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/Support/ErrorHandling.h"
40 #include "llvm/ADT/DenseMap.h"
41 #include "llvm/ADT/PostOrderIterator.h"
42 #include "llvm/ADT/SCCIterator.h"
43 #include "llvm/ADT/SmallPtrSet.h"
44 #include "llvm/ADT/SmallVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Target/TargetInstrInfo.h"
47 #include "llvm/Target/TargetLowering.h"
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");
59 /// \brief A structure for storing a weighted edge.
61 /// This stores an edge and its weight, computed as the product of the
62 /// frequency that the starting block is entered with the probability of
63 /// a particular exit block.
65 BlockFrequency EdgeFrequency;
66 MachineBasicBlock *From, *To;
68 bool operator<(const WeightedEdge &RHS) const {
69 return EdgeFrequency < RHS.EdgeFrequency;
76 /// \brief Type for our function-wide basic block -> block chain mapping.
77 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
81 /// \brief A chain of blocks which will be laid out contiguously.
83 /// This is the datastructure representing a chain of consecutive blocks that
84 /// are profitable to layout together in order to maximize fallthrough
85 /// probabilities. We also can use a block chain to represent a sequence of
86 /// basic blocks which have some external (correctness) requirement for
87 /// sequential layout.
89 /// Eventually, the block chains will form a directed graph over the function.
90 /// We provide an SCC-supporting-iterator in order to quicky build and walk the
91 /// SCCs of block chains within a function.
93 /// The block chains also have support for calculating and caching probability
94 /// information related to the chain itself versus other chains. This is used
95 /// for ranking during the final layout of block chains.
97 /// \brief The sequence of blocks belonging to this chain.
99 /// This is the sequence of blocks for a particular chain. These will be laid
100 /// out in-order within the function.
101 SmallVector<MachineBasicBlock *, 4> Blocks;
103 /// \brief A handle to the function-wide basic block to block chain mapping.
105 /// This is retained in each block chain to simplify the computation of child
106 /// block chains for SCC-formation and iteration. We store the edges to child
107 /// basic blocks, and map them back to their associated chains using this
109 BlockToChainMapType &BlockToChain;
112 /// \brief Construct a new BlockChain.
114 /// This builds a new block chain representing a single basic block in the
115 /// function. It also registers itself as the chain that block participates
116 /// in with the BlockToChain mapping.
117 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
118 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
119 assert(BB && "Cannot create a chain with a null basic block");
120 BlockToChain[BB] = this;
123 /// \brief Iterator over blocks within the chain.
124 typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator;
126 /// \brief Beginning of blocks within the chain.
127 iterator begin() const { return Blocks.begin(); }
129 /// \brief End of blocks within the chain.
130 iterator end() const { return Blocks.end(); }
132 /// \brief Merge a block chain into this one.
134 /// This routine merges a block chain into this one. It takes care of forming
135 /// a contiguous sequence of basic blocks, updating the edge list, and
136 /// updating the block -> chain mapping. It does not free or tear down the
137 /// old chain, but the old chain's block list is no longer valid.
138 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
140 assert(!Blocks.empty());
142 // Fast path in case we don't have a chain already.
144 assert(!BlockToChain[BB]);
145 Blocks.push_back(BB);
146 BlockToChain[BB] = this;
150 assert(BB == *Chain->begin());
151 assert(Chain->begin() != Chain->end());
153 // Update the incoming blocks to point to this chain, and add them to the
155 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
157 Blocks.push_back(*BI);
158 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
159 BlockToChain[*BI] = this;
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 TargetLowering *TLI;
191 /// \brief Allocator and owner of BlockChain structures.
193 /// We build BlockChains lazily by merging together high probability BB
194 /// sequences acording to the "Algo2" in the paper mentioned at the top of
195 /// the file. To reduce malloc traffic, we allocate them using this slab-like
196 /// allocator, and destroy them after the pass completes.
197 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
199 /// \brief Function wide BasicBlock to BlockChain mapping.
201 /// This mapping allows efficiently moving from any given basic block to the
202 /// BlockChain it participates in, if any. We use it to, among other things,
203 /// allow implicitly defining edges between chains as the existing edges
204 /// between basic blocks.
