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 *>::const_iterator iterator;
107 /// \brief Beginning of blocks within the chain.
108 iterator begin() const { return Blocks.begin(); }
110 /// \brief End of blocks within the chain.
111 iterator end() const { 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, const 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;
144 /// \brief Count of predecessors within the loop currently being processed.
146 /// This count is updated at each loop we process to represent the number of
147 /// in-loop predecessors of this chain.
148 unsigned LoopPredecessors;
153 class MachineBlockPlacement : public MachineFunctionPass {
154 /// \brief A typedef for a block filter set.
155 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
157 /// \brief A handle to the branch probability pass.
158 const MachineBranchProbabilityInfo *MBPI;
160 /// \brief A handle to the function-wide block frequency pass.
161 const MachineBlockFrequencyInfo *MBFI;
163 /// \brief A handle to the loop info.
164 const MachineLoopInfo *MLI;
166 /// \brief A handle to the target's instruction info.
167 const TargetInstrInfo *TII;
169 /// \brief A handle to the target's lowering info.
170 const TargetLowering *TLI;
172 /// \brief Allocator and owner of BlockChain structures.
174 /// We build BlockChains lazily by merging together high probability BB
175 /// sequences acording to the "Algo2" in the paper mentioned at the top of
176 /// the file. To reduce malloc traffic, we allocate them using this slab-like
177 /// allocator, and destroy them after the pass completes.
178 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
180 /// \brief Function wide BasicBlock to BlockChain mapping.
182 /// This mapping allows efficiently moving from any given basic block to the
183 /// BlockChain it participates in, if any. We use it to, among other things,
184 /// allow implicitly defining edges between chains as the existing edges
185 /// between basic blocks.
186 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
188 void markChainSuccessors(const BlockChain &Chain,
189 const MachineBasicBlock *LoopHeaderBB,
190 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
191 const BlockFilterSet *BlockFilter = 0) const;
192 MachineBasicBlock *selectBestSuccessor(const MachineBasicBlock *BB,
193 const BlockChain &Chain, const BlockFilterSet *BlockFilter) const;
194 MachineBasicBlock *selectBestCandidateBlock(
195 const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
196 const BlockFilterSet *BlockFilter) const;
197 MachineBasicBlock *getFirstUnplacedBlock(
199 const BlockChain &PlacedChain,
200 MachineFunction::iterator &PrevUnplacedBlockIt,
201 const BlockFilterSet *BlockFilter) const;
202 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
203 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
204 const BlockFilterSet *BlockFilter = 0) const;
205 MachineBasicBlock *findBestLoopTop(MachineFunction &F,
207 const BlockFilterSet &LoopBlockSet) const;
208 void buildLoopChains(MachineFunction &F, MachineLoop &L) const;
209 void buildCFGChains(MachineFunction &F);
210 void AlignLoops(MachineFunction &F);
213 static char ID; // Pass identification, replacement for typeid
214 MachineBlockPlacement() : MachineFunctionPass(ID) {
215 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
218 bool runOnMachineFunction(MachineFunction &F);
220 void getAnalysisUsage(AnalysisUsage &AU) const {
221 AU.addRequired<MachineBranchProbabilityInfo>();
222 AU.addRequired<MachineBlockFrequencyInfo>();
223 AU.addRequired<MachineLoopInfo>();
224 MachineFunctionPass::getAnalysisUsage(AU);
227 const char *getPassName() const { return "Block Placement"; }
231 char MachineBlockPlacement::ID = 0;
232 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
233 "Branch Probability Basic Block Placement", false, false)
234 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
235 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
236 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
237 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
238 "Branch Probability Basic Block Placement", false, false)
240 FunctionPass *llvm::createMachineBlockPlacementPass() {
241 return new MachineBlockPlacement();
245 /// \brief Helper to print the name of a MBB.
247 /// Only used by debug logging.
