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 // FIXME: Due to the performance of the probability and weight routines in
338 // the MBPI analysis, we manually compute probabilities using the edge
339 // weights. This is suboptimal as it means that the somewhat subtle
340 // definition of edge weight semantics is encoded here as well. We should
341 // improve the MBPI interface to effeciently support query patterns such as
343 uint32_t BestWeight = 0;
344 uint32_t WeightScale = 0;
345 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
346 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
347 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
350 if (BlockFilter && !BlockFilter->count(*SI))
352 BlockChain &SuccChain = *BlockToChain[*SI];
353 if (&SuccChain == &Chain) {
354 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
358 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
359 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
361 // Only consider successors which are either "hot", or wouldn't violate
362 // any CFG constraints.
363 if (SuccChain.LoopPredecessors != 0 && SuccProb < HotProb) {
364 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
368 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
370 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
372 if (BestSucc && BestWeight >= SuccWeight)
375 BestWeight = SuccWeight;
381 /// \brief Predicate struct to detect blocks already placed.
382 class IsBlockPlaced {
383 const BlockChain &PlacedChain;
384 const BlockToChainMapType &BlockToChain;
387 IsBlockPlaced(const BlockChain &PlacedChain,
388 const BlockToChainMapType &BlockToChain)
389 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
391 bool operator()(MachineBasicBlock *BB) const {
392 return BlockToChain.lookup(BB) == &PlacedChain;
397 /// \brief Select the best block from a worklist.
399 /// This looks through the provided worklist as a list of candidate basic
400 /// blocks and select the most profitable one to place. The definition of
401 /// profitable only really makes sense in the context of a loop. This returns
402 /// the most frequently visited block in the worklist, which in the case of
403 /// a loop, is the one most desirable to be physically close to the rest of the
404 /// loop body in order to improve icache behavior.
406 /// \returns The best block found, or null if none are viable.
407 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
408 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
409 const BlockFilterSet *BlockFilter) {
410 // Once we need to walk the worklist looking for a candidate, cleanup the
411 // worklist of already placed entries.
412 // FIXME: If this shows up on profiles, it could be folded (at the cost of
413 // some code complexity) into the loop below.
414 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
415 IsBlockPlaced(Chain, BlockToChain)),
418 MachineBasicBlock *BestBlock = 0;
419 BlockFrequency BestFreq;
420 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
421 WBE = WorkList.end();
423 assert(!BlockFilter || BlockFilter->count(*WBI));
424 BlockChain &SuccChain = *BlockToChain[*WBI];
425 if (&SuccChain == &Chain) {
426 DEBUG(dbgs() << " " << getBlockName(*WBI)
427 << " -> Already merged!\n");
430 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
432 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
433 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
435 if (BestBlock && BestFreq >= CandidateFreq)
438 BestFreq = CandidateFreq;
443 /// \brief Retrieve the first unplaced basic block.
445 /// This routine is called when we are unable to use the CFG to walk through
446 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
447 /// We walk through the sequence of blocks, starting from the
448 /// LastUnplacedBlockIdx. We update this index to avoid re-scanning the entire
449 /// sequence on repeated calls to this routine.
450 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
451 const BlockChain &PlacedChain,
452 ArrayRef<MachineBasicBlock *> Blocks,
453 unsigned &PrevUnplacedBlockIdx) {
454 for (unsigned i = PrevUnplacedBlockIdx, e = Blocks.size(); i != e; ++i) {
455 MachineBasicBlock *BB = Blocks[i];
456 if (BlockToChain[BB] != &PlacedChain) {
457 PrevUnplacedBlockIdx = i;
464 void MachineBlockPlacement::buildChain(
465 MachineBasicBlock *BB,
467 ArrayRef<MachineBasicBlock *> Blocks,
468 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
469 const BlockFilterSet *BlockFilter) {
471 assert(BlockToChain[BB] == &Chain);
472 assert(*Chain.begin() == BB);
473 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
474 unsigned PrevUnplacedBlockIdx = 0;
476 MachineBasicBlock *LoopHeaderBB = BB;
477 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
478 BB = *llvm::prior(Chain.end());
481 assert(BlockToChain[BB] == &Chain);
482 assert(*llvm::prior(Chain.end()) == BB);
483 MachineBasicBlock *BestSucc = 0;
485 // Check for unreasonable branches, and forcibly merge the existing layout
486 // successor for them. We can handle cases that AnalyzeBranch can't: jump
487 // tables etc are fine. The case we want to handle specially is when there
488 // is potential fallthrough, but the branch cannot be analyzed. This
489 // includes blocks without terminators as well as other cases.
