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 absence 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/Passes.h"
30 #include "llvm/ADT/DenseMap.h"
31 #include "llvm/ADT/SmallPtrSet.h"
32 #include "llvm/ADT/SmallVector.h"
33 #include "llvm/ADT/Statistic.h"
34 #include "llvm/CodeGen/MachineBasicBlock.h"
35 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
36 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
37 #include "llvm/CodeGen/MachineFunction.h"
38 #include "llvm/CodeGen/MachineFunctionPass.h"
39 #include "llvm/CodeGen/MachineLoopInfo.h"
40 #include "llvm/CodeGen/MachineModuleInfo.h"
41 #include "llvm/Support/Allocator.h"
42 #include "llvm/Support/CommandLine.h"
43 #include "llvm/Support/Debug.h"
44 #include "llvm/Target/TargetInstrInfo.h"
45 #include "llvm/Target/TargetLowering.h"
49 STATISTIC(NumCondBranches, "Number of conditional branches");
50 STATISTIC(NumUncondBranches, "Number of uncondittional branches");
51 STATISTIC(CondBranchTakenFreq,
52 "Potential frequency of taking conditional branches");
53 STATISTIC(UncondBranchTakenFreq,
54 "Potential frequency of taking unconditional branches");
56 static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
57 cl::desc("Force the alignment of all "
58 "blocks in the function."),
59 cl::init(0), cl::Hidden);
61 // FIXME: Find a good default for this flag and remove the flag.
62 static cl::opt<unsigned>
63 ExitBlockBias("block-placement-exit-block-bias",
64 cl::desc("Block frequency percentage a loop exit block needs "
65 "over the original exit to be considered the new exit."),
66 cl::init(0), cl::Hidden);
70 /// \brief Type for our function-wide basic block -> block chain mapping.
71 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
75 /// \brief A chain of blocks which will be laid out contiguously.
77 /// This is the datastructure representing a chain of consecutive blocks that
78 /// are profitable to layout together in order to maximize fallthrough
79 /// probabilities and code locality. We also can use a block chain to represent
80 /// a sequence of basic blocks which have some external (correctness)
81 /// requirement for sequential layout.
83 /// Chains can be built around a single basic block and can be merged to grow
84 /// them. They participate in a block-to-chain mapping, which is updated
85 /// automatically as chains are merged together.
87 /// \brief The sequence of blocks belonging to this chain.
89 /// This is the sequence of blocks for a particular chain. These will be laid
90 /// out in-order within the function.
91 SmallVector<MachineBasicBlock *, 4> Blocks;
93 /// \brief A handle to the function-wide basic block to block chain mapping.
95 /// This is retained in each block chain to simplify the computation of child
96 /// block chains for SCC-formation and iteration. We store the edges to child
97 /// basic blocks, and map them back to their associated chains using this
99 BlockToChainMapType &BlockToChain;
102 /// \brief Construct a new BlockChain.
104 /// This builds a new block chain representing a single basic block in the
105 /// function. It also registers itself as the chain that block participates
106 /// in with the BlockToChain mapping.
107 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
108 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
109 assert(BB && "Cannot create a chain with a null basic block");
110 BlockToChain[BB] = this;
113 /// \brief Iterator over blocks within the chain.
114 typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
116 /// \brief Beginning of blocks within the chain.
117 iterator begin() { return Blocks.begin(); }
119 /// \brief End of blocks within the chain.
120 iterator end() { return Blocks.end(); }
122 /// \brief Merge a block chain into this one.
124 /// This routine merges a block chain into this one. It takes care of forming
125 /// a contiguous sequence of basic blocks, updating the edge list, and
126 /// updating the block -> chain mapping. It does not free or tear down the
127 /// old chain, but the old chain's block list is no longer valid.
128 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
130 assert(!Blocks.empty());
132 // Fast path in case we don't have a chain already.
134 assert(!BlockToChain[BB]);
135 Blocks.push_back(BB);
136 BlockToChain[BB] = this;
140 assert(BB == *Chain->begin());
141 assert(Chain->begin() != Chain->end());
143 // Update the incoming blocks to point to this chain, and add them to the
145 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
147 Blocks.push_back(*BI);
148 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
149 BlockToChain[*BI] = this;
154 /// \brief Dump the blocks in this chain.
155 void dump() LLVM_ATTRIBUTE_USED {
156 for (iterator I = begin(), E = end(); I != E; ++I)
161 /// \brief Count of predecessors within the loop currently being processed.