248 static std::string getBlockName(const MachineBasicBlock *BB) {
250 raw_string_ostream OS(Result);
251 OS << "BB#" << BB->getNumber()
252 << " (derived from LLVM BB '" << BB->getName() << "')";
257 /// \brief Helper to print the number of a MBB.
259 /// Only used by debug logging.
260 static std::string getBlockNum(const MachineBasicBlock *BB) {
262 raw_string_ostream OS(Result);
263 OS << "BB#" << BB->getNumber();
269 /// \brief Mark a chain's successors as having one fewer preds.
271 /// When a chain is being merged into the "placed" chain, this routine will
272 /// quickly walk the successors of each block in the chain and mark them as
273 /// having one fewer active predecessor. It also adds any successors of this
274 /// chain which reach the zero-predecessor state to the worklist passed in.
275 void MachineBlockPlacement::markChainSuccessors(
276 const BlockChain &Chain,
277 const MachineBasicBlock *LoopHeaderBB,
278 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
279 const BlockFilterSet *BlockFilter) const {
280 // Walk all the blocks in this chain, marking their successors as having
281 // a predecessor placed.
282 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
284 // Add any successors for which this is the only un-placed in-loop
285 // predecessor to the worklist as a viable candidate for CFG-neutral
286 // placement. No subsequent placement of this block will violate the CFG
287 // shape, so we get to use heuristics to choose a favorable placement.
288 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
289 SE = (*CBI)->succ_end();
291 if (BlockFilter && !BlockFilter->count(*SI))
293 BlockChain &SuccChain = *BlockToChain.lookup(*SI);
294 // Disregard edges within a fixed chain, or edges to the loop header.
295 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
298 // This is a cross-chain edge that is within the loop, so decrement the
299 // loop predecessor count of the destination chain.
300 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
301 BlockWorkList.push_back(*SuccChain.begin());
306 /// \brief Select the best successor for a block.
308 /// This looks across all successors of a particular block and attempts to
309 /// select the "best" one to be the layout successor. It only considers direct
310 /// successors which also pass the block filter. It will attempt to avoid
311 /// breaking CFG structure, but cave and break such structures in the case of
312 /// very hot successor edges.
314 /// \returns The best successor block found, or null if none are viable.
315 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
316 const MachineBasicBlock *BB, const BlockChain &Chain,
317 const BlockFilterSet *BlockFilter) const {
318 const BranchProbability HotProb(4, 5); // 80%
320 MachineBasicBlock *BestSucc = 0;
321 // FIXME: Due to the performance of the probability and weight routines in
322 // the MBPI analysis, we manually compute probabilities using the edge
323 // weights. This is suboptimal as it means that the somewhat subtle
324 // definition of edge weight semantics is encoded here as well. We should
325 // improve the MBPI interface to effeciently support query patterns such as
327 uint32_t BestWeight = 0;
328 uint32_t WeightScale = 0;
329 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
330 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
331 for (MachineBasicBlock::const_succ_iterator SI = BB->succ_begin(),
332 SE = BB->succ_end(); SI != SE; ++SI) {
333 if (BlockFilter && !BlockFilter->count(*SI))
335 const BlockChain &SuccChain = *BlockToChain.lookup(*SI);
336 if (&SuccChain == &Chain) {
337 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
340 if (*SI != *SuccChain.begin()) {
341 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
345 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
346 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
348 // Only consider successors which are either "hot", or wouldn't violate
349 // any CFG constraints.
350 if (SuccChain.LoopPredecessors != 0) {
351 if (SuccProb < HotProb) {
352 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
356 // Make sure that a hot successor doesn't have a globally more important
358 BlockFrequency CandidateEdgeFreq
359 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
360 bool BadCFGConflict = false;
361 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
362 PE = (*SI)->pred_end();
364 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
365 BlockToChain.lookup(*PI) == &Chain)
367 BlockFrequency PredEdgeFreq
368 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
369 if (PredEdgeFreq >= CandidateEdgeFreq) {
370 BadCFGConflict = true;
374 if (BadCFGConflict) {
375 DEBUG(dbgs() << " " << getBlockName(*SI)
376 << " -> non-cold CFG conflict\n");
381 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
383 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
385 if (BestSucc && BestWeight >= SuccWeight)
388 BestWeight = SuccWeight;
394 /// \brief Predicate struct to detect blocks already placed.