491 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
492 if (TII->AnalyzeBranch(*BB, TBB, FBB, Cond) && BB->canFallThrough()) {
493 MachineFunction::iterator I(BB), NextI(llvm::next(I));
494 // Ensure that the layout successor is a viable block, as we know that
495 // fallthrough is a possibility.
496 assert(NextI != BB->getParent()->end());
497 assert(!BlockFilter || BlockFilter->count(NextI));
501 // Otherwise, look for the best viable successor if there is one to place
502 // immediately after this block.
504 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
506 // If an immediate successor isn't available, look for the best viable
507 // block among those we've identified as not violating the loop's CFG at
508 // this point. This won't be a fallthrough, but it will increase locality.
510 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
513 BestSucc = getFirstUnplacedBlock(Chain, Blocks, PrevUnplacedBlockIdx);
517 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
518 "layout successor until the CFG reduces\n");
521 // Place this block, updating the datastructures to reflect its placement.
522 BlockChain &SuccChain = *BlockToChain[BestSucc];
523 // Zero out LoopPredecessors for the successor we're about to merge in case
524 // we selected a successor that didn't fit naturally into the CFG.
525 SuccChain.LoopPredecessors = 0;
526 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
527 << " to " << getBlockNum(BestSucc) << "\n");
528 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
529 Chain.merge(BestSucc, &SuccChain);
530 BB = *llvm::prior(Chain.end());
533 DEBUG(dbgs() << "Finished forming chain for header block "
534 << getBlockNum(*Chain.begin()) << "\n");
537 /// \brief Forms basic block chains from the natural loop structures.
539 /// These chains are designed to preserve the existing *structure* of the code
540 /// as much as possible. We can then stitch the chains together in a way which
541 /// both preserves the topological structure and minimizes taken conditional
543 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
545 // First recurse through any nested loops, building chains for those inner
547 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
548 buildLoopChains(F, **LI);
550 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
551 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
552 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
554 // FIXME: This is a really lame way of walking the chains in the loop: we
555 // walk the blocks, and use a set to prevent visiting a particular chain
557 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
558 for (MachineLoop::block_iterator BI = L.block_begin(),
561 BlockChain &Chain = *BlockToChain[*BI];
562 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin())
565 assert(Chain.LoopPredecessors == 0);
566 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
568 assert(BlockToChain[*BCI] == &Chain);
569 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
570 PE = (*BCI)->pred_end();
572 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
574 ++Chain.LoopPredecessors;
578 if (Chain.LoopPredecessors == 0)
579 BlockWorkList.push_back(*BI);
582 buildChain(*L.block_begin(), LoopChain, L.getBlocks(), BlockWorkList,
586 // Crash at the end so we get all of the debugging output first.
587 bool BadLoop = false;
588 if (LoopChain.LoopPredecessors) {
590 dbgs() << "Loop chain contains a block without its preds placed!\n"
591 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
592 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
594 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
596 if (!LoopBlockSet.erase(*BCI)) {
598 dbgs() << "Loop chain contains a block not contained by the loop!\n"
599 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
600 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
601 << " Bad block: " << getBlockName(*BCI) << "\n";
604 if (!LoopBlockSet.empty()) {
606 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
607 LBE = LoopBlockSet.end();
609 dbgs() << "Loop contains blocks never placed into a chain!\n"
610 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
611 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
612 << " Bad block: " << getBlockName(*LBI) << "\n";
614 assert(!BadLoop && "Detected problems with the placement of this loop.");
618 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
619 // Ensure that every BB in the function has an associated chain to simplify
620 // the assumptions of the remaining algorithm.
621 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
623 new (ChainAllocator.Allocate()) BlockChain(BlockToChain, &*FI);
625 // Build any loop-based chains.
626 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
628 buildLoopChains(F, **LI);
630 // We need a vector of blocks so that buildChain can handle unnatural CFG
631 // constructs by searching for unplaced blocks and just concatenating them.
632 SmallVector<MachineBasicBlock *, 16> Blocks;
633 Blocks.reserve(F.size());
635 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
637 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
638 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
639 MachineBasicBlock *BB = &*FI;
640 Blocks.push_back(BB);
641 BlockChain &Chain = *BlockToChain[BB];
642 if (!UpdatedPreds.insert(&Chain))
645 assert(Chain.LoopPredecessors == 0);
646 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
648 assert(BlockToChain[*BCI] == &Chain);
649 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
650 PE = (*BCI)->pred_end();
652 if (BlockToChain[*PI] == &Chain)
654 ++Chain.LoopPredecessors;
658 if (Chain.LoopPredecessors == 0)
659 BlockWorkList.push_back(BB);
662 BlockChain &FunctionChain = *BlockToChain[&F.front()];
663 buildChain(&F.front(), FunctionChain, Blocks, BlockWorkList);
665 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
667 // Crash at the end so we get all of the debugging output first.
