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(
219 const BlockChain &PlacedChain,
220 MachineFunction::iterator &PrevUnplacedBlockIt,
221 const BlockFilterSet *BlockFilter);
222 void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
223 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
224 const BlockFilterSet *BlockFilter = 0);
225 void buildLoopChains(MachineFunction &F, MachineLoop &L);
226 void buildCFGChains(MachineFunction &F);
227 void AlignLoops(MachineFunction &F);
230 static char ID; // Pass identification, replacement for typeid
231 MachineBlockPlacement() : MachineFunctionPass(ID) {
232 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
235 bool runOnMachineFunction(MachineFunction &F);
237 void getAnalysisUsage(AnalysisUsage &AU) const {
238 AU.addRequired<MachineBranchProbabilityInfo>();
239 AU.addRequired<MachineBlockFrequencyInfo>();
240 AU.addRequired<MachineLoopInfo>();
241 MachineFunctionPass::getAnalysisUsage(AU);
244 const char *getPassName() const { return "Block Placement"; }
248 char MachineBlockPlacement::ID = 0;
249 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
250 "Branch Probability Basic Block Placement", false, false)
251 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
252 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
253 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
254 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
255 "Branch Probability Basic Block Placement", false, false)
257 FunctionPass *llvm::createMachineBlockPlacementPass() {
258 return new MachineBlockPlacement();
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(*SI);
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 effeciently 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 assert(!BlockFilter || BlockFilter->count(*WBI));
455 BlockChain &SuccChain = *BlockToChain[*WBI];
456 if (&SuccChain == &Chain) {
457 DEBUG(dbgs() << " " << getBlockName(*WBI)
458 << " -> Already merged!\n");
461 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
463 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
464 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq
466 if (BestBlock && BestFreq >= CandidateFreq)
469 BestFreq = CandidateFreq;
474 /// \brief Retrieve the first unplaced basic block.
476 /// This routine is called when we are unable to use the CFG to walk through
477 /// all of the basic blocks and form a chain due to unnatural loops in the CFG.
478 /// We walk through the function's blocks in order, starting from the
479 /// LastUnplacedBlockIt. We update this iterator on each call to avoid
480 /// re-scanning the entire sequence on repeated calls to this routine.
481 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
482 MachineFunction &F, const BlockChain &PlacedChain,
483 MachineFunction::iterator &PrevUnplacedBlockIt,
484 const BlockFilterSet *BlockFilter) {
485 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
487 if (BlockFilter && !BlockFilter->count(I))
489 if (BlockToChain[I] != &PlacedChain) {
490 PrevUnplacedBlockIt = I;
497 void MachineBlockPlacement::buildChain(
498 MachineBasicBlock *BB,
500 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
501 const BlockFilterSet *BlockFilter) {
503 assert(BlockToChain[BB] == &Chain);
504 assert(*Chain.begin() == BB);
505 MachineFunction &F = *BB->getParent();
506 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
508 MachineBasicBlock *LoopHeaderBB = BB;
509 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
510 BB = *llvm::prior(Chain.end());
513 assert(BlockToChain[BB] == &Chain);
514 assert(*llvm::prior(Chain.end()) == BB);
515 MachineBasicBlock *BestSucc = 0;
517 // Look for the best viable successor if there is one to place immediately
519 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 Forms basic block chains from the natural loop structures.
555 /// These chains are designed to preserve the existing *structure* of the code
556 /// as much as possible. We can then stitch the chains together in a way which
557 /// both preserves the topological structure and minimizes taken conditional
559 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
561 // First recurse through any nested loops, building chains for those inner
563 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
564 buildLoopChains(F, **LI);
566 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
567 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
568 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
570 // FIXME: This is a really lame way of walking the chains in the loop: we
571 // walk the blocks, and use a set to prevent visiting a particular chain
573 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
574 for (MachineLoop::block_iterator BI = L.block_begin(),
577 BlockChain &Chain = *BlockToChain[*BI];
578 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin())
581 assert(Chain.LoopPredecessors == 0);
582 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
584 assert(BlockToChain[*BCI] == &Chain);
585 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
586 PE = (*BCI)->pred_end();
588 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
590 ++Chain.LoopPredecessors;
594 if (Chain.LoopPredecessors == 0)
595 BlockWorkList.push_back(*BI);
598 buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet);
601 // Crash at the end so we get all of the debugging output first.
602 bool BadLoop = false;
603 if (LoopChain.LoopPredecessors) {
605 dbgs() << "Loop chain contains a block without its preds placed!\n"
606 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
607 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
609 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
611 if (!LoopBlockSet.erase(*BCI)) {
612 // We don't mark the loop as bad here because there are real situations
613 // where this can occur. For example, with an unanalyzable fallthrough
614 // from a loop block to a non-loop block.
615 // FIXME: Such constructs shouldn't exist. Track them down and fix them.
616 dbgs() << "Loop chain contains a block not contained by the loop!\n"
617 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
618 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
619 << " Bad block: " << getBlockName(*BCI) << "\n";
622 if (!LoopBlockSet.empty()) {
624 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
625 LBE = LoopBlockSet.end();
627 dbgs() << "Loop contains blocks never placed into a chain!\n"
628 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
629 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
630 << " Bad block: " << getBlockName(*LBI) << "\n";
632 assert(!BadLoop && "Detected problems with the placement of this loop.");
636 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
637 // Ensure that every BB in the function has an associated chain to simplify
638 // the assumptions of the remaining algorithm.
639 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
640 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
641 MachineBasicBlock *BB = FI;
642 BlockChain *&Chain = BlockToChain[BB];
643 Chain = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
644 // Also, merge any blocks which we cannot reason about and must preserve
645 // the exact fallthrough behavior for.