395 class IsBlockPlaced {
396 const BlockChain &PlacedChain;
397 const BlockToChainMapType &BlockToChain;
400 IsBlockPlaced(const BlockChain &PlacedChain,
401 const BlockToChainMapType &BlockToChain)
402 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
404 bool operator()(MachineBasicBlock *BB) const {
405 return BlockToChain.lookup(BB) == &PlacedChain;
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 const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
422 const BlockFilterSet *BlockFilter) const {
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 IsBlockPlaced(Chain, BlockToChain)),
431 MachineBasicBlock *BestBlock = 0;
432 BlockFrequency BestFreq;
433 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
434 WBE = WorkList.end();
436 assert(!BlockFilter || BlockFilter->count(*WBI));
437 const BlockChain &SuccChain = *BlockToChain.lookup(*WBI);
438 if (&SuccChain == &Chain) {
439 DEBUG(dbgs() << " " << getBlockName(*WBI)
440 << " -> Already merged!\n");
443 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
445 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
446 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
448 if (BestBlock && BestFreq >= CandidateFreq)
451 BestFreq = CandidateFreq;
456 /// \brief Retrieve the first unplaced basic block.
458 /// This routine is called when we are unable to use the CFG to walk through
459 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
460 /// We walk through the function's blocks in order, starting from the
461 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
462 /// re-scanning the entire sequence on repeated calls to this routine.
463 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
464 MachineFunction &F, const BlockChain &PlacedChain,
465 MachineFunction::iterator &PrevUnplacedBlockIt,
466 const BlockFilterSet *BlockFilter) const {
467 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
469 if (BlockFilter && !BlockFilter->count(I))
471 if (BlockToChain.lookup(I) != &PlacedChain) {
472 PrevUnplacedBlockIt = I;
473 // Now select the head of the chain to which the unplaced block belongs
474 // as the block to place. This will force the entire chain to be placed,
475 // and satisfies the requirements of merging chains.
476 return *BlockToChain.lookup(I)->begin();
482 void MachineBlockPlacement::buildChain(
483 MachineBasicBlock *BB,
485 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
486 const BlockFilterSet *BlockFilter) const {
488 assert(BlockToChain.lookup(BB) == &Chain);
489 MachineFunction &F = *BB->getParent();
490 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
492 MachineBasicBlock *LoopHeaderBB = BB;
493 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
494 BB = *llvm::prior(Chain.end());
497 assert(BlockToChain.lookup(BB) == &Chain);
498 assert(*llvm::prior(Chain.end()) == BB);
499 MachineBasicBlock *BestSucc = 0;
501 // Look for the best viable successor if there is one to place immediately
503 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.lookup(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 = *llvm::prior(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 /// This routine implements the logic to analyze the loop looking for the best
540 /// block to layout at the top of the loop. Typically this is done to maximize
541 /// fallthrough opportunities.
543 MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
545 const BlockFilterSet &LoopBlockSet) const {
546 BlockFrequency BestExitEdgeFreq;
547 MachineBasicBlock *ExitingBB = 0;
548 MachineBasicBlock *LoopingBB = 0;
549 // If there are exits to outer loops, loop rotation can severely limit
550 // fallthrough opportunites unless it selects such an exit. Keep a set of
551 // blocks where rotating to exit with that block will reach an outer loop.
552 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
554 DEBUG(dbgs() << "Finding best loop exit for: "
555 << getBlockName(L.getHeader()) << "\n");
556 for (MachineLoop::block_iterator I = L.block_begin(),
559 const BlockChain &Chain = *BlockToChain.lookup(*I);
560 // Ensure that this block is at the end of a chain; otherwise it could be
561 // mid-way through an inner loop or a successor of an analyzable branch.
