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 *>::iterator iterator;
125 typedef SmallVectorImpl<MachineBasicBlock *>::reverse_iterator
128 /// \brief Beginning of blocks within the chain.
129 iterator begin() { return Blocks.begin(); }
130 reverse_iterator rbegin() { return Blocks.rbegin(); }
132 /// \brief End of blocks within the chain.
133 iterator end() { return Blocks.end(); }
134 reverse_iterator rend() { return Blocks.rend(); }
136 /// \brief Merge a block chain into this one.
138 /// This routine merges a block chain into this one. It takes care of forming
139 /// a contiguous sequence of basic blocks, updating the edge list, and
140 /// updating the block -> chain mapping. It does not free or tear down the
141 /// old chain, but the old chain's block list is no longer valid.
142 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
144 assert(!Blocks.empty());
146 // Fast path in case we don't have a chain already.
148 assert(!BlockToChain[BB]);
149 Blocks.push_back(BB);
150 BlockToChain[BB] = this;
154 assert(BB == *Chain->begin());
155 assert(Chain->begin() != Chain->end());
157 // Update the incoming blocks to point to this chain, and add them to the
159 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
161 Blocks.push_back(*BI);
162 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
163 BlockToChain[*BI] = this;
167 /// \brief Count of predecessors within the loop currently being processed.
169 /// This count is updated at each loop we process to represent the number of
170 /// in-loop predecessors of this chain.
171 unsigned LoopPredecessors;
176 class MachineBlockPlacement : public MachineFunctionPass {
177 /// \brief A typedef for a block filter set.
178 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
180 /// \brief A handle to the branch probability pass.
181 const MachineBranchProbabilityInfo *MBPI;
183 /// \brief A handle to the function-wide block frequency pass.
184 const MachineBlockFrequencyInfo *MBFI;
186 /// \brief A handle to the loop info.
187 const MachineLoopInfo *MLI;
189 /// \brief A handle to the target's instruction info.
190 const TargetInstrInfo *TII;
192 /// \brief A handle to the target's lowering info.
193 const TargetLowering *TLI;
195 /// \brief Allocator and owner of BlockChain structures.
197 /// We build BlockChains lazily by merging together high probability BB
198 /// sequences acording to the "Algo2" in the paper mentioned at the top of
199 /// the file. To reduce malloc traffic, we allocate them using this slab-like
200 /// allocator, and destroy them after the pass completes.
201 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
203 /// \brief Function wide BasicBlock to BlockChain mapping.
205 /// This mapping allows efficiently moving from any given basic block to the
206 /// BlockChain it participates in, if any. We use it to, among other things,
207 /// allow implicitly defining edges between chains as the existing edges
208 /// between basic blocks.
209 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
211 void markChainSuccessors(BlockChain &Chain,
212 MachineBasicBlock *LoopHeaderBB,
213 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
214 const BlockFilterSet *BlockFilter = 0);
215 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
217 const BlockFilterSet *BlockFilter);
218 MachineBasicBlock *selectBestCandidateBlock(
219 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
220 const BlockFilterSet *BlockFilter);
221 MachineBasicBlock *getFirstUnplacedBlock(
223 const BlockChain &PlacedChain,
224 MachineFunction::iterator &PrevUnplacedBlockIt,
225 const BlockFilterSet *BlockFilter);
226 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
227 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
228 const BlockFilterSet *BlockFilter = 0);
229 void rotateLoop(MachineLoop &L, BlockChain &LoopChain,
230 const BlockFilterSet &LoopBlockSet);
231 void buildLoopChains(MachineFunction &F, MachineLoop &L);
232 void buildCFGChains(MachineFunction &F);
233 void AlignLoops(MachineFunction &F);
236 static char ID; // Pass identification, replacement for typeid
237 MachineBlockPlacement() : MachineFunctionPass(ID) {
238 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
241 bool runOnMachineFunction(MachineFunction &F);
243 void getAnalysisUsage(AnalysisUsage &AU) const {
244 AU.addRequired<MachineBranchProbabilityInfo>();
245 AU.addRequired<MachineBlockFrequencyInfo>();
246 AU.addRequired<MachineLoopInfo>();
247 MachineFunctionPass::getAnalysisUsage(AU);
250 const char *getPassName() const { return "Block Placement"; }
254 char MachineBlockPlacement::ID = 0;
255 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
256 "Branch Probability Basic Block Placement", false, false)
257 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
258 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
259 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
260 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
261 "Branch Probability Basic Block Placement", false, false)
263 FunctionPass *llvm::createMachineBlockPlacementPass() {
264 return new MachineBlockPlacement();
268 /// \brief Helper to print the name of a MBB.
