back(*BI);
139 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
140 BlockToChain[*BI] = this;
145 /// \brief Dump the blocks in this chain.
146 void dump() LLVM_ATTRIBUTE_USED {
147 for (iterator I = begin(), E = end(); I != E; ++I)
152 /// \brief Count of predecessors within the loop currently being processed.
154 /// This count is updated at each loop we process to represent the number of
155 /// in-loop predecessors of this chain.
156 unsigned LoopPredecessors;
161 class MachineBlockPlacement : public MachineFunctionPass {
162 /// \brief A typedef for a block filter set.
163 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
165 /// \brief A handle to the branch probability pass.
166 const MachineBranchProbabilityInfo *MBPI;
168 /// \brief A handle to the function-wide block frequency pass.
169 const MachineBlockFrequencyInfo *MBFI;
171 /// \brief A handle to the loop info.
172 const MachineLoopInfo *MLI;
174 /// \brief A handle to the target's instruction info.
175 const TargetInstrInfo *TII;
177 /// \brief A handle to the target's lowering info.
178 const TargetLowering *TLI;
180 /// \brief Allocator and owner of BlockChain structures.
182 /// We build BlockChains lazily by merging together high probability BB
183 /// sequences acording to the "Algo2" in the paper mentioned at the top of
184 /// the file. To reduce malloc traffic, we allocate them using this slab-like
185 /// allocator, and destroy them after the pass completes.
186 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
188 /// \brief Function wide BasicBlock to BlockChain mapping.
190 /// This mapping allows efficiently moving from any given basic block to the
191 /// BlockChain it participates in, if any. We use it to, among other things,
192 /// allow implicitly defining edges between chains as the existing edges
193 /// between basic blocks.
194 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
196 void markChainSuccessors(BlockChain &Chain,
197 MachineBasicBlock *LoopHeaderBB,
198 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
199 const BlockFilterSet *BlockFilter = 0);
200 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
202 const BlockFilterSet *BlockFilter);
203 MachineBasicBlock *selectBestCandidateBlock(
204 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
205 const BlockFilterSet *BlockFilter);
206 MachineBasicBlock *getFirstUnplacedBlock(
208 const BlockChain &PlacedChain,
209 MachineFunction::iterator &PrevUnplacedBlockIt,
210 const BlockFilterSet *BlockFilter);
211 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
212 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
213 const BlockFilterSet *BlockFilter = 0);
214 MachineBasicBlock *findBestLoopExit(MachineFunction &F,
216 const BlockFilterSet &LoopBlockSet);
217 void buildLoopChains(MachineFunction &F, MachineLoop &L);
218 void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
219 const BlockFilterSet &LoopBlockSet);
220 void buildCFGChains(MachineFunction &F);
223 static char ID; // Pass identification, replacement for typeid
224 MachineBlockPlacement() : MachineFunctionPass(ID) {
225 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
228 bool runOnMachineFunction(MachineFunction &F);
230 void getAnalysisUsage(AnalysisUsage &AU) const {
231 AU.addRequired<MachineBranchProbabilityInfo>();
232 AU.addRequired<MachineBlockFrequencyInfo>();
233 AU.addRequired<MachineLoopInfo>();
234 MachineFunctionPass::getAnalysisUsage(AU);
239 char MachineBlockPlacement::ID = 0;
240 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
241 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
242 "Branch Probability Basic Block Placement", false, false)
243 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
244 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
245 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
246 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
247 "Branch Probability Basic Block Placement", false, false)
250 /// \brief Helper to print the name of a MBB.
252 /// Only used by debug logging.
253 static std::string getBlockName(MachineBasicBlock *BB) {
255 raw_string_ostream OS(Result);
256 OS << "BB#" << BB->getNumber()
257 << " (derived from LLVM BB '" << BB->getName() << "')";
262 /// \brief Helper to print the number of a MBB.
264 /// Only used by debug logging.
265 static std::string getBlockNum(MachineBasicBlock *BB) {
267 raw_string_ostream OS(Result);
268 OS << "BB#" << BB->getNumber();
274 /// \brief Mark a chain's successors as having one fewer preds.
276 /// When a chain is being merged into the "placed" chain, this routine will
277 /// quickly walk the successors of each block in the chain and mark them as
278 /// having one fewer active predecessor. It also adds any successors of this
279 /// chain which reach the zero-predecessor state to the worklist passed in.