163 /// This count is updated at each loop we process to represent the number of
164 /// in-loop predecessors of this chain.
165 unsigned LoopPredecessors;
170 class MachineBlockPlacement : public MachineFunctionPass {
171 /// \brief A typedef for a block filter set.
172 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
174 /// \brief A handle to the branch probability pass.
175 const MachineBranchProbabilityInfo *MBPI;
177 /// \brief A handle to the function-wide block frequency pass.
178 const MachineBlockFrequencyInfo *MBFI;
180 /// \brief A handle to the loop info.
181 const MachineLoopInfo *MLI;
183 /// \brief A handle to the target's instruction info.
184 const TargetInstrInfo *TII;
186 /// \brief A handle to the target's lowering info.
187 const TargetLoweringBase *TLI;
189 /// \brief Allocator and owner of BlockChain structures.
191 /// We build BlockChains lazily while processing the loop structure of
192 /// a function. To reduce malloc traffic, we allocate them using this
193 /// slab-like allocator, and destroy them after the pass completes. An
194 /// important guarantee is that this allocator produces stable pointers to
196 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
198 /// \brief Function wide BasicBlock to BlockChain mapping.
200 /// This mapping allows efficiently moving from any given basic block to the
201 /// BlockChain it participates in, if any. We use it to, among other things,
202 /// allow implicitly defining edges between chains as the existing edges
203 /// between basic blocks.
204 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
206 void markChainSuccessors(BlockChain &Chain,
207 MachineBasicBlock *LoopHeaderBB,
208 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
209 const BlockFilterSet *BlockFilter = 0);
210 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
212 const BlockFilterSet *BlockFilter);
213 MachineBasicBlock *selectBestCandidateBlock(
214 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
215 const BlockFilterSet *BlockFilter);
216 MachineBasicBlock *getFirstUnplacedBlock(
218 const BlockChain &PlacedChain,
219 MachineFunction::iterator &PrevUnplacedBlockIt,
220 const BlockFilterSet *BlockFilter);
221 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
222 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
223 const BlockFilterSet *BlockFilter = 0);
224 MachineBasicBlock *findBestLoopTop(MachineLoop &L,
225 const BlockFilterSet &LoopBlockSet);
226 MachineBasicBlock *findBestLoopExit(MachineFunction &F,
228 const BlockFilterSet &LoopBlockSet);
229 void buildLoopChains(MachineFunction &F, MachineLoop &L);
230 void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
231 const BlockFilterSet &LoopBlockSet);
232 void buildCFGChains(MachineFunction &F);
235 static char ID; // Pass identification, replacement for typeid
236 MachineBlockPlacement() : MachineFunctionPass(ID) {
237 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
240 bool runOnMachineFunction(MachineFunction &F);
242 void getAnalysisUsage(AnalysisUsage &AU) const {
243 AU.addRequired<MachineBranchProbabilityInfo>();
244 AU.addRequired<MachineBlockFrequencyInfo>();
245 AU.addRequired<MachineLoopInfo>();
246 MachineFunctionPass::getAnalysisUsage(AU);
251 char MachineBlockPlacement::ID = 0;
252 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
253 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
254 "Branch Probability Basic Block Placement", false, false)
255 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
256 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
257 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
258 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
259 "Branch Probability Basic Block Placement", false, false)
262 /// \brief Helper to print the name of a MBB.
264 /// Only used by debug logging.
265 static std::string getBlockName(MachineBasicBlock *BB) {
267 raw_string_ostream OS(Result);
268 OS << "BB#" << BB->getNumber()
269 << " (derived from LLVM BB '" << BB->getName() << "')";
274 /// \brief Helper to print the number of a MBB.
276 /// Only used by debug logging.
277 static std::string getBlockNum(MachineBasicBlock *BB) {
279 raw_string_ostream OS(Result);
280 OS << "BB#" << BB->getNumber();
286 /// \brief Mark a chain's successors as having one fewer preds.
288 /// When a chain is being merged into the "placed" chain, this routine will
289 /// quickly walk the successors of each block in the chain and mark them as
290 /// having one fewer active predecessor. It also adds any successors of this
291 /// chain which reach the zero-predecessor state to the worklist passed in.
292 void MachineBlockPlacement::markChainSuccessors(
294 MachineBasicBlock *LoopHeaderBB,
295 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
296 const BlockFilterSet *BlockFilter) {
297 // Walk all the blocks in this chain, marking their successors as having
298 // a predecessor placed.