270 /// Only used by debug logging.
271 static std::string getBlockName(MachineBasicBlock *BB) {
273 raw_string_ostream OS(Result);
274 OS << "BB#" << BB->getNumber()
275 << " (derived from LLVM BB '" << BB->getName() << "')";
280 /// \brief Helper to print the number of a MBB.
282 /// Only used by debug logging.
283 static std::string getBlockNum(MachineBasicBlock *BB) {
285 raw_string_ostream OS(Result);
286 OS << "BB#" << BB->getNumber();
292 /// \brief Mark a chain's successors as having one fewer preds.
294 /// When a chain is being merged into the "placed" chain, this routine will
295 /// quickly walk the successors of each block in the chain and mark them as
296 /// having one fewer active predecessor. It also adds any successors of this
297 /// chain which reach the zero-predecessor state to the worklist passed in.
298 void MachineBlockPlacement::markChainSuccessors(
300 MachineBasicBlock *LoopHeaderBB,
301 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
302 const BlockFilterSet *BlockFilter) {
303 // Walk all the blocks in this chain, marking their successors as having
304 // a predecessor placed.
305 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
307 // Add any successors for which this is the only un-placed in-loop
308 // predecessor to the worklist as a viable candidate for CFG-neutral
309 // placement. No subsequent placement of this block will violate the CFG
310 // shape, so we get to use heuristics to choose a favorable placement.
311 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
312 SE = (*CBI)->succ_end();
314 if (BlockFilter && !BlockFilter->count(*SI))
316 BlockChain &SuccChain = *BlockToChain[*SI];
317 // Disregard edges within a fixed chain, or edges to the loop header.
318 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
321 // This is a cross-chain edge that is within the loop, so decrement the
322 // loop predecessor count of the destination chain.
323 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
324 BlockWorkList.push_back(*SuccChain.begin());
329 /// \brief Select the best successor for a block.
331 /// This looks across all successors of a particular block and attempts to
332 /// select the "best" one to be the layout successor. It only considers direct
333 /// successors which also pass the block filter. It will attempt to avoid
334 /// breaking CFG structure, but cave and break such structures in the case of
335 /// very hot successor edges.
337 /// \returns The best successor block found, or null if none are viable.
338 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
339 MachineBasicBlock *BB, BlockChain &Chain,
340 const BlockFilterSet *BlockFilter) {
341 const BranchProbability HotProb(4, 5); // 80%
343 MachineBasicBlock *BestSucc = 0;
344 // FIXME: Due to the performance of the probability and weight routines in
345 // the MBPI analysis, we manually compute probabilities using the edge
346 // weights. This is suboptimal as it means that the somewhat subtle
347 // definition of edge weight semantics is encoded here as well. We should
348 // improve the MBPI interface to effeciently support query patterns such as
350 uint32_t BestWeight = 0;
351 uint32_t WeightScale = 0;
352 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
353 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
354 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
357 if (BlockFilter && !BlockFilter->count(*SI))
359 BlockChain &SuccChain = *BlockToChain[*SI];
360 if (&SuccChain == &Chain) {
361 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
364 if (*SI != *SuccChain.begin()) {
365 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
369 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
370 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
372 // Only consider successors which are either "hot", or wouldn't violate
373 // any CFG constraints.