668 bool BadFunc = false;
669 FunctionBlockSetType FunctionBlockSet;
670 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
671 FunctionBlockSet.insert(FI);
673 for (BlockChain::iterator BCI = FunctionChain.begin(),
674 BCE = FunctionChain.end();
676 if (!FunctionBlockSet.erase(*BCI)) {
678 dbgs() << "Function chain contains a block not in the function!\n"
679 << " Bad block: " << getBlockName(*BCI) << "\n";
682 if (!FunctionBlockSet.empty()) {
684 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
685 FBE = FunctionBlockSet.end();
687 dbgs() << "Function contains blocks never placed into a chain!\n"
688 << " Bad block: " << getBlockName(*FBI) << "\n";
690 assert(!BadFunc && "Detected problems with the block placement.");
693 // Splice the blocks into place.
694 MachineFunction::iterator InsertPos = F.begin();
695 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
696 for (BlockChain::iterator BI = FunctionChain.begin(),
697 BE = FunctionChain.end();
699 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
701 << getBlockName(*BI) << "\n");
702 if (InsertPos != MachineFunction::iterator(*BI))
703 F.splice(InsertPos, *BI);
707 // Update the terminator of the previous block.
708 if (BI == FunctionChain.begin())
710 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
712 // FIXME: It would be awesome of updateTerminator would just return rather
713 // than assert when the branch cannot be analyzed in order to remove this
716 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
717 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
718 PrevBB->updateTerminator();
721 // Fixup the last block.
723 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
724 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
725 F.back().updateTerminator();
728 /// \brief Recursive helper to align a loop and any nested loops.
729 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
730 // Recurse through nested loops.
731 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
732 AlignLoop(F, *I, Align);
734 L->getTopBlock()->setAlignment(Align);
737 /// \brief Align loop headers to target preferred alignments.
738 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
739 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
742 unsigned Align = TLI->getPrefLoopAlignment();
744 return; // Don't care about loop alignment.
746 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
747 AlignLoop(F, *I, Align);
750 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
751 // Check for single-block functions and skip them.
752 if (llvm::next(F.begin()) == F.end())
755 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
756 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
757 MLI = &getAnalysis<MachineLoopInfo>();
758 TII = F.getTarget().getInstrInfo();
759 TLI = F.getTarget().getTargetLowering();
760 assert(BlockToChain.empty());
765 BlockToChain.clear();
767 // We always return true as we have no way to track whether the final order
768 // differs from the original order.
773 /// \brief A pass to compute block placement statistics.
775 /// A separate pass to compute interesting statistics for evaluating block
776 /// placement. This is separate from the actual placement pass so that they can
777 /// be computed in the absense of any placement transformations or when using
778 /// alternative placement strategies.
779 class MachineBlockPlacementStats : public MachineFunctionPass {
780 /// \brief A handle to the branch probability pass.
781 const MachineBranchProbabilityInfo *MBPI;
783 /// \brief A handle to the function-wide block frequency pass.
784 const MachineBlockFrequencyInfo *MBFI;
787 static char ID; // Pass identification, replacement for typeid
788 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
789 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
792 bool runOnMachineFunction(MachineFunction &F);
794 void getAnalysisUsage(AnalysisUsage &AU) const {
795 AU.addRequired<MachineBranchProbabilityInfo>();
796 AU.addRequired<MachineBlockFrequencyInfo>();
797 AU.setPreservesAll();
798 MachineFunctionPass::getAnalysisUsage(AU);
801 const char *getPassName() const { return "Block Placement Stats"; }
805 char MachineBlockPlacementStats::ID = 0;
806 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
807 "Basic Block Placement Stats", false, false)
808 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
809 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
810 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
811 "Basic Block Placement Stats", false, false)
813 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
814 return new MachineBlockPlacementStats();
817 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
818 // Check for single-block functions and skip them.
819 if (llvm::next(F.begin()) == F.end())
822 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
823 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
825 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
826 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
827 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
829 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
830 : UncondBranchTakenFreq;
831 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
834 // Skip if this successor is a fallthrough.
835 if (I->isLayoutSuccessor(*SI))
838 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
840 BranchTakenFreq += EdgeFreq.getFrequency();