205 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
207 void markChainSuccessors(BlockChain &Chain,
208 MachineBasicBlock *LoopHeaderBB,
209 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
210 const BlockFilterSet *BlockFilter = 0);
211 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
213 const BlockFilterSet *BlockFilter);
214 MachineBasicBlock *selectBestCandidateBlock(
215 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
216 const BlockFilterSet *BlockFilter);
217 MachineBasicBlock *getFirstUnplacedBlock(const BlockChain &PlacedChain,
218 ArrayRef<MachineBasicBlock *> Blocks,
219 unsigned &PrevUnplacedBlockIdx);
220 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
221 ArrayRef<MachineBasicBlock *> Blocks,
222 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
223 const BlockFilterSet *BlockFilter = 0);
224 void buildLoopChains(MachineFunction &F, MachineLoop &L);
225 void buildCFGChains(MachineFunction &F);
226 void AlignLoops(MachineFunction &F);
229 static char ID; // Pass identification, replacement for typeid
230 MachineBlockPlacement() : MachineFunctionPass(ID) {
231 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
234 bool runOnMachineFunction(MachineFunction &F);
236 void getAnalysisUsage(AnalysisUsage &AU) const {
237 AU.addRequired<MachineBranchProbabilityInfo>();
238 AU.addRequired<MachineBlockFrequencyInfo>();
239 AU.addRequired<MachineLoopInfo>();
240 MachineFunctionPass::getAnalysisUsage(AU);
243 const char *getPassName() const { return "Block Placement"; }
247 char MachineBlockPlacement::ID = 0;
248 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
249 "Branch Probability Basic Block Placement", false, false)
250 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
251 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
252 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
253 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
254 "Branch Probability Basic Block Placement", false, false)
256 FunctionPass *llvm::createMachineBlockPlacementPass() {
257 return new MachineBlockPlacement();
261 /// \brief Helper to print the name of a MBB.
263 /// Only used by debug logging.
264 static std::string getBlockName(MachineBasicBlock *BB) {
266 raw_string_ostream OS(Result);
267 OS << "BB#" << BB->getNumber()
268 << " (derived from LLVM BB '" << BB->getName() << "')";
273 /// \brief Helper to print the number of a MBB.
275 /// Only used by debug logging.
276 static std::string getBlockNum(MachineBasicBlock *BB) {
278 raw_string_ostream OS(Result);
279 OS << "BB#" << BB->getNumber();
285 /// \brief Mark a chain's successors as having one fewer preds.
287 /// When a chain is being merged into the "placed" chain, this routine will
288 /// quickly walk the successors of each block in the chain and mark them as
289 /// having one fewer active predecessor. It also adds any successors of this
290 /// chain which reach the zero-predecessor state to the worklist passed in.
291 void MachineBlockPlacement::markChainSuccessors(
293 MachineBasicBlock *LoopHeaderBB,
294 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
295 const BlockFilterSet *BlockFilter) {
296 // Walk all the blocks in this chain, marking their successors as having
297 // a predecessor placed.
298 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
300 // Add any successors for which this is the only un-placed in-loop
301 // predecessor to the worklist as a viable candidate for CFG-neutral
302 // placement. No subsequent placement of this block will violate the CFG
303 // shape, so we get to use heuristics to choose a favorable placement.
304 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
305 SE = (*CBI)->succ_end();
307 if (BlockFilter && !BlockFilter->count(*SI))
309 BlockChain &SuccChain = *BlockToChain[*SI];
310 // Disregard edges within a fixed chain, or edges to the loop header.
311 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
314 // This is a cross-chain edge that is within the loop, so decrement the
315 // loop predecessor count of the destination chain.
316 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
317 BlockWorkList.push_back(*SI);
322 /// \brief Select the best successor for a block.
324 /// This looks across all successors of a particular block and attempts to
325 /// select the "best" one to be the layout successor. It only considers direct
326 /// successors which also pass the block filter. It will attempt to avoid
327 /// breaking CFG structure, but cave and break such structures in the case of
328 /// very hot successor edges.
330 /// \returns The best successor block found, or null if none are viable.