280 void MachineBlockPlacement::markChainSuccessors(
282 MachineBasicBlock *LoopHeaderBB,
283 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
284 const BlockFilterSet *BlockFilter) {
285 // Walk all the blocks in this chain, marking their successors as having
286 // a predecessor placed.
287 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
289 // Add any successors for which this is the only un-placed in-loop
290 // predecessor to the worklist as a viable candidate for CFG-neutral
291 // placement. No subsequent placement of this block will violate the CFG
292 // shape, so we get to use heuristics to choose a favorable placement.
293 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
294 SE = (*CBI)->succ_end();
296 if (BlockFilter && !BlockFilter->count(*SI))
298 BlockChain &SuccChain = *BlockToChain[*SI];
299 // Disregard edges within a fixed chain, or edges to the loop header.
300 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
303 // This is a cross-chain edge that is within the loop, so decrement the
304 // loop predecessor count of the destination chain.
305 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
306 BlockWorkList.push_back(*SuccChain.begin());
311 /// \brief Select the best successor for a block.
313 /// This looks across all successors of a particular block and attempts to
314 /// select the "best" one to be the layout successor. It only considers direct
315 /// successors which also pass the block filter. It will attempt to avoid
316 /// breaking CFG structure, but cave and break such structures in the case of
317 /// very hot successor edges.
319 /// \returns The best successor block found, or null if none are viable.
320 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
321 MachineBasicBlock *BB, BlockChain &Chain,
322 const BlockFilterSet *BlockFilter) {
323 const BranchProbability HotProb(4, 5); // 80%
325 MachineBasicBlock *BestSucc = 0;
326 // FIXME: Due to the performance of the probability and weight routines in
327 // the MBPI analysis, we manually compute probabilities using the edge
328 // weights. This is suboptimal as it means that the somewhat subtle
329 // definition of edge weight semantics is encoded here as well. We should
330 // improve the MBPI interface to effeciently support query patterns such as
332 uint32_t BestWeight = 0;
333 uint32_t WeightScale = 0;
334 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
335 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
336 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
339 if (BlockFilter && !BlockFilter->count(*SI))
341 BlockChain &SuccChain = *BlockToChain[*SI];
342 if (&SuccChain == &Chain) {
343 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
346 if (*SI != *SuccChain.begin()) {
347 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
351 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
352 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
354 // Only consider successors which are either "hot", or wouldn't violate
355 // any CFG constraints.
356 if (SuccChain.LoopPredecessors != 0) {
357 if (SuccProb < HotProb) {
358 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
362 // Make sure that a hot successor doesn't have a globally more important
364 BlockFrequency CandidateEdgeFreq
365 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
366 bool BadCFGConflict = false;
367 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
368 PE = (*SI)->pred_end();
370 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
371 BlockToChain[*PI] == &Chain)
373 BlockFrequency PredEdgeFreq
374 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
375 if (PredEdgeFreq >= CandidateEdgeFreq) {
376 BadCFGConflict = true;
380 if (BadCFGConflict) {
381 DEBUG(dbgs() << " " << getBlockName(*SI)
382 << " -> non-cold CFG conflict\n");
387 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
389 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
391 if (BestSucc && BestWeight >= SuccWeight)
394 BestWeight = SuccWeight;
400 /// \brief Predicate struct to detect blocks already placed.
401 class IsBlockPlaced {
402 const BlockChain &PlacedChain;
403 const BlockToChainMapType &BlockToChain;
406 IsBlockPlaced(const BlockChain &PlacedChain,
407 const BlockToChainMapType &BlockToChain)
408 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
410 bool operator()(MachineBasicBlock *BB) const {
411 return BlockToChain.lookup(BB) == &PlacedChain;
416 /// \brief Select the best block from a worklist.
418 /// This looks through the provided worklist as a list of candidate basic
419 /// blocks and select the most profitable one to place. The definition of
420 /// profitable only really makes sense in the context of a loop. This returns
421 /// the most frequently visited block in the worklist, which in the case of
422 /// a loop, is the one most desirable to be physically close to the rest of the
423 /// loop body in order to improve icache behavior.
425 /// \returns The best block found, or null if none are viable.
426 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
427 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
428 const BlockFilterSet *BlockFilter) {
429 // Once we need to walk the worklist looking for a candidate, cleanup the
430 // worklist of already placed entries.
431 // FIXME: If this shows up on profiles, it could be folded (at the cost of
432 // some code complexity) into the loop below.