299 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
301 // Add any successors for which this is the only un-placed in-loop
302 // predecessor to the worklist as a viable candidate for CFG-neutral
303 // placement. No subsequent placement of this block will violate the CFG
304 // shape, so we get to use heuristics to choose a favorable placement.
305 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
306 SE = (*CBI)->succ_end();
308 if (BlockFilter && !BlockFilter->count(*SI))
310 BlockChain &SuccChain = *BlockToChain[*SI];
311 // Disregard edges within a fixed chain, or edges to the loop header.
312 if (&Chain == &SuccChain || *SI == LoopHeaderBB)
315 // This is a cross-chain edge that is within the loop, so decrement the
316 // loop predecessor count of the destination chain.
317 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
318 BlockWorkList.push_back(*SuccChain.begin());
323 /// \brief Select the best successor for a block.
325 /// This looks across all successors of a particular block and attempts to
326 /// select the "best" one to be the layout successor. It only considers direct
327 /// successors which also pass the block filter. It will attempt to avoid
328 /// breaking CFG structure, but cave and break such structures in the case of
329 /// very hot successor edges.
331 /// \returns The best successor block found, or null if none are viable.
332 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
333 MachineBasicBlock *BB, BlockChain &Chain,
334 const BlockFilterSet *BlockFilter) {
335 const BranchProbability HotProb(4, 5); // 80%
337 MachineBasicBlock *BestSucc = 0;
338 // FIXME: Due to the performance of the probability and weight routines in
339 // the MBPI analysis, we manually compute probabilities using the edge
340 // weights. This is suboptimal as it means that the somewhat subtle
341 // definition of edge weight semantics is encoded here as well. We should
342 // improve the MBPI interface to efficiently support query patterns such as
344 uint32_t BestWeight = 0;
345 uint32_t WeightScale = 0;
346 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
347 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
348 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
351 if (BlockFilter && !BlockFilter->count(*SI))
353 BlockChain &SuccChain = *BlockToChain[*SI];
354 if (&SuccChain == &Chain) {
355 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
358 if (*SI != *SuccChain.begin()) {
359 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
363 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
364 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
366 // Only consider successors which are either "hot", or wouldn't violate
367 // any CFG constraints.
368 if (SuccChain.LoopPredecessors != 0) {
369 if (SuccProb < HotProb) {
370 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
374 // Make sure that a hot successor doesn't have a globally more important
376 BlockFrequency CandidateEdgeFreq
377 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
378 bool BadCFGConflict = false;
379 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
380 PE = (*SI)->pred_end();
382 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
383 BlockToChain[*PI] == &Chain)
385 BlockFrequency PredEdgeFreq
386 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
387 if (PredEdgeFreq >= CandidateEdgeFreq) {
388 BadCFGConflict = true;
392 if (BadCFGConflict) {
393 DEBUG(dbgs() << " " << getBlockName(*SI)
394 << " -> non-cold CFG conflict\n");
399 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
401 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
403 if (BestSucc && BestWeight >= SuccWeight)
406 BestWeight = SuccWeight;
412 /// \brief Predicate struct to detect blocks already placed.
413 class IsBlockPlaced {
414 const BlockChain &PlacedChain;
415 const BlockToChainMapType &BlockToChain;
418 IsBlockPlaced(const BlockChain &PlacedChain,
419 const BlockToChainMapType &BlockToChain)
420 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
422 bool operator()(MachineBasicBlock *BB) const {
423 return BlockToChain.lookup(BB) == &PlacedChain;
428 /// \brief Select the best block from a worklist.
430 /// This looks through the provided worklist as a list of candidate basic
431 /// blocks and select the most profitable one to place. The definition of
432 /// profitable only really makes sense in the context of a loop. This returns
433 /// the most frequently visited block in the worklist, which in the case of
434 /// a loop, is the one most desirable to be physically close to the rest of the
435 /// loop body in order to improve icache behavior.
437 /// \returns The best block found, or null if none are viable.
438 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
439 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
440 const BlockFilterSet *BlockFilter) {
441 // Once we need to walk the worklist looking for a candidate, cleanup the
442 // worklist of already placed entries.
443 // FIXME: If this shows up on profiles, it could be folded (at the cost of
444 // some code complexity) into the loop below.
445 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
446 IsBlockPlaced(Chain, BlockToChain)),
449 MachineBasicBlock *BestBlock = 0;
450 BlockFrequency BestFreq;
451 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
452 WBE = WorkList.end();
454 BlockChain &SuccChain = *BlockToChain[*WBI];
455 if (&SuccChain == &Chain) {
456 DEBUG(dbgs() << " " << getBlockName(*WBI)
457 << " -> Already merged!\n");
460 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
462 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
463 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
465 if (BestBlock && BestFreq >= CandidateFreq)
468 BestFreq = CandidateFreq;
473 /// \brief Retrieve the first unplaced basic block.