374 if (SuccChain.LoopPredecessors != 0) {
375 if (SuccProb < HotProb) {
376 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
380 // Make sure that a hot successor doesn't have a globally more important
382 BlockFrequency CandidateEdgeFreq
383 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
384 bool BadCFGConflict = false;
385 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
386 PE = (*SI)->pred_end();
388 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
389 BlockToChain[*PI] == &Chain)
391 BlockFrequency PredEdgeFreq
392 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
393 if (PredEdgeFreq >= CandidateEdgeFreq) {
394 BadCFGConflict = true;
398 if (BadCFGConflict) {
399 DEBUG(dbgs() << " " << getBlockName(*SI)
400 << " -> non-cold CFG conflict\n");
405 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
407 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
409 if (BestSucc && BestWeight >= SuccWeight)
412 BestWeight = SuccWeight;
418 /// \brief Predicate struct to detect blocks already placed.
419 class IsBlockPlaced {
420 const BlockChain &PlacedChain;
421 const BlockToChainMapType &BlockToChain;
424 IsBlockPlaced(const BlockChain &PlacedChain,
425 const BlockToChainMapType &BlockToChain)
426 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
428 bool operator()(MachineBasicBlock *BB) const {
429 return BlockToChain.lookup(BB) == &PlacedChain;
434 /// \brief Select the best block from a worklist.
436 /// This looks through the provided worklist as a list of candidate basic
437 /// blocks and select the most profitable one to place. The definition of
438 /// profitable only really makes sense in the context of a loop. This returns
439 /// the most frequently visited block in the worklist, which in the case of
440 /// a loop, is the one most desirable to be physically close to the rest of the
441 /// loop body in order to improve icache behavior.
443 /// \returns The best block found, or null if none are viable.
444 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
445 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
446 const BlockFilterSet *BlockFilter) {
447 // Once we need to walk the worklist looking for a candidate, cleanup the
448 // worklist of already placed entries.
449 // FIXME: If this shows up on profiles, it could be folded (at the cost of
450 // some code complexity) into the loop below.
451 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
452 IsBlockPlaced(Chain, BlockToChain)),
455 MachineBasicBlock *BestBlock = 0;
456 BlockFrequency BestFreq;
457 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
458 WBE = WorkList.end();
460 assert(!BlockFilter || BlockFilter->count(*WBI));
461 BlockChain &SuccChain = *BlockToChain[*WBI];
462 if (&SuccChain == &Chain) {
463 DEBUG(dbgs() << " " << getBlockName(*WBI)
464 << " -> Already merged!\n");
467 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
469 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
470 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
472 if (BestBlock && BestFreq >= CandidateFreq)
475 BestFreq = CandidateFreq;
480 /// \brief Retrieve the first unplaced basic block.
482 /// This routine is called when we are unable to use the CFG to walk through
483 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
484 /// We walk through the function's blocks in order, starting from the
485 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
486 /// re-scanning the entire sequence on repeated calls to this routine.
487 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
488 MachineFunction &F, const BlockChain &PlacedChain,
489 MachineFunction::iterator &PrevUnplacedBlockIt,
490 const BlockFilterSet *BlockFilter) {
491 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
493 if (BlockFilter && !BlockFilter->count(I))
495 if (BlockToChain[I] != &PlacedChain) {
496 PrevUnplacedBlockIt = I;
497 // Now select the head of the chain to which the unplaced block belongs
498 // as the block to place. This will force the entire chain to be placed,
499 // and satisfies the requirements of merging chains.
500 return *BlockToChain[I]->begin();
506 void MachineBlockPlacement::buildChain(
507 MachineBasicBlock *BB,
509 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
510 const BlockFilterSet *BlockFilter) {
512 assert(BlockToChain[BB] == &Chain);
513 MachineFunction &F = *BB->getParent();
514 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
516 MachineBasicBlock *LoopHeaderBB = BB;
517 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
518 BB = *llvm::prior(Chain.end());
521 assert(BlockToChain[BB] == &Chain);
522 assert(*llvm::prior(Chain.end()) == BB);
523 MachineBasicBlock *BestSucc = 0;
525 // Look for the best viable successor if there is one to place immediately
527 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
529 // If an immediate successor isn't available, look for the best viable
530 // block among those we've identified as not violating the loop's CFG at
531 // this point. This won't be a fallthrough, but it will increase locality.
533 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
536 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
541 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
542 "layout successor until the CFG reduces\n");
545 // Place this block, updating the datastructures to reflect its placement.
546 BlockChain &SuccChain = *BlockToChain[BestSucc];
547 // Zero out LoopPredecessors for the successor we're about to merge in case
548 // we selected a successor that didn't fit naturally into the CFG.