331 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
332 MachineBasicBlock *BB, BlockChain &Chain,
333 const BlockFilterSet *BlockFilter) {
334 const BranchProbability HotProb(4, 5); // 80%
336 MachineBasicBlock *BestSucc = 0;
337 BranchProbability BestProb = BranchProbability::getZero();
338 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
339 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
342 if (BlockFilter && !BlockFilter->count(*SI))
344 BlockChain &SuccChain = *BlockToChain[*SI];
345 if (&SuccChain == &Chain) {
346 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
350 BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
352 // Only consider successors which are either "hot", or wouldn't violate
353 // any CFG constraints.
354 if (SuccChain.LoopPredecessors != 0 && SuccProb < HotProb) {
355 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
359 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
361 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
363 if (BestSucc && BestProb >= SuccProb)
371 /// \brief Select the best block from a worklist.
373 /// This looks through the provided worklist as a list of candidate basic
374 /// blocks and select the most profitable one to place. The definition of
375 /// profitable only really makes sense in the context of a loop. This returns
376 /// the most frequently visited block in the worklist, which in the case of
377 /// a loop, is the one most desirable to be physically close to the rest of the
378 /// loop body in order to improve icache behavior.
380 /// \returns The best block found, or null if none are viable.
381 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
382 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
383 const BlockFilterSet *BlockFilter) {
384 MachineBasicBlock *BestBlock = 0;
385 BlockFrequency BestFreq;
386 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
387 WBE = WorkList.end();
389 if (BlockFilter && !BlockFilter->count(*WBI))
391 BlockChain &SuccChain = *BlockToChain[*WBI];
392 if (&SuccChain == &Chain) {
393 DEBUG(dbgs() << " " << getBlockName(*WBI)
394 << " -> Already merged!\n");
397 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
399 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
400 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
402 if (BestBlock && BestFreq >= CandidateFreq)
405 BestFreq = CandidateFreq;
410 /// \brief Retrieve the first unplaced basic block.
412 /// This routine is called when we are unable to use the CFG to walk through
413 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
414 /// We walk through the sequence of blocks, starting from the
415 /// LastUnplacedBlockIdx. We update this index to avoid re-scanning the entire
416 /// sequence on repeated calls to this routine.
417 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
418 const BlockChain &PlacedChain,
419 ArrayRef<MachineBasicBlock *> Blocks,
420 unsigned &PrevUnplacedBlockIdx) {
421 for (unsigned i = PrevUnplacedBlockIdx, e = Blocks.size(); i != e; ++i) {
422 MachineBasicBlock *BB = Blocks[i];
423 if (BlockToChain[BB] != &PlacedChain) {
424 PrevUnplacedBlockIdx = i;
431 void MachineBlockPlacement::buildChain(
432 MachineBasicBlock *BB,
434 ArrayRef<MachineBasicBlock *> Blocks,
435 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
436 const BlockFilterSet *BlockFilter) {
438 assert(BlockToChain[BB] == &Chain);
439 assert(*Chain.begin() == BB);
440 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
441 unsigned PrevUnplacedBlockIdx = 0;
443 MachineBasicBlock *LoopHeaderBB = BB;
444 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
445 BB = *llvm::prior(Chain.end());
448 assert(BlockToChain[BB] == &Chain);
449 assert(*llvm::prior(Chain.end()) == BB);
450 MachineBasicBlock *BestSucc = 0;
452 // Check for unreasonable branches, and forcibly merge the existing layout
453 // successor for them. We can handle cases that AnalyzeBranch can't: jump
454 // tables etc are fine. The case we want to handle specially is when there
455 // is potential fallthrough, but the branch cannot be analyzed. This
456 // includes blocks without terminators as well as other cases.
458 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
459 if (TII->AnalyzeBranch(*BB, TBB, FBB, Cond) && BB->canFallThrough()) {
460 MachineFunction::iterator I(BB), NextI(llvm::next(I));
461 // Ensure that the layout successor is a viable block, as we know that
462 // fallthrough is a possibility.
463 assert(NextI != BB->getParent()->end());
464 assert(!BlockFilter || BlockFilter->count(NextI));
468 // Otherwise, look for the best viable successor if there is one to place
469 // immediately after this block.
471 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
473 // If an immediate successor isn't available, look for the best viable
474 // block among those we've identified as not violating the loop's CFG at
475 // this point. This won't be a fallthrough, but it will increase locality.