433 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
434 IsBlockPlaced(Chain, BlockToChain)),
437 MachineBasicBlock *BestBlock = 0;
438 BlockFrequency BestFreq;
439 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
440 WBE = WorkList.end();
442 BlockChain &SuccChain = *BlockToChain[*WBI];
443 if (&SuccChain == &Chain) {
444 DEBUG(dbgs() << " " << getBlockName(*WBI)
445 << " -> Already merged!\n");
448 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
450 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
451 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
453 if (BestBlock && BestFreq >= CandidateFreq)
456 BestFreq = CandidateFreq;
461 /// \brief Retrieve the first unplaced basic block.
463 /// This routine is called when we are unable to use the CFG to walk through
464 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
465 /// We walk through the function's blocks in order, starting from the
466 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
467 /// re-scanning the entire sequence on repeated calls to this routine.
468 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
469 MachineFunction &F, const BlockChain &PlacedChain,
470 MachineFunction::iterator &PrevUnplacedBlockIt,
471 const BlockFilterSet *BlockFilter) {
472 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
474 if (BlockFilter && !BlockFilter->count(I))
476 if (BlockToChain[I] != &PlacedChain) {
477 PrevUnplacedBlockIt = I;
478 // Now select the head of the chain to which the unplaced block belongs
479 // as the block to place. This will force the entire chain to be placed,
480 // and satisfies the requirements of merging chains.
481 return *BlockToChain[I]->begin();
487 void MachineBlockPlacement::buildChain(
488 MachineBasicBlock *BB,
490 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
491 const BlockFilterSet *BlockFilter) {
493 assert(BlockToChain[BB] == &Chain);
494 MachineFunction &F = *BB->getParent();
495 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
497 MachineBasicBlock *LoopHeaderBB = BB;
498 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
499 BB = *llvm::prior(Chain.end());
502 assert(BlockToChain[BB] == &Chain);
503 assert(*llvm::prior(Chain.end()) == BB);
504 MachineBasicBlock *BestSucc = 0;
506 // Look for the best viable successor if there is one to place immediately
508 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
510 // If an immediate successor isn't available, look for the best viable
511 // block among those we've identified as not violating the loop's CFG at
512 // this point. This won't be a fallthrough, but it will increase locality.
514 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
517 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
522 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
523 "layout successor until the CFG reduces\n");
526 // Place this block, updating the datastructures to reflect its placement.
527 BlockChain &SuccChain = *BlockToChain[BestSucc];
528 // Zero out LoopPredecessors for the successor we're about to merge in case
529 // we selected a successor that didn't fit naturally into the CFG.
530 SuccChain.LoopPredecessors = 0;
531 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
532 << " to " << getBlockNum(BestSucc) << "\n");
533 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
534 Chain.merge(BestSucc, &SuccChain);
535 BB = *llvm::prior(Chain.end());
538 DEBUG(dbgs() << "Finished forming chain for header block "
539 << getBlockNum(*Chain.begin()) << "\n");
542 /// \brief Find the best loop top block for layout.
544 /// This routine implements the logic to analyze the loop looking for the best
545 /// block to layout at the top of the loop. Typically this is done to maximize
546 /// fallthrough opportunities.
548 MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
550 const BlockFilterSet &LoopBlockSet) {
551 // We don't want to layout the loop linearly in all cases. If the loop header
552 // is just a normal basic block in the loop, we want to look for what block
553 // within the loop is the best one to layout at the top. However, if the loop
554 // header has be pre-merged into a chain due to predecessors not having
555 // analyzable branches, *and* the predecessor it is merged with is *not* part
556 // of the loop, rotating the header into the middle of the loop will create
557 // a non-contiguous range of blocks which is Very Bad. So start with the
558 // header and only rotate if safe.
559 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
560 if (!LoopBlockSet.count(*HeaderChain.begin()))
563 BlockFrequency BestExitEdgeFreq;
564 unsigned BestExitLoopDepth = 0;
565 MachineBasicBlock *ExitingBB = 0;
566 // If there are exits to outer loops, loop rotation can severely limit
567 // fallthrough opportunites unless it selects such an exit. Keep a set of
568 // blocks where rotating to exit with that block will reach an outer loop.
