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;
491 // Now select the head of the chain to which the unplaced block belongs
492 // as the block to place. This will force the entire chain to be placed,
493 // and satisfies the requirements of merging chains.
494 return *BlockToChain[I]->begin();
500 void MachineBlockPlacement::buildChain(
501 MachineBasicBlock *BB,
503 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
504 const BlockFilterSet *BlockFilter) {
506 assert(BlockToChain[BB] == &Chain);
507 assert(*Chain.begin() == BB);
508 MachineFunction &F = *BB->getParent();
509 MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
511 MachineBasicBlock *LoopHeaderBB = BB;
512 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
513 BB = *llvm::prior(Chain.end());
516 assert(BlockToChain[BB] == &Chain);
517 assert(*llvm::prior(Chain.end()) == BB);
518 MachineBasicBlock *BestSucc = 0;
520 // Look for the best viable successor if there is one to place immediately
522 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
524 // If an immediate successor isn't available, look for the best viable
525 // block among those we've identified as not violating the loop's CFG at
526 // this point. This won't be a fallthrough, but it will increase locality.
528 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
531 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
536 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
537 "layout successor until the CFG reduces\n");
540 // Place this block, updating the datastructures to reflect its placement.
541 BlockChain &SuccChain = *BlockToChain[BestSucc];
542 // Zero out LoopPredecessors for the successor we're about to merge in case
543 // we selected a successor that didn't fit naturally into the CFG.
544 SuccChain.LoopPredecessors = 0;
545 DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
546 << " to " << getBlockNum(BestSucc) << "\n");
547 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
548 Chain.merge(BestSucc, &SuccChain);
549 BB = *llvm::prior(Chain.end());
552 DEBUG(dbgs() << "Finished forming chain for header block "
553 << getBlockNum(*Chain.begin()) << "\n");
556 /// \brief Forms basic block chains from the natural loop structures.
558 /// These chains are designed to preserve the existing *structure* of the code
559 /// as much as possible. We can then stitch the chains together in a way which
560 /// both preserves the topological structure and minimizes taken conditional
562 void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
564 // First recurse through any nested loops, building chains for those inner
566 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
567 buildLoopChains(F, **LI);
569 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
570 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
571 BlockChain &LoopChain = *BlockToChain[L.getHeader()];
573 // FIXME: This is a really lame way of walking the chains in the loop: we
574 // walk the blocks, and use a set to prevent visiting a particular chain
576 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
577 for (MachineLoop::block_iterator BI = L.block_begin(),
580 BlockChain &Chain = *BlockToChain[*BI];
581 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin())
584 assert(Chain.LoopPredecessors == 0);
585 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
587 assert(BlockToChain[*BCI] == &Chain);
588 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
589 PE = (*BCI)->pred_end();
591 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
593 ++Chain.LoopPredecessors;
597 if (Chain.LoopPredecessors == 0)
598 BlockWorkList.push_back(*BI);
601 buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet);
604 // Crash at the end so we get all of the debugging output first.
605 bool BadLoop = false;
606 if (LoopChain.LoopPredecessors) {
608 dbgs() << "Loop chain contains a block without its preds placed!\n"
609 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
610 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
612 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
614 if (!LoopBlockSet.erase(*BCI)) {
615 // We don't mark the loop as bad here because there are real situations
616 // where this can occur. For example, with an unanalyzable fallthrough
617 // from a loop block to a non-loop block.
618 // FIXME: Such constructs shouldn't exist. Track them down and fix them.
619 dbgs() << "Loop chain contains a block not contained by the loop!\n"
620 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
621 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
622 << " Bad block: " << getBlockName(*BCI) << "\n";
625 if (!LoopBlockSet.empty()) {
627 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
628 LBE = LoopBlockSet.end();
630 dbgs() << "Loop contains blocks never placed into a chain!\n"
631 << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
632 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
633 << " Bad block: " << getBlockName(*LBI) << "\n";
635 assert(!BadLoop && "Detected problems with the placement of this loop.");
639 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
640 // Ensure that every BB in the function has an associated chain to simplify
641 // the assumptions of the remaining algorithm.
642 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
643 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
644 MachineBasicBlock *BB = FI;
645 BlockChain *&Chain = BlockToChain[BB];
646 Chain = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
647 // Also, merge any blocks which we cannot reason about and must preserve
648 // the exact fallthrough behavior for.
651 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
652 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
655 MachineFunction::iterator NextFI(llvm::next(FI));
656 MachineBasicBlock *NextBB = NextFI;
657 // Ensure that the layout successor is a viable block, as we know that
658 // fallthrough is a possibility.
659 assert(NextFI != FE && "Can't fallthrough past the last block.");
660 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
661 << getBlockName(BB) << " -> " << getBlockName(NextBB)
663 Chain->merge(NextBB, 0);
669 // Build any loop-based chains.