477 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
480 BestSucc = getFirstUnplacedBlock(Chain, Blocks, PrevUnplacedBlockIdx);
484 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
485 "layout successor until the CFG reduces\n");
488 // Place this block, updating the datastructures to reflect its placement.
489 BlockChain &SuccChain = *BlockToChain[BestSucc];
490 // Zero out LoopPredecessors for the successor we're about to merge in case
491 // we selected a successor that didn't fit naturally into the CFG.
492 SuccChain.LoopPredecessors = 0;
493 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
494 << " to " << getBlockNum(BestSucc) << "\n");
495 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
496 Chain.merge(BestSucc, &SuccChain);
497 BB = *llvm::prior(Chain.end());
500 DEBUG(dbgs() << "Finished forming chain for header block "
501 << getBlockNum(*Chain.begin()) << "\n");
504 /// \brief Forms basic block chains from the natural loop structures.
506 /// These chains are designed to preserve the existing *structure* of the code
507 /// as much as possible. We can then stitch the chains together in a way which
508 /// both preserves the topological structure and minimizes taken conditional
510 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
512 // First recurse through any nested loops, building chains for those inner
514 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
515 buildLoopChains(F, **LI);
517 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
518 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
519 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
521 // FIXME: This is a really lame way of walking the chains in the loop: we
522 // walk the blocks, and use a set to prevent visiting a particular chain
524 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
525 for (MachineLoop::block_iterator BI = L.block_begin(),
528 BlockChain &Chain = *BlockToChain[*BI];
529 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin())
532 assert(Chain.LoopPredecessors == 0);
533 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
535 assert(BlockToChain[*BCI] == &Chain);
536 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
537 PE = (*BCI)->pred_end();
539 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
541 ++Chain.LoopPredecessors;
545 if (Chain.LoopPredecessors == 0)
546 BlockWorkList.push_back(*BI);
549 buildChain(*L.block_begin(), LoopChain, L.getBlocks(), BlockWorkList,
553 // Crash at the end so we get all of the debugging output first.
554 bool BadLoop = false;
555 if (LoopChain.LoopPredecessors) {
557 dbgs() << "Loop chain contains a block without its preds placed!\n"
558 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
559 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
561 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
563 if (!LoopBlockSet.erase(*BCI)) {
565 dbgs() << "Loop chain contains a block not contained by the loop!\n"
566 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
567 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
568 << " Bad block: " << getBlockName(*BCI) << "\n";
571 if (!LoopBlockSet.empty()) {
573 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
574 LBE = LoopBlockSet.end();
576 dbgs() << "Loop contains blocks never placed into a chain!\n"
577 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
578 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
579 << " Bad block: " << getBlockName(*LBI) << "\n";
581 assert(!BadLoop && "Detected problems with the placement of this loop.");
585 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
586 // Ensure that every BB in the function has an associated chain to simplify
587 // the assumptions of the remaining algorithm.
588 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
590 new (ChainAllocator.Allocate()) BlockChain(BlockToChain, &*FI);
592 // Build any loop-based chains.
593 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
595 buildLoopChains(F, **LI);
597 // We need a vector of blocks so that buildChain can handle unnatural CFG
598 // constructs by searching for unplaced blocks and just concatenating them.
599 SmallVector<MachineBasicBlock *, 16> Blocks;
600 Blocks.reserve(F.size());
602 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
604 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
605 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
606 MachineBasicBlock *BB = &*FI;
607 Blocks.push_back(BB);
608 BlockChain &Chain = *BlockToChain[BB];
609 if (!UpdatedPreds.insert(&Chain))
612 assert(Chain.LoopPredecessors == 0);
613 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
615 assert(BlockToChain[*BCI] == &Chain);
616 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
617 PE = (*BCI)->pred_end();
619 if (BlockToChain[*PI] == &Chain)
621 ++Chain.LoopPredecessors;
625 if (Chain.LoopPredecessors == 0)
626 BlockWorkList.push_back(BB);
629 BlockChain &FunctionChain = *BlockToChain[&F.front()];
630 buildChain(&F.front(), FunctionChain, Blocks, BlockWorkList);
632 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
634 // Crash at the end so we get all of the debugging output first.