475 /// This routine is called when we are unable to use the CFG to walk through
476 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
477 /// We walk through the function's blocks in order, starting from the
478 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
479 /// re-scanning the entire sequence on repeated calls to this routine.
480 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
481 MachineFunction &F, const BlockChain &PlacedChain,
482 MachineFunction::iterator &PrevUnplacedBlockIt,
483 const BlockFilterSet *BlockFilter) {
484 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
486 if (BlockFilter && !BlockFilter->count(I))
488 if (BlockToChain[I] != &PlacedChain) {
489 PrevUnplacedBlockIt = I;
490 // Now select the head of the chain to which the unplaced block belongs
491 // as the block to place. This will force the entire chain to be placed,
492 // and satisfies the requirements of merging chains.
493 return *BlockToChain[I]->begin();
499 void MachineBlockPlacement::buildChain(
500 MachineBasicBlock *BB,
502 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
503 const BlockFilterSet *BlockFilter) {
505 assert(BlockToChain[BB] == &Chain);
506 MachineFunction &F = *BB->getParent();
507 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
509 MachineBasicBlock *LoopHeaderBB = BB;
510 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
511 BB = *llvm::prior(Chain.end());
514 assert(BlockToChain[BB] == &Chain);
515 assert(*llvm::prior(Chain.end()) == BB);
517 // Look for the best viable successor if there is one to place immediately
519 MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
521 // If an immediate successor isn't available, look for the best viable
522 // block among those we've identified as not violating the loop's CFG at
523 // this point. This won't be a fallthrough, but it will increase locality.
525 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
528 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
533 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
534 "layout successor until the CFG reduces\n");
537 // Place this block, updating the datastructures to reflect its placement.
538 BlockChain &SuccChain = *BlockToChain[BestSucc];
539 // Zero out LoopPredecessors for the successor we're about to merge in case
540 // we selected a successor that didn't fit naturally into the CFG.
541 SuccChain.LoopPredecessors = 0;
542 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
543 << " to " << getBlockNum(BestSucc) << "\n");
544 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
545 Chain.merge(BestSucc, &SuccChain);
546 BB = *llvm::prior(Chain.end());
549 DEBUG(dbgs() << "Finished forming chain for header block "
550 << getBlockNum(*Chain.begin()) << "\n");
553 /// \brief Find the best loop top block for layout.
555 /// Look for a block which is strictly better than the loop header for laying
556 /// out at the top of the loop. This looks for one and only one pattern:
557 /// a latch block with no conditional exit. This block will cause a conditional
558 /// jump around it or will be the bottom of the loop if we lay it out in place,
559 /// but if it it doesn't end up at the bottom of the loop for any reason,
560 /// rotation alone won't fix it. Because such a block will always result in an
561 /// unconditional jump (for the backedge) rotating it in front of the loop
562 /// header is always profitable.
564 MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
565 const BlockFilterSet &LoopBlockSet) {
566 // Check that the header hasn't been fused with a preheader block due to
567 // crazy branches. If it has, we need to start with the header at the top to
568 // prevent pulling the preheader into the loop body.
569 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
570 if (!LoopBlockSet.count(*HeaderChain.begin()))
571 return L.getHeader();
573 DEBUG(dbgs() << "Finding best loop top for: "
574 << getBlockName(L.getHeader()) << "\n");
576 BlockFrequency BestPredFreq;
577 MachineBasicBlock *BestPred = 0;
578 for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
579 PE = L.getHeader()->pred_end();
581 MachineBasicBlock *Pred = *PI;
582 if (!LoopBlockSet.count(Pred))
584 DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
585 << Pred->succ_size() << " successors, "
586 << MBFI->getBlockFreq(Pred) << " freq\n");
587 if (Pred->succ_size() > 1)
590 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
591 if (!BestPred || PredFreq > BestPredFreq ||
592 (!(PredFreq < BestPredFreq) &&
593 Pred->isLayoutSuccessor(L.getHeader()))) {
595 BestPredFreq = PredFreq;
599 // If no direct predecessor is fine, just use the loop header.
601 return L.getHeader();
603 // Walk backwards through any straight line of predecessors.
