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"
52 /// \brief A structure for storing a weighted edge.
54 /// This stores an edge and its weight, computed as the product of the
55 /// frequency that the starting block is entered with the probability of
56 /// a particular exit block.
58 BlockFrequency EdgeFrequency;
59 MachineBasicBlock *From, *To;
61 bool operator<(const WeightedEdge &RHS) const {
62 return EdgeFrequency < RHS.EdgeFrequency;
69 /// \brief Type for our function-wide basic block -> block chain mapping.
70 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
74 /// \brief A chain of blocks which will be laid out contiguously.
76 /// This is the datastructure representing a chain of consecutive blocks that
77 /// are profitable to layout together in order to maximize fallthrough
78 /// probabilities. We also can use a block chain to represent a sequence of
79 /// basic blocks which have some external (correctness) requirement for
80 /// sequential layout.
82 /// Eventually, the block chains will form a directed graph over the function.
83 /// We provide an SCC-supporting-iterator in order to quicky build and walk the
84 /// SCCs of block chains within a function.
86 /// The block chains also have support for calculating and caching probability
87 /// information related to the chain itself versus other chains. This is used
88 /// for ranking during the final layout of block chains.
90 /// \brief The sequence of blocks belonging to this chain.
92 /// This is the sequence of blocks for a particular chain. These will be laid
93 /// out in-order within the function.
94 SmallVector<MachineBasicBlock *, 4> Blocks;
96 /// \brief A handle to the function-wide basic block to block chain mapping.
98 /// This is retained in each block chain to simplify the computation of child
99 /// block chains for SCC-formation and iteration. We store the edges to child
100 /// basic blocks, and map them back to their associated chains using this
102 BlockToChainMapType &BlockToChain;
105 /// \brief Construct a new BlockChain.
107 /// This builds a new block chain representing a single basic block in the
108 /// function. It also registers itself as the chain that block participates
109 /// in with the BlockToChain mapping.
110 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
111 : Blocks(1, BB), BlockToChain(BlockToChain) {
112 assert(BB && "Cannot create a chain with a null basic block");
113 BlockToChain[BB] = this;
116 /// \brief Iterator over blocks within the chain.
117 typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator;
119 /// \brief Beginning of blocks within the chain.
120 iterator begin() const { return Blocks.begin(); }
122 /// \brief End of blocks within the chain.
123 iterator end() const { return Blocks.end(); }
125 /// \brief Merge a block chain into this one.
127 /// This routine merges a block chain into this one. It takes care of forming
128 /// a contiguous sequence of basic blocks, updating the edge list, and
129 /// updating the block -> chain mapping. It does not free or tear down the
130 /// old chain, but the old chain's block list is no longer valid.
131 void merge(MachineBasicBlock *BB, BlockChain *Chain) {
133 assert(!Blocks.empty());
134 assert(Blocks.back()->isSuccessor(BB));
136 // Fast path in case we don't have a chain already.
138 assert(!BlockToChain[BB]);
139 Blocks.push_back(BB);
140 BlockToChain[BB] = this;
144 assert(BB == *Chain->begin());
145 assert(Chain->begin() != Chain->end());
147 // Update the incoming blocks to point to this chain, and add them to the
149 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
151 Blocks.push_back(*BI);
152 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
153 BlockToChain[*BI] = this;
160 class MachineBlockPlacement : public MachineFunctionPass {
161 /// \brief A typedef for a block filter set.
162 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
164 /// \brief A handle to the branch probability pass.
165 const MachineBranchProbabilityInfo *MBPI;
167 /// \brief A handle to the function-wide block frequency pass.
168 const MachineBlockFrequencyInfo *MBFI;
170 /// \brief A handle to the loop info.
171 const MachineLoopInfo *MLI;
173 /// \brief A handle to the target's instruction info.
174 const TargetInstrInfo *TII;
176 /// \brief A handle to the target's lowering info.
177 const TargetLowering *TLI;
179 /// \brief Allocator and owner of BlockChain structures.
181 /// We build BlockChains lazily by merging together high probability BB
182 /// sequences acording to the "Algo2" in the paper mentioned at the top of
183 /// the file. To reduce malloc traffic, we allocate them using this slab-like
184 /// allocator, and destroy them after the pass completes.
185 SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
187 /// \brief Function wide BasicBlock to BlockChain mapping.
189 /// This mapping allows efficiently moving from any given basic block to the
190 /// BlockChain it participates in, if any. We use it to, among other things,
191 /// allow implicitly defining edges between chains as the existing edges
192 /// between basic blocks.
193 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
195 BlockChain *CreateChain(MachineBasicBlock *BB);
196 void mergeSuccessor(MachineBasicBlock *BB, BlockChain *Chain,
197 BlockFilterSet *Filter = 0);
198 void buildLoopChains(MachineFunction &F, MachineLoop &L);
199 void buildCFGChains(MachineFunction &F);
200 void placeChainsTopologically(MachineFunction &F);
201 void AlignLoops(MachineFunction &F);
204 static char ID; // Pass identification, replacement for typeid
205 MachineBlockPlacement() : MachineFunctionPass(ID) {
206 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
209 bool runOnMachineFunction(MachineFunction &F);
211 void getAnalysisUsage(AnalysisUsage &AU) const {
212 AU.addRequired<MachineBranchProbabilityInfo>();
213 AU.addRequired<MachineBlockFrequencyInfo>();
214 AU.addRequired<MachineLoopInfo>();
215 MachineFunctionPass::getAnalysisUsage(AU);
218 const char *getPassName() const { return "Block Placement"; }
222 char MachineBlockPlacement::ID = 0;
223 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
224 "Branch Probability Basic Block Placement", false, false)
225 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
226 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
227 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
228 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
229 "Branch Probability Basic Block Placement", false, false)
231 FunctionPass *llvm::createMachineBlockPlacementPass() {
232 return new MachineBlockPlacement();
236 /// \brief Helper to print the name of a MBB.