635 bool BadFunc = false;
636 FunctionBlockSetType FunctionBlockSet;
637 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
638 FunctionBlockSet.insert(FI);
640 for (BlockChain::iterator BCI = FunctionChain.begin(),
641 BCE = FunctionChain.end();
643 if (!FunctionBlockSet.erase(*BCI)) {
645 dbgs() << "Function chain contains a block not in the function!\n"
646 << " Bad block: " << getBlockName(*BCI) << "\n";
649 if (!FunctionBlockSet.empty()) {
651 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
652 FBE = FunctionBlockSet.end();
654 dbgs() << "Function contains blocks never placed into a chain!\n"
655 << " Bad block: " << getBlockName(*FBI) << "\n";
657 assert(!BadFunc && "Detected problems with the block placement.");
660 // Splice the blocks into place.
661 MachineFunction::iterator InsertPos = F.begin();
662 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
663 for (BlockChain::iterator BI = FunctionChain.begin(),
664 BE = FunctionChain.end();
666 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
668 << getBlockName(*BI) << "\n");
669 if (InsertPos != MachineFunction::iterator(*BI))
670 F.splice(InsertPos, *BI);
674 // Update the terminator of the previous block.
675 if (BI == FunctionChain.begin())
677 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
679 // FIXME: It would be awesome of updateTerminator would just return rather
680 // than assert when the branch cannot be analyzed in order to remove this
683 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
684 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
685 PrevBB->updateTerminator();
688 // Fixup the last block.
690 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
691 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
692 F.back().updateTerminator();
695 /// \brief Recursive helper to align a loop and any nested loops.
696 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
697 // Recurse through nested loops.
698 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
699 AlignLoop(F, *I, Align);
701 L->getTopBlock()->setAlignment(Align);
704 /// \brief Align loop headers to target preferred alignments.
705 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
706 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
709 unsigned Align = TLI->getPrefLoopAlignment();
711 return; // Don't care about loop alignment.
713 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
714 AlignLoop(F, *I, Align);
717 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
718 // Check for single-block functions and skip them.
719 if (llvm::next(F.begin()) == F.end())
722 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
723 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
724 MLI = &getAnalysis<MachineLoopInfo>();
725 TII = F.getTarget().getInstrInfo();
726 TLI = F.getTarget().getTargetLowering();
727 assert(BlockToChain.empty());
732 BlockToChain.clear();
734 // We always return true as we have no way to track whether the final order
735 // differs from the original order.
740 /// \brief A pass to compute block placement statistics.
742 /// A separate pass to compute interesting statistics for evaluating block
743 /// placement. This is separate from the actual placement pass so that they can
744 /// be computed in the absense of any placement transformations or when using
745 /// alternative placement strategies.
746 class MachineBlockPlacementStats : public MachineFunctionPass {
747 /// \brief A handle to the branch probability pass.
748 const MachineBranchProbabilityInfo *MBPI;
750 /// \brief A handle to the function-wide block frequency pass.
751 const MachineBlockFrequencyInfo *MBFI;
754 static char ID; // Pass identification, replacement for typeid
755 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
756 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
759 bool runOnMachineFunction(MachineFunction &F);
761 void getAnalysisUsage(AnalysisUsage &AU) const {
762 AU.addRequired<MachineBranchProbabilityInfo>();
763 AU.addRequired<MachineBlockFrequencyInfo>();
764 AU.setPreservesAll();
765 MachineFunctionPass::getAnalysisUsage(AU);
768 const char *getPassName() const { return "Block Placement Stats"; }
772 char MachineBlockPlacementStats::ID = 0;
773 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
774 "Basic Block Placement Stats", false, false)
775 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
776 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
777 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
778 "Basic Block Placement Stats", false, false)
780 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
781 return new MachineBlockPlacementStats();
784 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
785 // Check for single-block functions and skip them.
786 if (llvm::next(F.begin()) == F.end())
789 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
790 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
792 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
793 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
794 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
796 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
797 : UncondBranchTakenFreq;
798 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
801 // Skip if this successor is a fallthrough.
802 if (I->isLayoutSuccessor(*SI))
805 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
807 BranchTakenFreq += EdgeFreq.getFrequency();