604 while (BestPred->pred_size() == 1 &&
605 (*BestPred->pred_begin())->succ_size() == 1 &&
606 *BestPred->pred_begin() != L.getHeader())
607 BestPred = *BestPred->pred_begin();
609 DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
614 /// \brief Find the best loop exiting block for layout.
616 /// This routine implements the logic to analyze the loop looking for the best
617 /// block to layout at the top of the loop. Typically this is done to maximize
618 /// fallthrough opportunities.
620 MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
622 const BlockFilterSet &LoopBlockSet) {
623 // We don't want to layout the loop linearly in all cases. If the loop header
624 // is just a normal basic block in the loop, we want to look for what block
625 // within the loop is the best one to layout at the top. However, if the loop
626 // header has be pre-merged into a chain due to predecessors not having
627 // analyzable branches, *and* the predecessor it is merged with is *not* part
628 // of the loop, rotating the header into the middle of the loop will create
629 // a non-contiguous range of blocks which is Very Bad. So start with the
630 // header and only rotate if safe.
631 BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
632 if (!LoopBlockSet.count(*HeaderChain.begin()))
635 BlockFrequency BestExitEdgeFreq;
636 unsigned BestExitLoopDepth = 0;
637 MachineBasicBlock *ExitingBB = 0;
638 // If there are exits to outer loops, loop rotation can severely limit
639 // fallthrough opportunites unless it selects such an exit. Keep a set of
640 // blocks where rotating to exit with that block will reach an outer loop.
641 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
643 DEBUG(dbgs() << "Finding best loop exit for: "
644 << getBlockName(L.getHeader()) << "\n");
645 for (MachineLoop::block_iterator I = L.block_begin(),
648 BlockChain &Chain = *BlockToChain[*I];
649 // Ensure that this block is at the end of a chain; otherwise it could be
650 // mid-way through an inner loop or a successor of an analyzable branch.
651 if (*I != *llvm::prior(Chain.end()))
654 // Now walk the successors. We need to establish whether this has a viable
655 // exiting successor and whether it has a viable non-exiting successor.
656 // We store the old exiting state and restore it if a viable looping
657 // successor isn't found.
658 MachineBasicBlock *OldExitingBB = ExitingBB;
659 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
660 bool HasLoopingSucc = false;
661 // FIXME: Due to the performance of the probability and weight routines in
662 // the MBPI analysis, we use the internal weights and manually compute the
663 // probabilities to avoid quadratic behavior.
664 uint32_t WeightScale = 0;
665 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
666 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
667 SE = (*I)->succ_end();
669 if ((*SI)->isLandingPad())
673 BlockChain &SuccChain = *BlockToChain[*SI];
674 // Don't split chains, either this chain or the successor's chain.
675 if (&Chain == &SuccChain) {
676 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
677 << getBlockName(*SI) << " (chain conflict)\n");
681 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
682 if (LoopBlockSet.count(*SI)) {
683 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
684 << getBlockName(*SI) << " (" << SuccWeight << ")\n");
685 HasLoopingSucc = true;
689 unsigned SuccLoopDepth = 0;
690 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
691 SuccLoopDepth = ExitLoop->getLoopDepth();
692 if (ExitLoop->contains(&L))
693 BlocksExitingToOuterLoop.insert(*I);
696 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
697 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
698 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
699 << getBlockName(*SI) << " [L:" << SuccLoopDepth
700 << "] (" << ExitEdgeFreq << ")\n");
701 // Note that we bias this toward an existing layout successor to retain
702 // incoming order in the absence of better information. The exit must have
703 // a frequency higher than the current exit before we consider breaking
705 BranchProbability Bias(100 - ExitBlockBias, 100);
706 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth ||
707 ExitEdgeFreq > BestExitEdgeFreq ||
708 ((*I)->isLayoutSuccessor(*SI) &&
709 !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
710 BestExitEdgeFreq = ExitEdgeFreq;
715 // Restore the old exiting state, no viable looping successor was found.
716 if (!HasLoopingSucc) {
717 ExitingBB = OldExitingBB;
718 BestExitEdgeFreq = OldBestExitEdgeFreq;
722 // Without a candidate exiting block or with only a single block in the
723 // loop, just use the loop header to layout the loop.
724 if (!ExitingBB || L.getNumBlocks() == 1)
727 // Also, if we have exit blocks which lead to outer loops but didn't select
728 // one of them as the exiting block we are rotating toward, disable loop
729 // rotation altogether.
730 if (!BlocksExitingToOuterLoop.empty() &&
731 !BlocksExitingToOuterLoop.count(ExitingBB))
734 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
738 /// \brief Attempt to rotate an exiting block to the bottom of the loop.