569 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
571 DEBUG(dbgs() << "Finding best loop exit for: "
572 << getBlockName(L.getHeader()) << "\n");
573 for (MachineLoop::block_iterator I = L.block_begin(),
576 BlockChain &Chain = *BlockToChain[*I];
577 // Ensure that this block is at the end of a chain; otherwise it could be
578 // mid-way through an inner loop or a successor of an analyzable branch.
579 if (*I != *llvm::prior(Chain.end()))
582 // Now walk the successors. We need to establish whether this has a viable
583 // exiting successor and whether it has a viable non-exiting successor.
584 // We store the old exiting state and restore it if a viable looping
585 // successor isn't found.
586 MachineBasicBlock *OldExitingBB = ExitingBB;
587 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
588 bool HasLoopingSucc = false;
589 // FIXME: Due to the performance of the probability and weight routines in
590 // the MBPI analysis, we use the internal weights and manually compute the
591 // probabilities to avoid quadratic behavior.
592 uint32_t WeightScale = 0;
593 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
594 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
595 SE = (*I)->succ_end();
597 if ((*SI)->isLandingPad())
601 BlockChain &SuccChain = *BlockToChain[*SI];
602 // Don't split chains, either this chain or the successor's chain.
603 if (&Chain == &SuccChain) {
604 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
605 << getBlockName(*SI) << " (chain conflict)\n");
609 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
610 if (LoopBlockSet.count(*SI)) {
611 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
612 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
613 HasLoopingSucc = true;
617 unsigned SuccLoopDepth = 0;
618 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
619 SuccLoopDepth = ExitLoop->getLoopDepth();
620 if (ExitLoop->contains(&L))
621 BlocksExitingToOuterLoop.insert(*I);
624 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
625 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
626 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
627 << getBlockName(*SI) << " [L:" << SuccLoopDepth
628 << "] (" << ExitEdgeFreq << ")\n");
629 // Note that we slightly bias this toward an existing layout successor to
630 // retain incoming order in the absence of better information.
631 // FIXME: Should we bias this more strongly? It's pretty weak.
632 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
633 ExitEdgeFreq > BestExitEdgeFreq ||
634 ((*I)->isLayoutSuccessor(*SI) &&
635 !(ExitEdgeFreq < BestExitEdgeFreq))) {
636 BestExitEdgeFreq = ExitEdgeFreq;
641 // Restore the old exiting state, no viable looping successor was found.
642 if (!HasLoopingSucc) {
643 ExitingBB = OldExitingBB;
644 BestExitEdgeFreq = OldBestExitEdgeFreq;
648 // Without a candidate exiting block or with only a single block in the
649 // loop, just use the loop header to layout the loop.
650 if (!ExitingBB || L.getNumBlocks() == 1)
653 // Also, if we have exit blocks which lead to outer loops but didn't select
654 // one of them as the exiting block we are rotating toward, disable loop
655 // rotation altogether.
656 if (!BlocksExitingToOuterLoop.empty() &&
657 !BlocksExitingToOuterLoop.count(ExitingBB))
660 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
664 /// \brief Attempt to rotate an exiting block to the bottom of the loop.
666 /// Once we have built a chain, try to rotate it to line up the hot exit block
667 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary
668 /// branches. For example, if the loop has fallthrough into its header and out
669 /// of its bottom already, don't rotate it.
670 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
671 MachineBasicBlock *ExitingBB,
672 const BlockFilterSet &LoopBlockSet) {
676 MachineBasicBlock *Top = *LoopChain.begin();
677 bool ViableTopFallthrough = false;
678 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
679 PE = Top->pred_end();
681 BlockChain *PredChain = BlockToChain[*PI];
682 if (!LoopBlockSet.count(*PI) &&
683 (!PredChain || *PI == *llvm::prior(PredChain->end()))) {
684 ViableTopFallthrough = true;
689 // If the header has viable fallthrough, check whether the current loop
690 // bottom is a viable exiting block. If so, bail out as rotating will
691 // introduce an unnecessary branch.
692 if (ViableTopFallthrough) {
693 MachineBasicBlock *Bottom = *llvm::prior(LoopChain.end());
694 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
695 SE = Bottom->succ_end();
697 BlockChain *SuccChain = BlockToChain[*SI];
698 if (!LoopBlockSet.count(*SI) &&
699 (!SuccChain || *SI == *SuccChain->begin()))
704 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
706 if (ExitIt == LoopChain.end())
709 std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
712 /// \brief Forms basic block chains from the natural loop structures.