549 SuccChain.LoopPredecessors = 0;
550 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
551 << " to " << getBlockNum(BestSucc) << "\n");
552 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
553 Chain.merge(BestSucc, &SuccChain);
554 BB = *llvm::prior(Chain.end());
557 DEBUG(dbgs() << "Finished forming chain for header block "
558 << getBlockNum(*Chain.begin()) << "\n");
561 /// \brief Attempt to rotate loop chains ending in an unconditional backedge.
563 /// This is a very conservative attempt to rotate unconditional backedge jumps
564 /// into fallthrough opportunities. It only attempts to perform the rotation
565 /// when it is trivial to find a block within the loop which has a conditional
566 /// loop exit. This block is then made the bottom of the chain, and the in-loop
567 /// fallthrough block the top. That turns a conditional branch out of the loop
568 /// into a conditional branch to the top of the loop while completely
569 /// eliminitating an unconditional branch within the loop.
570 void MachineBlockPlacement::rotateLoop(MachineLoop &L,
571 BlockChain &LoopChain,
572 const BlockFilterSet &LoopBlockSet) {
573 MachineBasicBlock *Header = *L.block_begin();
574 // Ensure that we have a chain of blocks which starts with the loop header.
575 // Otherwise, rotating the blocks might break an analyzable branch.
576 if (Header != *LoopChain.begin())
579 // We only support rotating the loop chain as a unit, so look directly at the
580 // back of the chain and check that it has a backedge.
581 MachineBasicBlock *Latch = *llvm::prior(LoopChain.end());
582 if (Latch == Header ||
583 !Latch->isSuccessor(Header))
586 // We need to analyze the branch and determine if rotating the loop will
587 // completely remove a branch. We bail if the analysis fails or we don't find
588 // an unconditional backedge. Note that this has to handle cases where the
589 // original order was rotated, and the chain has un-done it. As a result,
590 // there may not (yet) be the uncondiationl branch, but we can tell whether
591 // one will be added when updating the terminators.
592 SmallVector<MachineOperand, 4> Cond;
593 MachineBasicBlock *TBB = 0, *FBB = 0;
594 if (TII->AnalyzeBranch(*Latch, TBB, FBB, Cond) || !Cond.empty())
597 // Next we need to find a split point. This rotate algorithm is *very*
598 // narrow, and it only tries to do the rotate when it can find a split point
599 // which is an analyzable branch that exits the loop. Splitting there allows
600 // us to form a fallthrough out of the loop and a conditional jump to the top
601 // of the loop after rotation.
602 MachineBasicBlock *NewBottom = 0, *NewTop = 0;
603 BlockChain::iterator SplitIt = LoopChain.end();
604 for (BlockChain::reverse_iterator I = llvm::next(LoopChain.rbegin()),
605 E = LoopChain.rend();
609 // Ensure that this is a block with a conditional branch which we can
610 // analyze and re-form after rotating the loop. While it might be tempting
611 // to use the TBB or FBB output parameters from this, they will often be
612 // lies as they are only correct after the terminators have been updated,
613 // and we are mid-chain formation.
614 if (TII->AnalyzeBranch(**I, TBB, FBB, Cond) || Cond.empty())
617 // See if this block is an exiting block from the loop. LoopInfo provides
618 // a nice API for this, but it's actuall N*M runtime where N is the number
619 // of blocks in the loop and M is the number of successors. We can
620 // eliminate the N by doing this ourselves.
621 // FIXME: The LoopInfo datastructure should be improved for these types of
623 MachineBasicBlock *ExitB = 0;
624 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(), SE = (*I)->succ_end();
626 if (!(*SI)->isLandingPad() && *SI != *I && !LoopBlockSet.count(*SI)) {
635 NewTop = *llvm::prior(I);
639 if (!NewBottom || !NewTop ||
640 SplitIt == LoopChain.end() || SplitIt == LoopChain.begin())
642 assert(BlockToChain[NewBottom] == &LoopChain);
643 assert(BlockToChain[NewTop] == &LoopChain);
644 assert(*SplitIt == NewTop);
646 // Rotate the chain and we're done.