562 if (*I != *llvm::prior(Chain.end()))
565 // Now walk the successors. We need to establish whether this has a viable
566 // exiting successor and whether it has a viable non-exiting successor.
567 // We store the old exiting state and restore it if a viable looping
568 // successor isn't found.
569 MachineBasicBlock *OldExitingBB = ExitingBB;
570 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
571 // We also compute and store the best looping successor for use in layout.
572 MachineBasicBlock *BestLoopSucc = 0;
573 // FIXME: Due to the performance of the probability and weight routines in
574 // the MBPI analysis, we use the internal weights. This is only valid
575 // because it is purely a ranking function, we don't care about anything
576 // but the relative values.
577 uint32_t BestLoopSuccWeight = 0;
578 // FIXME: We also manually compute the probabilities to avoid quadratic
580 uint32_t WeightScale = 0;
581 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
582 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
583 SE = (*I)->succ_end();
585 if ((*SI)->isLandingPad())
589 const BlockChain &SuccChain = *BlockToChain.lookup(*SI);
590 // Don't split chains, either this chain or the successor's chain.
591 if (&Chain == &SuccChain || *SI != *SuccChain.begin()) {
592 DEBUG(dbgs() << " " << (LoopBlockSet.count(*SI) ? "looping: "
594 << getBlockName(*I) << " -> "
595 << getBlockName(*SI) << " (chain conflict)\n");
599 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
600 if (LoopBlockSet.count(*SI)) {
601 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
602 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
603 if (BestLoopSucc && BestLoopSuccWeight >= SuccWeight)
607 BestLoopSuccWeight = SuccWeight;
611 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
612 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
613 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
614 << getBlockName(*SI) << " (" << ExitEdgeFreq << ")\n");
615 // Note that we slightly bias this toward an existing layout successor to
616 // retain incoming order in the absence of better information.
617 // FIXME: Should we bias this more strongly? It's pretty weak.
618 if (!ExitingBB || ExitEdgeFreq > BestExitEdgeFreq ||
619 ((*I)->isLayoutSuccessor(*SI) &&
620 !(ExitEdgeFreq < BestExitEdgeFreq))) {
621 BestExitEdgeFreq = ExitEdgeFreq;
625 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI))
626 if (ExitLoop->contains(&L))
627 BlocksExitingToOuterLoop.insert(*I);
630 // Restore the old exiting state, no viable looping successor was found.
632 ExitingBB = OldExitingBB;
633 BestExitEdgeFreq = OldBestExitEdgeFreq;
637 // If this was best exiting block thus far, also record the looping block.
639 LoopingBB = BestLoopSucc;
641 // Without a candidate exitting block or with only a single block in the
642 // loop, just use the loop header to layout the loop.
643 if (!ExitingBB || L.getNumBlocks() == 1)
644 return L.getHeader();
646 // Also, if we have exit blocks which lead to outer loops but didn't select
647 // one of them as the exiting block we are rotating toward, disable loop
648 // rotation altogether.
649 if (!BlocksExitingToOuterLoop.empty() &&
650 !BlocksExitingToOuterLoop.count(ExitingBB))
651 return L.getHeader();
653 assert(LoopingBB && "All successors of a loop block are exit blocks!");
654 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
655 DEBUG(dbgs() << " Best top block: " << getBlockName(LoopingBB) << "\n");
659 /// \brief Forms basic block chains from the natural loop structures.