740 /// Once we have built a chain, try to rotate it to line up the hot exit block
741 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary
742 /// branches. For example, if the loop has fallthrough into its header and out
743 /// of its bottom already, don't rotate it.
744 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
745 MachineBasicBlock *ExitingBB,
746 const BlockFilterSet &LoopBlockSet) {
750 MachineBasicBlock *Top = *LoopChain.begin();
751 bool ViableTopFallthrough = false;
752 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
753 PE = Top->pred_end();
755 BlockChain *PredChain = BlockToChain[*PI];
756 if (!LoopBlockSet.count(*PI) &&
757 (!PredChain || *PI == *llvm::prior(PredChain->end()))) {
758 ViableTopFallthrough = true;
763 // If the header has viable fallthrough, check whether the current loop
764 // bottom is a viable exiting block. If so, bail out as rotating will
765 // introduce an unnecessary branch.
766 if (ViableTopFallthrough) {
767 MachineBasicBlock *Bottom = *llvm::prior(LoopChain.end());
768 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
769 SE = Bottom->succ_end();
771 BlockChain *SuccChain = BlockToChain[*SI];
772 if (!LoopBlockSet.count(*SI) &&
773 (!SuccChain || *SI == *SuccChain->begin()))
778 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
780 if (ExitIt == LoopChain.end())
783 std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
786 /// \brief Forms basic block chains from the natural loop structures.
788 /// These chains are designed to preserve the existing *structure* of the code
789 /// as much as possible. We can then stitch the chains together in a way which
790 /// both preserves the topological structure and minimizes taken conditional
792 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
794 // First recurse through any nested loops, building chains for those inner
796 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
797 buildLoopChains(F, **LI);
799 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
800 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
802 // First check to see if there is an obviously preferable top block for the
803 // loop. This will default to the header, but may end up as one of the
804 // predecessors to the header if there is one which will result in strictly
805 // fewer branches in the loop body.
806 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
808 // If we selected just the header for the loop top, look for a potentially
809 // profitable exit block in the event that rotating the loop can eliminate
810 // branches by placing an exit edge at the bottom.
811 MachineBasicBlock *ExitingBB = 0;
812 if (LoopTop == L.getHeader())
813 ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
815 BlockChain &LoopChain = *BlockToChain[LoopTop];
817 // FIXME: This is a really lame way of walking the chains in the loop: we
818 // walk the blocks, and use a set to prevent visiting a particular chain
820 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
821 assert(LoopChain.LoopPredecessors == 0);
822 UpdatedPreds.insert(&LoopChain);
823 for (MachineLoop::block_iterator BI = L.block_begin(),
826 BlockChain &Chain = *BlockToChain[*BI];
827 if (!UpdatedPreds.insert(&Chain))
830 assert(Chain.LoopPredecessors == 0);
831 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
833 assert(BlockToChain[*BCI] == &Chain);
834 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
835 PE = (*BCI)->pred_end();
837 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
839 ++Chain.LoopPredecessors;
843 if (Chain.LoopPredecessors == 0)
844 BlockWorkList.push_back(*Chain.begin());
847 buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
848 rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
851 // Crash at the end so we get all of the debugging output first.
852 bool BadLoop = false;
853 if (LoopChain.LoopPredecessors) {
855 dbgs() << "Loop chain contains a block without its preds placed!\n"
856 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
857 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
859 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
861 dbgs() << " ... " << getBlockName(*BCI) << "\n";
862 if (!LoopBlockSet.erase(*BCI)) {
863 // We don't mark the loop as bad here because there are real situations
864 // where this can occur. For example, with an unanalyzable fallthrough
865 // from a loop block to a non-loop block or vice versa.
866 dbgs() << "Loop chain contains a block not contained by the loop!\n"
867 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
868 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
869 << " Bad block: " << getBlockName(*BCI) << "\n";
873 if (!LoopBlockSet.empty()) {
875 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
876 LBE = LoopBlockSet.end();
878 dbgs() << "Loop contains blocks never placed into a chain!\n"
879 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
880 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
881 << " Bad block: " << getBlockName(*LBI) << "\n";
883 assert(!BadLoop && "Detected problems with the placement of this loop.");
887 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
888 // Ensure that every BB in the function has an associated chain to simplify
889 // the assumptions of the remaining algorithm.
890 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
891 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
892 MachineBasicBlock *BB = FI;
894 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
895 // Also, merge any blocks which we cannot reason about and must preserve
896 // the exact fallthrough behavior for.