647 DEBUG(dbgs() << "Rotating loop headed by: " << getBlockName(Header) << "\n"
648 << " new top: " << getBlockName(NewTop) << "\n"
649 << " new bottom: " << getBlockName(NewBottom) << "\n");
650 std::rotate(LoopChain.begin(), SplitIt, LoopChain.end());
653 /// \brief Forms basic block chains from the natural loop structures.
655 /// These chains are designed to preserve the existing *structure* of the code
656 /// as much as possible. We can then stitch the chains together in a way which
657 /// both preserves the topological structure and minimizes taken conditional
659 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
661 // First recurse through any nested loops, building chains for those inner
663 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
664 buildLoopChains(F, **LI);
666 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
667 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
668 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
670 // FIXME: This is a really lame way of walking the chains in the loop: we
671 // walk the blocks, and use a set to prevent visiting a particular chain
673 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
674 for (MachineLoop::block_iterator BI = L.block_begin(),
677 BlockChain &Chain = *BlockToChain[*BI];
678 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin())
681 assert(Chain.LoopPredecessors == 0);
682 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
684 assert(BlockToChain[*BCI] == &Chain);
685 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
686 PE = (*BCI)->pred_end();
688 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
690 ++Chain.LoopPredecessors;
694 if (Chain.LoopPredecessors == 0)
695 BlockWorkList.push_back(*Chain.begin());
698 buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet);
699 rotateLoop(L, LoopChain, LoopBlockSet);
702 // Crash at the end so we get all of the debugging output first.
703 bool BadLoop = false;
704 if (LoopChain.LoopPredecessors) {
706 dbgs() << "Loop chain contains a block without its preds placed!\n"
707 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
708 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
710 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
712 if (!LoopBlockSet.erase(*BCI)) {
713 // We don't mark the loop as bad here because there are real situations
714 // where this can occur. For example, with an unanalyzable fallthrough
715 // from a loop block to a non-loop block or vice versa.
716 dbgs() << "Loop chain contains a block not contained by the loop!\n"
717 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
718 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
719 << " Bad block: " << getBlockName(*BCI) << "\n";
722 if (!LoopBlockSet.empty()) {
724 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
725 LBE = LoopBlockSet.end();
727 dbgs() << "Loop contains blocks never placed into a chain!\n"
728 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
729 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
730 << " Bad block: " << getBlockName(*LBI) << "\n";
732 assert(!BadLoop && "Detected problems with the placement of this loop.");
736 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
737 // Ensure that every BB in the function has an associated chain to simplify
738 // the assumptions of the remaining algorithm.
739 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
740 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
741 MachineBasicBlock *BB = FI;
743 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
744 // Also, merge any blocks which we cannot reason about and must preserve
745 // the exact fallthrough behavior for.
748 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
749 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
752 MachineFunction::iterator NextFI(llvm::next(FI));
753 MachineBasicBlock *NextBB = NextFI;
754 // Ensure that the layout successor is a viable block, as we know that
755 // fallthrough is a possibility.
756 assert(NextFI != FE && "Can't fallthrough past the last block.");
757 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
758 << getBlockName(BB) << " -> " << getBlockName(NextBB)
760 Chain->merge(NextBB, 0);
766 // Build any loop-based chains.
767 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
769 buildLoopChains(F, **LI);
771 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
773 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
774 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
775 MachineBasicBlock *BB = &*FI;
776 BlockChain &Chain = *BlockToChain[BB];
777 if (!UpdatedPreds.insert(&Chain))
780 assert(Chain.LoopPredecessors == 0);
781 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
783 assert(BlockToChain[*BCI] == &Chain);
784 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
785 PE = (*BCI)->pred_end();
787 if (BlockToChain[*PI] == &Chain)
789 ++Chain.LoopPredecessors;
793 if (Chain.LoopPredecessors == 0)
794 BlockWorkList.push_back(*Chain.begin());
797 BlockChain &FunctionChain = *BlockToChain[&F.front()];
798 buildChain(&F.front(), FunctionChain, BlockWorkList);
800 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
802 // Crash at the end so we get all of the debugging output first.