661 /// These chains are designed to preserve the existing *structure* of the code
662 /// as much as possible. We can then stitch the chains together in a way which
663 /// both preserves the topological structure and minimizes taken conditional
665 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
666 MachineLoop &L) const {
667 // First recurse through any nested loops, building chains for those inner
669 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
670 buildLoopChains(F, **LI);
672 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
673 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
675 MachineBasicBlock *LayoutTop = findBestLoopTop(F, L, LoopBlockSet);
676 BlockChain &LoopChain = *BlockToChain.lookup(LayoutTop);
678 // FIXME: This is a really lame way of walking the chains in the loop: we
679 // walk the blocks, and use a set to prevent visiting a particular chain
681 SmallPtrSet<const BlockChain *, 4> UpdatedPreds;
682 assert(LoopChain.LoopPredecessors == 0);
683 UpdatedPreds.insert(&LoopChain);
684 for (MachineLoop::block_iterator BI = L.block_begin(),
687 BlockChain &Chain = *BlockToChain.lookup(*BI);
688 if (!UpdatedPreds.insert(&Chain))
691 assert(Chain.LoopPredecessors == 0);
692 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
694 assert(BlockToChain.lookup(*BCI) == &Chain);
695 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
696 PE = (*BCI)->pred_end();
698 if (BlockToChain.lookup(*PI) == &Chain || !LoopBlockSet.count(*PI))
700 ++Chain.LoopPredecessors;
704 if (Chain.LoopPredecessors == 0)
705 BlockWorkList.push_back(*Chain.begin());
708 buildChain(LayoutTop, LoopChain, BlockWorkList, &LoopBlockSet);
711 // Crash at the end so we get all of the debugging output first.
712 bool BadLoop = false;
713 if (LoopChain.LoopPredecessors) {
715 dbgs() << "Loop chain contains a block without its preds placed!\n"
716 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
717 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
719 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
721 if (!LoopBlockSet.erase(*BCI)) {
722 // We don't mark the loop as bad here because there are real situations
723 // where this can occur. For example, with an unanalyzable fallthrough
724 // from a loop block to a non-loop block or vice versa.
725 dbgs() << "Loop chain contains a block not contained by the loop!\n"
726 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
727 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
728 << " Bad block: " << getBlockName(*BCI) << "\n";
731 if (!LoopBlockSet.empty()) {
733 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
734 LBE = LoopBlockSet.end();
736 dbgs() << "Loop contains blocks never placed into a chain!\n"
737 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
738 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
739 << " Bad block: " << getBlockName(*LBI) << "\n";
741 assert(!BadLoop && "Detected problems with the placement of this loop.");
745 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
746 // Ensure that every BB in the function has an associated chain to simplify
747 // the assumptions of the remaining algorithm.
748 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
749 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
750 MachineBasicBlock *BB = FI;
752 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
753 // Also, merge any blocks which we cannot reason about and must preserve
754 // the exact fallthrough behavior for.
757 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
758 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
761 MachineFunction::iterator NextFI(llvm::next(FI));
762 MachineBasicBlock *NextBB = NextFI;
763 // Ensure that the layout successor is a viable block, as we know that
764 // fallthrough is a possibility.
765 assert(NextFI != FE && "Can't fallthrough past the last block.");
766 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
767 << getBlockName(BB) << " -> " << getBlockName(NextBB)
769 Chain->merge(NextBB, 0);
775 // Build any loop-based chains.
776 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
778 buildLoopChains(F, **LI);
780 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
782 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
783 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
784 MachineBasicBlock *BB = &*FI;
785 BlockChain &Chain = *BlockToChain[BB];
786 if (!UpdatedPreds.insert(&Chain))
789 assert(Chain.LoopPredecessors == 0);
790 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
792 assert(BlockToChain[*BCI] == &Chain);
793 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
794 PE = (*BCI)->pred_end();
796 if (BlockToChain[*PI] == &Chain)
798 ++Chain.LoopPredecessors;
802 if (Chain.LoopPredecessors == 0)
803 BlockWorkList.push_back(*Chain.begin());
806 BlockChain &FunctionChain = *BlockToChain[&F.front()];
807 buildChain(&F.front(), FunctionChain, BlockWorkList);
809 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
811 // Crash at the end so we get all of the debugging output first.