899 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
900 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
903 MachineFunction::iterator NextFI(llvm::next(FI));
904 MachineBasicBlock *NextBB = NextFI;
905 // Ensure that the layout successor is a viable block, as we know that
906 // fallthrough is a possibility.
907 assert(NextFI != FE && "Can't fallthrough past the last block.");
908 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
909 << getBlockName(BB) << " -> " << getBlockName(NextBB)
911 Chain->merge(NextBB, 0);
917 // Build any loop-based chains.
918 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
920 buildLoopChains(F, **LI);
922 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
924 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
925 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
926 MachineBasicBlock *BB = &*FI;
927 BlockChain &Chain = *BlockToChain[BB];
928 if (!UpdatedPreds.insert(&Chain))
931 assert(Chain.LoopPredecessors == 0);
932 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
934 assert(BlockToChain[*BCI] == &Chain);
935 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
936 PE = (*BCI)->pred_end();
938 if (BlockToChain[*PI] == &Chain)
940 ++Chain.LoopPredecessors;
944 if (Chain.LoopPredecessors == 0)
945 BlockWorkList.push_back(*Chain.begin());
948 BlockChain &FunctionChain = *BlockToChain[&F.front()];
949 buildChain(&F.front(), FunctionChain, BlockWorkList);
951 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
953 // Crash at the end so we get all of the debugging output first.
954 bool BadFunc = false;
955 FunctionBlockSetType FunctionBlockSet;
956 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
957 FunctionBlockSet.insert(FI);
959 for (BlockChain::iterator BCI = FunctionChain.begin(),
960 BCE = FunctionChain.end();
962 if (!FunctionBlockSet.erase(*BCI)) {
964 dbgs() << "Function chain contains a block not in the function!\n"
965 << " Bad block: " << getBlockName(*BCI) << "\n";
968 if (!FunctionBlockSet.empty()) {
970 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
971 FBE = FunctionBlockSet.end();
973 dbgs() << "Function contains blocks never placed into a chain!\n"
974 << " Bad block: " << getBlockName(*FBI) << "\n";
976 assert(!BadFunc && "Detected problems with the block placement.");
979 // Splice the blocks into place.
980 MachineFunction::iterator InsertPos = F.begin();
981 for (BlockChain::iterator BI = FunctionChain.begin(),
982 BE = FunctionChain.end();
984 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
986 << getBlockName(*BI) << "\n");
987 if (InsertPos != MachineFunction::iterator(*BI))
988 F.splice(InsertPos, *BI);
992 // Update the terminator of the previous block.
993 if (BI == FunctionChain.begin())
995 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
997 // FIXME: It would be awesome of updateTerminator would just return rather
998 // than assert when the branch cannot be analyzed in order to remove this
1001 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
1002 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
1003 // The "PrevBB" is not yet updated to reflect current code layout, so,
1004 // o. it may fall-through to a block without explict "goto" instruction
1005 // before layout, and no longer fall-through it after layout; or
1006 // o. just opposite.
1008 // AnalyzeBranch() may return erroneous value for FBB when these two
1009 // situations take place. For the first scenario FBB is mistakenly set
1010 // NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
1011 // is mistakenly pointing to "*BI".
1013 bool needUpdateBr = true;
1014 if (!Cond.empty() && (!FBB || FBB == *BI)) {
1015 PrevBB->updateTerminator();
1016 needUpdateBr = false;
1019 if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
1020 // FIXME: This should never take place.
1025 // If PrevBB has a two-way branch, try to re-order the branches
1026 // such that we branch to the successor with higher weight first.
1027 if (TBB && !Cond.empty() && FBB &&
1028 MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
1029 !TII->ReverseBranchCondition(Cond)) {
1030 DEBUG(dbgs() << "Reverse order of the two branches: "
1031 << getBlockName(PrevBB) << "\n");
1032 DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
1033 << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
1034 DebugLoc dl; // FIXME: this is nowhere
1035 TII->RemoveBranch(*PrevBB);
1036 TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
1037 needUpdateBr = true;
1040 PrevBB->updateTerminator();
1044 // Fixup the last block.
1046 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
1047 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
1048 F.back().updateTerminator();
1050 // Walk through the backedges of the function now that we have fully laid out
1051 // the basic blocks and align the destination of each backedge. We don't rely
1052 // exclusively on the loop info here so that we can align backedges in
1053 // unnatural CFGs and backedges that were introduced purely because of the
1054 // loop rotations done during this layout pass.
1055 if (F.getFunction()->getAttributes().