803 bool BadFunc = false;
804 FunctionBlockSetType FunctionBlockSet;
805 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
806 FunctionBlockSet.insert(FI);
808 for (BlockChain::iterator BCI = FunctionChain.begin(),
809 BCE = FunctionChain.end();
811 if (!FunctionBlockSet.erase(*BCI)) {
813 dbgs() << "Function chain contains a block not in the function!\n"
814 << " Bad block: " << getBlockName(*BCI) << "\n";
817 if (!FunctionBlockSet.empty()) {
819 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
820 FBE = FunctionBlockSet.end();
822 dbgs() << "Function contains blocks never placed into a chain!\n"
823 << " Bad block: " << getBlockName(*FBI) << "\n";
825 assert(!BadFunc && "Detected problems with the block placement.");
828 // Splice the blocks into place.
829 MachineFunction::iterator InsertPos = F.begin();
830 for (BlockChain::iterator BI = FunctionChain.begin(),
831 BE = FunctionChain.end();
833 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
835 << getBlockName(*BI) << "\n");
836 if (InsertPos != MachineFunction::iterator(*BI))
837 F.splice(InsertPos, *BI);
841 // Update the terminator of the previous block.
842 if (BI == FunctionChain.begin())
844 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
846 // FIXME: It would be awesome of updateTerminator would just return rather
847 // than assert when the branch cannot be analyzed in order to remove this
850 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
851 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
852 PrevBB->updateTerminator();
855 // Fixup the last block.
857 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
858 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
859 F.back().updateTerminator();
862 /// \brief Recursive helper to align a loop and any nested loops.
863 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
864 // Recurse through nested loops.
865 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
866 AlignLoop(F, *I, Align);
868 L->getTopBlock()->setAlignment(Align);
871 /// \brief Align loop headers to target preferred alignments.
872 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
873 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
876 unsigned Align = TLI->getPrefLoopAlignment();
878 return; // Don't care about loop alignment.
880 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
881 AlignLoop(F, *I, Align);
884 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
885 // Check for single-block functions and skip them.
886 if (llvm::next(F.begin()) == F.end())
889 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
890 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
891 MLI = &getAnalysis<MachineLoopInfo>();
892 TII = F.getTarget().getInstrInfo();
893 TLI = F.getTarget().getTargetLowering();
894 assert(BlockToChain.empty());
899 BlockToChain.clear();
900 ChainAllocator.DestroyAll();
902 // We always return true as we have no way to track whether the final order
903 // differs from the original order.
908 /// \brief A pass to compute block placement statistics.
910 /// A separate pass to compute interesting statistics for evaluating block
911 /// placement. This is separate from the actual placement pass so that they can
912 /// be computed in the absense of any placement transformations or when using
913 /// alternative placement strategies.
914 class MachineBlockPlacementStats : public MachineFunctionPass {
915 /// \brief A handle to the branch probability pass.
916 const MachineBranchProbabilityInfo *MBPI;
918 /// \brief A handle to the function-wide block frequency pass.
919 const MachineBlockFrequencyInfo *MBFI;
922 static char ID; // Pass identification, replacement for typeid
923 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
924 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
927 bool runOnMachineFunction(MachineFunction &F);
929 void getAnalysisUsage(AnalysisUsage &AU) const {
930 AU.addRequired<MachineBranchProbabilityInfo>();
931 AU.addRequired<MachineBlockFrequencyInfo>();
932 AU.setPreservesAll();
933 MachineFunctionPass::getAnalysisUsage(AU);
936 const char *getPassName() const { return "Block Placement Stats"; }
940 char MachineBlockPlacementStats::ID = 0;
941 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
942 "Basic Block Placement Stats", false, false)
943 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
944 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
945 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
946 "Basic Block Placement Stats", false, false)
948 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
949 return new MachineBlockPlacementStats();
952 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
953 // Check for single-block functions and skip them.
954 if (llvm::next(F.begin()) == F.end())
957 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
958 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
960 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
961 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
962 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
964 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
965 : UncondBranchTakenFreq;
966 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
969 // Skip if this successor is a fallthrough.
970 if (I->isLayoutSuccessor(*SI))
973 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
975 BranchTakenFreq += EdgeFreq.getFrequency();