648 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
649 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
652 MachineFunction::iterator NextFI(llvm::next(FI));
653 MachineBasicBlock *NextBB = NextFI;
654 // Ensure that the layout successor is a viable block, as we know that
655 // fallthrough is a possibility.
656 assert(NextFI != FE && "Can't fallthrough past the last block.");
657 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
658 << getBlockName(BB) << " -> " << getBlockName(NextBB)
660 Chain->merge(NextBB, 0);
666 // Build any loop-based chains.
667 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
669 buildLoopChains(F, **LI);
671 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
673 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
674 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
675 MachineBasicBlock *BB = &*FI;
676 BlockChain &Chain = *BlockToChain[BB];
677 if (!UpdatedPreds.insert(&Chain))
680 assert(Chain.LoopPredecessors == 0);
681 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
683 assert(BlockToChain[*BCI] == &Chain);
684 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
685 PE = (*BCI)->pred_end();
687 if (BlockToChain[*PI] == &Chain)
689 ++Chain.LoopPredecessors;
693 if (Chain.LoopPredecessors == 0)
694 BlockWorkList.push_back(BB);
697 BlockChain &FunctionChain = *BlockToChain[&F.front()];
698 buildChain(&F.front(), FunctionChain, BlockWorkList);
700 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
702 // Crash at the end so we get all of the debugging output first.
703 bool BadFunc = false;
704 FunctionBlockSetType FunctionBlockSet;
705 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
706 FunctionBlockSet.insert(FI);
708 for (BlockChain::iterator BCI = FunctionChain.begin(),
709 BCE = FunctionChain.end();
711 if (!FunctionBlockSet.erase(*BCI)) {
713 dbgs() << "Function chain contains a block not in the function!\n"
714 << " Bad block: " << getBlockName(*BCI) << "\n";
717 if (!FunctionBlockSet.empty()) {
719 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
720 FBE = FunctionBlockSet.end();
722 dbgs() << "Function contains blocks never placed into a chain!\n"
723 << " Bad block: " << getBlockName(*FBI) << "\n";
725 assert(!BadFunc && "Detected problems with the block placement.");
728 // Splice the blocks into place.
729 MachineFunction::iterator InsertPos = F.begin();
730 for (BlockChain::iterator BI = FunctionChain.begin(),
731 BE = FunctionChain.end();
733 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
735 << getBlockName(*BI) << "\n");
736 if (InsertPos != MachineFunction::iterator(*BI))
737 F.splice(InsertPos, *BI);
741 // Update the terminator of the previous block.
742 if (BI == FunctionChain.begin())
744 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
746 // FIXME: It would be awesome of updateTerminator would just return rather
747 // than assert when the branch cannot be analyzed in order to remove this
750 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
751 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
752 PrevBB->updateTerminator();
755 // Fixup the last block.
757 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
758 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
759 F.back().updateTerminator();
762 /// \brief Recursive helper to align a loop and any nested loops.
763 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
764 // Recurse through nested loops.
765 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
766 AlignLoop(F, *I, Align);
768 L->getTopBlock()->setAlignment(Align);
771 /// \brief Align loop headers to target preferred alignments.
772 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
773 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
776 unsigned Align = TLI->getPrefLoopAlignment();
778 return; // Don't care about loop alignment.
780 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
781 AlignLoop(F, *I, Align);
784 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
785 // Check for single-block functions and skip them.
786 if (llvm::next(F.begin()) == F.end())
789 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
790 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
791 MLI = &getAnalysis<MachineLoopInfo>();
792 TII = F.getTarget().getInstrInfo();
793 TLI = F.getTarget().getTargetLowering();
794 assert(BlockToChain.empty());
799 BlockToChain.clear();
800 ChainAllocator.DestroyAll();
802 // We always return true as we have no way to track whether the final order
803 // differs from the original order.
808 /// \brief A pass to compute block placement statistics.
810 /// A separate pass to compute interesting statistics for evaluating block
811 /// placement. This is separate from the actual placement pass so that they can
812 /// be computed in the absense of any placement transformations or when using
813 /// alternative placement strategies.
814 class MachineBlockPlacementStats : public MachineFunctionPass {
815 /// \brief A handle to the branch probability pass.
816 const MachineBranchProbabilityInfo *MBPI;
818 /// \brief A handle to the function-wide block frequency pass.
819 const MachineBlockFrequencyInfo *MBFI;
822 static char ID; // Pass identification, replacement for typeid
823 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
824 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
827 bool runOnMachineFunction(MachineFunction &F);
829 void getAnalysisUsage(AnalysisUsage &AU) const {
830 AU.addRequired<MachineBranchProbabilityInfo>();
831 AU.addRequired<MachineBlockFrequencyInfo>();
832 AU.setPreservesAll();
833 MachineFunctionPass::getAnalysisUsage(AU);
836 const char *getPassName() const { return "Block Placement Stats"; }
840 char MachineBlockPlacementStats::ID = 0;
841 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
842 "Basic Block Placement Stats", false, false)
843 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
844 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
845 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
846 "Basic Block Placement Stats", false, false)
848 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
849 return new MachineBlockPlacementStats();
852 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
853 // Check for single-block functions and skip them.
854 if (llvm::next(F.begin()) == F.end())
857 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
858 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
860 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
861 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
862 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
864 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
865 : UncondBranchTakenFreq;
866 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
869 // Skip if this successor is a fallthrough.
870 if (I->isLayoutSuccessor(*SI))
873 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
875 BranchTakenFreq += EdgeFreq.getFrequency();