1056 hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize))
1058 unsigned Align = TLI->getPrefLoopAlignment();
1060 return; // Don't care about loop alignment.
1061 if (FunctionChain.begin() == FunctionChain.end())
1062 return; // Empty chain.
1064 const BranchProbability ColdProb(1, 5); // 20%
1065 BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
1066 BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
1067 for (BlockChain::iterator BI = llvm::next(FunctionChain.begin()),
1068 BE = FunctionChain.end();
1070 // Don't align non-looping basic blocks. These are unlikely to execute
1071 // enough times to matter in practice. Note that we'll still handle
1072 // unnatural CFGs inside of a natural outer loop (the common case) and
1074 MachineLoop *L = MLI->getLoopFor(*BI);
1078 // If the block is cold relative to the function entry don't waste space
1080 BlockFrequency Freq = MBFI->getBlockFreq(*BI);
1081 if (Freq < WeightedEntryFreq)
1084 // If the block is cold relative to its loop header, don't align it
1085 // regardless of what edges into the block exist.
1086 MachineBasicBlock *LoopHeader = L->getHeader();
1087 BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
1088 if (Freq < (LoopHeaderFreq * ColdProb))
1091 // Check for the existence of a non-layout predecessor which would benefit
1092 // from aligning this block.
1093 MachineBasicBlock *LayoutPred = *llvm::prior(BI);
1095 // Force alignment if all the predecessors are jumps. We already checked
1096 // that the block isn't cold above.
1097 if (!LayoutPred->isSuccessor(*BI)) {
1098 (*BI)->setAlignment(Align);
1102 // Align this block if the layout predecessor's edge into this block is
1103 // cold relative to the block. When this is true, other predecessors make up
1104 // all of the hot entries into the block and thus alignment is likely to be
1106 BranchProbability LayoutProb = MBPI->getEdgeProbability(LayoutPred, *BI);
1107 BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
1108 if (LayoutEdgeFreq <= (Freq * ColdProb))
1109 (*BI)->setAlignment(Align);
1113 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
1114 // Check for single-block functions and skip them.
1115 if (llvm::next(F.begin()) == F.end())
1118 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1119 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1120 MLI = &getAnalysis<MachineLoopInfo>();
1121 TII = F.getTarget().getInstrInfo();
1122 TLI = F.getTarget().getTargetLowering();
1123 assert(BlockToChain.empty());
1127 BlockToChain.clear();
1128 ChainAllocator.DestroyAll();
1131 // Align all of the blocks in the function to a specific alignment.
1132 for (MachineFunction::iterator FI = F.begin(), FE = F.end();
1134 FI->setAlignment(AlignAllBlock);
1136 // We always return true as we have no way to track whether the final order
1137 // differs from the original order.
1142 /// \brief A pass to compute block placement statistics.
1144 /// A separate pass to compute interesting statistics for evaluating block
1145 /// placement. This is separate from the actual placement pass so that they can
1146 /// be computed in the absence of any placement transformations or when using
1147 /// alternative placement strategies.
1148 class MachineBlockPlacementStats : public MachineFunctionPass {
1149 /// \brief A handle to the branch probability pass.
1150 const MachineBranchProbabilityInfo *MBPI;
1152 /// \brief A handle to the function-wide block frequency pass.
1153 const MachineBlockFrequencyInfo *MBFI;
1156 static char ID; // Pass identification, replacement for typeid
1157 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
1158 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
1161 bool runOnMachineFunction(MachineFunction &F);
1163 void getAnalysisUsage(AnalysisUsage &AU) const {
1164 AU.addRequired<MachineBranchProbabilityInfo>();
1165 AU.addRequired<MachineBlockFrequencyInfo>();
1166 AU.setPreservesAll();
1167 MachineFunctionPass::getAnalysisUsage(AU);
1172 char MachineBlockPlacementStats::ID = 0;
1173 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
1174 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
1175 "Basic Block Placement Stats", false, false)
1176 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
1177 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
1178 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
1179 "Basic Block Placement Stats", false, false)
1181 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
1182 // Check for single-block functions and skip them.
1183 if (llvm::next(F.begin()) == F.end())
1186 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
1187 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
1189 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
1190 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
1191 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
1192 : NumUncondBranches;
1193 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
1194 : UncondBranchTakenFreq;
1195 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
1198 // Skip if this successor is a fallthrough.
1199 if (I->isLayoutSuccessor(*SI))
1202 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
1204 BranchTakenFreq += EdgeFreq.getFrequency();