670 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
672 buildLoopChains(F, **LI);
674 SmallVector<MachineBasicBlock *, 16> BlockWorkList;
676 SmallPtrSet<BlockChain *, 4> UpdatedPreds;
677 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
678 MachineBasicBlock *BB = &*FI;
679 BlockChain &Chain = *BlockToChain[BB];
680 if (!UpdatedPreds.insert(&Chain))
683 assert(Chain.LoopPredecessors == 0);
684 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
686 assert(BlockToChain[*BCI] == &Chain);
687 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
688 PE = (*BCI)->pred_end();
690 if (BlockToChain[*PI] == &Chain)
692 ++Chain.LoopPredecessors;
696 if (Chain.LoopPredecessors == 0)
697 BlockWorkList.push_back(BB);
700 BlockChain &FunctionChain = *BlockToChain[&F.front()];
701 buildChain(&F.front(), FunctionChain, BlockWorkList);
703 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
705 // Crash at the end so we get all of the debugging output first.
706 bool BadFunc = false;
707 FunctionBlockSetType FunctionBlockSet;
708 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
709 FunctionBlockSet.insert(FI);
711 for (BlockChain::iterator BCI = FunctionChain.begin(),
712 BCE = FunctionChain.end();
714 if (!FunctionBlockSet.erase(*BCI)) {
716 dbgs() << "Function chain contains a block not in the function!\n"
717 << " Bad block: " << getBlockName(*BCI) << "\n";
720 if (!FunctionBlockSet.empty()) {
722 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
723 FBE = FunctionBlockSet.end();
725 dbgs() << "Function contains blocks never placed into a chain!\n"
726 << " Bad block: " << getBlockName(*FBI) << "\n";
728 assert(!BadFunc && "Detected problems with the block placement.");
731 // Splice the blocks into place.
732 MachineFunction::iterator InsertPos = F.begin();
733 for (BlockChain::iterator BI = FunctionChain.begin(),
734 BE = FunctionChain.end();
736 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
738 << getBlockName(*BI) << "\n");
739 if (InsertPos != MachineFunction::iterator(*BI))
740 F.splice(InsertPos, *BI);
744 // Update the terminator of the previous block.
745 if (BI == FunctionChain.begin())
747 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI));
749 // FIXME: It would be awesome of updateTerminator would just return rather
750 // than assert when the branch cannot be analyzed in order to remove this
753 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
754 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond))
755 PrevBB->updateTerminator();
758 // Fixup the last block.
760 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
761 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
762 F.back().updateTerminator();
765 /// \brief Recursive helper to align a loop and any nested loops.
766 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
767 // Recurse through nested loops.
768 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
769 AlignLoop(F, *I, Align);
771 L->getTopBlock()->setAlignment(Align);
774 /// \brief Align loop headers to target preferred alignments.
775 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
776 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
779 unsigned Align = TLI->getPrefLoopAlignment();
781 return; // Don't care about loop alignment.
783 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
784 AlignLoop(F, *I, Align);
787 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
788 // Check for single-block functions and skip them.
789 if (llvm::next(F.begin()) == F.end())
792 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
793 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
794 MLI = &getAnalysis<MachineLoopInfo>();
795 TII = F.getTarget().getInstrInfo();
796 TLI = F.getTarget().getTargetLowering();
797 assert(BlockToChain.empty());
802 BlockToChain.clear();
803 ChainAllocator.DestroyAll();
805 // We always return true as we have no way to track whether the final order
806 // differs from the original order.
811 /// \brief A pass to compute block placement statistics.
813 /// A separate pass to compute interesting statistics for evaluating block
814 /// placement. This is separate from the actual placement pass so that they can
815 /// be computed in the absense of any placement transformations or when using
816 /// alternative placement strategies.
817 class MachineBlockPlacementStats : public MachineFunctionPass {
818 /// \brief A handle to the branch probability pass.
819 const MachineBranchProbabilityInfo *MBPI;
821 /// \brief A handle to the function-wide block frequency pass.
822 const MachineBlockFrequencyInfo *MBFI;
825 static char ID; // Pass identification, replacement for typeid
826 MachineBlockPlacementStats() : MachineFunctionPass(ID) {
827 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
830 bool runOnMachineFunction(MachineFunction &F);
832 void getAnalysisUsage(AnalysisUsage &AU) const {
833 AU.addRequired<MachineBranchProbabilityInfo>();
834 AU.addRequired<MachineBlockFrequencyInfo>();
835 AU.setPreservesAll();
836 MachineFunctionPass::getAnalysisUsage(AU);
839 const char *getPassName() const { return "Block Placement Stats"; }
843 char MachineBlockPlacementStats::ID = 0;
844 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
845 "Basic Block Placement Stats", false, false)
846 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
847 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
848 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
849 "Basic Block Placement Stats", false, false)
851 FunctionPass *llvm::createMachineBlockPlacementStatsPass() {
852 return new MachineBlockPlacementStats();
855 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
856 // Check for single-block functions and skip them.
857 if (llvm::next(F.begin()) == F.end())
860 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
861 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
863 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
864 BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
865 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
867 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
868 : UncondBranchTakenFreq;
869 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
872 // Skip if this successor is a fallthrough.
873 if (I->isLayoutSuccessor(*SI))
876 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
878 BranchTakenFreq += EdgeFreq.getFrequency();