812 bool BadFunc = false;
813 FunctionBlockSetType FunctionBlockSet;
814 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
815 FunctionBlockSet.insert(FI);
817 for (BlockChain::iterator BCI = FunctionChain.begin(),
818 BCE = FunctionChain.end();
820 if (!FunctionBlockSet.erase(*BCI)) {
822 dbgs() << "Function chain contains a block not in the function!\n"
823 << " Bad block: " << getBlockName(*BCI) << "\n";
826 if (!FunctionBlockSet.empty()) {
828 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
829 FBE = FunctionBlockSet.end();
831 dbgs() << "Function contains blocks never placed into a chain!\n"
832 << " Bad block: " << getBlockName(*FBI) << "\n";
834 assert(!BadFunc && "Detected problems with the block placement.");
837 // Splice the blocks into place.
838 MachineFunction::iterator InsertPos = F.begin();
839 for (BlockChain::iterator BI = FunctionChain.begin(),
840 BE = FunctionChain.end();
842 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
844 << getBlockName(*BI) << "\n");
845 if (InsertPos != MachineFunction::iterator(*BI))
846 F.splice(InsertPos, *BI);
850 // Update the terminator of the previous block.
851 if (BI == FunctionChain.begin())
853 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
855 // FIXME: It would be awesome of updateTerminator would just return rather
856 // than assert when the branch cannot be analyzed in order to remove this
859 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
860 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
861 PrevBB->updateTerminator();
864 // Fixup the last block.
866 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
867 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
868 F.back().updateTerminator();
871 /// \brief Recursive helper to align a loop and any nested loops.
872 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
873 // Recurse through nested loops.
874 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
875 AlignLoop(F, *I, Align);
877 L->getTopBlock()->setAlignment(Align);
880 /// \brief Align loop headers to target preferred alignments.
881 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
882 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
885 unsigned Align = TLI->getPrefLoopAlignment();
887 return; // Don't care about loop alignment.
889 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
890 AlignLoop(F, *I, Align);
893 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
894 // Check for single-block functions and skip them.
895 if (llvm::next(F.begin()) == F.end())
898 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
899 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
900 MLI = &getAnalysis<MachineLoopInfo>();
901 TII = F.getTarget().getInstrInfo();
902 TLI = F.getTarget().getTargetLowering();
903 assert(BlockToChain.empty());
908 BlockToChain.clear();
909 ChainAllocator.DestroyAll();
911 // We always return true as we have no way to track whether the final order
912 // differs from the original order.
917 /// \brief A pass to compute block placement statistics.
919 /// A separate pass to compute interesting statistics for evaluating block
920 /// placement. This is separate from the actual placement pass so that they can
921 /// be computed in the absense of any placement transformations or when using
922 /// alternative placement strategies.
923 class MachineBlockPlacementStats : public MachineFunctionPass {
924 /// \brief A handle to the branch probability pass.
925 const MachineBranchProbabilityInfo *MBPI;
927 /// \brief A handle to the function-wide block frequency pass.
928 const MachineBlockFrequencyInfo *MBFI;
931 static char ID; // Pass identification, replacement for typeid
932 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
933 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
936 bool runOnMachineFunction(MachineFunction &F);
938 void getAnalysisUsage(AnalysisUsage &AU) const {
939 AU.addRequired<MachineBranchProbabilityInfo>();
940 AU.addRequired<MachineBlockFrequencyInfo>();
941 AU.setPreservesAll();
942 MachineFunctionPass::getAnalysisUsage(AU);
945 const char *getPassName() const { return "Block Placement Stats"; }
949 char MachineBlockPlacementStats::ID = 0;
950 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
951 "Basic Block Placement Stats", false, false)
952 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
953 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
954 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
955 "Basic Block Placement Stats", false, false)
957 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
958 return new MachineBlockPlacementStats();
961 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
962 // Check for single-block functions and skip them.
963 if (llvm::next(F.begin()) == F.end())
966 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
967 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
969 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
970 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
971 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
973 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
974 : UncondBranchTakenFreq;
975 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
978 // Skip if this successor is a fallthrough.
979 if (I->isLayoutSuccessor(*SI))
982 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
984 BranchTakenFreq += EdgeFreq.getFrequency();