714 /// These chains are designed to preserve the existing *structure* of the code
715 /// as much as possible. We can then stitch the chains together in a way which
716 /// both preserves the topological structure and minimizes taken conditional
718 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
720 // First recurse through any nested loops, building chains for those inner
722 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
723 buildLoopChains(F, **LI);
725 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
726 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
728 MachineBasicBlock *ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
729 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
731 // FIXME: This is a really lame way of walking the chains in the loop: we
732 // walk the blocks, and use a set to prevent visiting a particular chain
734 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
735 assert(LoopChain.LoopPredecessors == 0);
736 UpdatedPreds.insert(&LoopChain);
737 for (MachineLoop::block_iterator BI = L.block_begin(),
740 BlockChain &Chain = *BlockToChain[*BI];
741 if (!UpdatedPreds.insert(&Chain))
744 assert(Chain.LoopPredecessors == 0);
745 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
747 assert(BlockToChain[*BCI] == &Chain);
748 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
749 PE = (*BCI)->pred_end();
751 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
753 ++Chain.LoopPredecessors;
757 if (Chain.LoopPredecessors == 0)
758 BlockWorkList.push_back(*Chain.begin());
761 buildChain(L.getHeader(), LoopChain, BlockWorkList, &LoopBlockSet);
762 rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
765 // Crash at the end so we get all of the debugging output first.
766 bool BadLoop = false;
767 if (LoopChain.LoopPredecessors) {
769 dbgs() << "Loop chain contains a block without its preds placed!\n"
770 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
771 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
773 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
775 dbgs() << " ... " << getBlockName(*BCI) << "\n";
776 if (!LoopBlockSet.erase(*BCI)) {
777 // We don't mark the loop as bad here because there are real situations
778 // where this can occur. For example, with an unanalyzable fallthrough
779 // from a loop block to a non-loop block or vice versa.
780 dbgs() << "Loop chain contains a block not contained by the loop!\n"
781 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
782 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
783 << " Bad block: " << getBlockName(*BCI) << "\n";
787 if (!LoopBlockSet.empty()) {
789 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
790 LBE = LoopBlockSet.end();
792 dbgs() << "Loop contains blocks never placed into a chain!\n"
793 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
794 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
795 << " Bad block: " << getBlockName(*LBI) << "\n";
797 assert(!BadLoop && "Detected problems with the placement of this loop.");
801 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
802 // Ensure that every BB in the function has an associated chain to simplify
803 // the assumptions of the remaining algorithm.
804 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
805 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
806 MachineBasicBlock *BB = FI;
808 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
809 // Also, merge any blocks which we cannot reason about and must preserve
810 // the exact fallthrough behavior for.
813 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
814 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
817 MachineFunction::iterator NextFI(llvm::next(FI));
818 MachineBasicBlock *NextBB = NextFI;
819 // Ensure that the layout successor is a viable block, as we know that
820 // fallthrough is a possibility.
821 assert(NextFI != FE && "Can't fallthrough past the last block.");
822 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
823 << getBlockName(BB) << " -> " << getBlockName(NextBB)
825 Chain->merge(NextBB, 0);
831 // Build any loop-based chains.
832 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
834 buildLoopChains(F, **LI);
836 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
838 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
839 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
840 MachineBasicBlock *BB = &*FI;
841 BlockChain &Chain = *BlockToChain[BB];
842 if (!UpdatedPreds.insert(&Chain))
845 assert(Chain.LoopPredecessors == 0);
846 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
848 assert(BlockToChain[*BCI] == &Chain);
849 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
850 PE = (*BCI)->pred_end();
852 if (BlockToChain[*PI] == &Chain)
854 ++Chain.LoopPredecessors;
858 if (Chain.LoopPredecessors == 0)
859 BlockWorkList.push_back(*Chain.begin());
862 BlockChain &FunctionChain = *BlockToChain[&F.front()];
863 buildChain(&F.front(), FunctionChain, BlockWorkList);
865 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
867 // Crash at the end so we get all of the debugging output first.
868 bool BadFunc = false;
869 FunctionBlockSetType FunctionBlockSet;
870 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
871 FunctionBlockSet.insert(FI);
873 for (BlockChain::iterator BCI = FunctionChain.begin(),
874 BCE = FunctionChain.end();
876 if (!FunctionBlockSet.erase(*BCI)) {
878 dbgs() << "Function chain contains a block not in the function!\n"
879 << " Bad block: " << getBlockName(*BCI) << "\n";
882 if (!FunctionBlockSet.empty()) {
884 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
885 FBE = FunctionBlockSet.end();
887 dbgs() << "Function contains blocks never placed into a chain!\n"
888 << " Bad block: " << getBlockName(*FBI) << "\n";
890 assert(!BadFunc && "Detected problems with the block placement.");
893 // Splice the blocks into place.
894 MachineFunction::iterator InsertPos = F.begin();
895 for (BlockChain::iterator BI = FunctionChain.begin(),
896 BE = FunctionChain.end();
898 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
900 << getBlockName(*BI) << "\n");
901 if (InsertPos != MachineFunction::iterator(*BI))
902 F.splice(InsertPos, *BI);
906 // Update the terminator of the previous block.
907 if (BI == FunctionChain.begin())
909 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
911 // FIXME: It would be awesome of updateTerminator would just return rather
912 // than assert when the branch cannot be analyzed in order to remove this
915 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
916 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
917 PrevBB->updateTerminator();
920 // Fixup the last block.
922 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
923 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
924 F.back().updateTerminator();
926 // Walk through the backedges of the function now that we have fully laid out
927 // the basic blocks and align the destination of each backedge. We don't rely
928 // on the loop info here so that we can align backedges in unnatural CFGs and
929 // backedges that were introduced purely because of the loop rotations done
930 // during this layout pass.
931 // FIXME: This isn't quite right, we shouldn't align backedges that result
932 // from blocks being sunken below the exit block for the function.
933 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
935 unsigned Align = TLI->getPrefLoopAlignment();
937 return; // Don't care about loop alignment.
939 SmallPtrSet<MachineBasicBlock *, 16> PreviousBlocks;
940 for (BlockChain::iterator BI = FunctionChain.begin(),
941 BE = FunctionChain.end();
943 PreviousBlocks.insert(*BI);
944 // Set alignment on the destination of all the back edges in the new
946 for (MachineBasicBlock::succ_iterator SI = (*BI)->succ_begin(),
947 SE = (*BI)->succ_end();
949 if (PreviousBlocks.count(*SI))
950 (*SI)->setAlignment(Align);
954 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
955 // Check for single-block functions and skip them.
956 if (llvm::next(F.begin()) == F.end())
959 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
960 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
961 MLI = &getAnalysis<MachineLoopInfo>();
962 TII = F.getTarget().getInstrInfo();
963 TLI = F.getTarget().getTargetLowering();
964 assert(BlockToChain.empty());
968 BlockToChain.clear();
969 ChainAllocator.DestroyAll();
971 // We always return true as we have no way to track whether the final order
972 // differs from the original order.
977 /// \brief A pass to compute block placement statistics.
979 /// A separate pass to compute interesting statistics for evaluating block
980 /// placement. This is separate from the actual placement pass so that they can
981 /// be computed in the absense of any placement transformations or when using
982 /// alternative placement strategies.
983 class MachineBlockPlacementStats : public MachineFunctionPass {
984 /// \brief A handle to the branch probability pass.
985 const MachineBranchProbabilityInfo *MBPI;
987 /// \brief A handle to the function-wide block frequency pass.
988 const MachineBlockFrequencyInfo *MBFI;
991 static char ID; // Pass identification, replacement for typeid
992 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
993 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
996 bool runOnMachineFunction(MachineFunction &F);
998 void getAnalysisUsage(AnalysisUsage &AU) const {
999 AU.addRequired<MachineBranchProbabilityInfo>();
1000 AU.addRequired<MachineBlockFrequencyInfo>();
1001 AU.setPreservesAll();
1002 MachineFunctionPass::getAnalysisUsage(AU);
1007 char MachineBlockPlacementStats::ID = 0;
1008 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
1009 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
1010 "Basic Block Placement Stats", false, false)
1011 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
1012 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
1013 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
1014 "Basic Block Placement Stats", false, false)
1016 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
1017 // Check for single-block functions and skip them.
1018 if (llvm::next(F.begin()) == F.end())
1021 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1022 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1024 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
1025 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
1026 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
1027 : NumUncondBranches;
1028 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
1029 : UncondBranchTakenFreq;
1030 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
1033 // Skip if this successor is a fallthrough.
1034 if (I->isLayoutSuccessor(*SI))
1037 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
1039 BranchTakenFreq += EdgeFreq.getFrequency();