238 /// Only used by debug logging.
239 static std::string getBlockName(MachineBasicBlock *BB) {
241 raw_string_ostream OS(Result);
242 OS << "BB#" << BB->getNumber()
243 << " (derived from LLVM BB '" << BB->getName() << "')";
248 /// \brief Helper to print the number of a MBB.
250 /// Only used by debug logging.
251 static std::string getBlockNum(MachineBasicBlock *BB) {
253 raw_string_ostream OS(Result);
254 OS << "BB#" << BB->getNumber();
260 /// \brief Helper to create a new chain for a single BB.
262 /// Takes care of growing the Chains, setting up the BlockChain object, and any
263 /// debug checking logic.
264 /// \returns A pointer to the new BlockChain.
265 BlockChain *MachineBlockPlacement::CreateChain(MachineBasicBlock *BB) {
267 new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
271 /// \brief Merge a chain with any viable successor.
273 /// This routine walks the predecessors of the current block, looking for
274 /// viable merge candidates. It has strict rules it uses to determine when
275 /// a predecessor can be merged with the current block, which center around
276 /// preserving the CFG structure. It performs the merge if any viable candidate
278 void MachineBlockPlacement::mergeSuccessor(MachineBasicBlock *BB,
280 BlockFilterSet *Filter) {
284 // If this block is not at the end of its chain, it cannot merge with any
286 if (Chain && *llvm::prior(Chain->end()) != BB)
289 // Walk through the successors looking for the highest probability edge.
290 MachineBasicBlock *Successor = 0;
291 BranchProbability BestProb = BranchProbability::getZero();
292 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
293 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
296 if (BB == *SI || (Filter && !Filter->count(*SI)))
299 BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
300 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb << "\n");
301 if (!Successor || SuccProb > BestProb || (!(SuccProb < BestProb) &&
302 BB->isLayoutSuccessor(*SI))) {
310 // Grab a chain if it exists already for this successor and make sure the
311 // successor is at the start of the chain as we can't merge mid-chain. Also,
312 // if the successor chain is the same as our chain, we're already merged.
313 BlockChain *SuccChain = BlockToChain[Successor];
314 if (SuccChain && (SuccChain == Chain || Successor != *SuccChain->begin()))
317 // We only merge chains across a CFG merge when the desired merge path is
318 // significantly hotter than the incoming edge. We define a hot edge more
319 // strictly than the BranchProbabilityInfo does, as the two predecessor
320 // blocks may have dramatically different incoming probabilities we need to
321 // account for. Therefor we use the "global" edge weight which is the
322 // branch's probability times the block frequency of the predecessor.
323 BlockFrequency MergeWeight = MBFI->getBlockFreq(BB);
324 MergeWeight *= MBPI->getEdgeProbability(BB, Successor);
325 // We only want to consider breaking the CFG when the merge weight is much
326 // higher (80% vs. 20%), so multiply it by 1/4. This will require the merged
327 // edge to be 4x more likely before we disrupt the CFG. This number matches
328 // the definition of "hot" in BranchProbabilityAnalysis (80% vs. 20%).
329 MergeWeight *= BranchProbability(1, 4);
330 for (MachineBasicBlock::pred_iterator PI = Successor->pred_begin(),
331 PE = Successor->pred_end();
333 if (BB == *PI || Successor == *PI) continue;
334 BlockFrequency PredWeight = MBFI->getBlockFreq(*PI);
335 PredWeight *= MBPI->getEdgeProbability(*PI, Successor);
337 // Return on the first predecessor we find which outstrips our merge weight.
338 if (MergeWeight < PredWeight)
340 DEBUG(dbgs() << "Breaking CFG edge!\n"
341 << " Edge from " << getBlockNum(BB) << " to "
342 << getBlockNum(Successor) << ": " << MergeWeight << "\n"
343 << " vs. " << getBlockNum(BB) << " to "
344 << getBlockNum(*PI) << ": " << PredWeight << "\n");
347 DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
348 << getBlockNum(Successor) << "\n");
349 Chain->merge(Successor, SuccChain);
352 /// \brief Forms basic block chains from the natural loop structures.
354 /// These chains are designed to preserve the existing *structure* of the code
355 /// as much as possible. We can then stitch the chains together in a way which
356 /// both preserves the topological structure and minimizes taken conditional
358 void MachineBlockPlacement::buildLoopChains(MachineFunction &F, MachineLoop &L) {
359 // First recurse through any nested loops, building chains for those inner
361 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
362 buildLoopChains(F, **LI);
364 SmallPtrSet<MachineBasicBlock *, 16> LoopBlockSet(L.block_begin(),
367 // Begin building up a set of chains of blocks within this loop which should
368 // remain contiguous. Some of the blocks already belong to a chain which
369 // represents an inner loop.
370 for (MachineLoop::block_iterator BI = L.block_begin(), BE = L.block_end();
372 MachineBasicBlock *BB = *BI;
373 BlockChain *Chain = BlockToChain[BB];
374 if (!Chain) Chain = CreateChain(BB);
375 mergeSuccessor(BB, Chain, &LoopBlockSet);
379 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
380 // First build any loop-based chains.
381 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
383 buildLoopChains(F, **LI);
385 // Now walk the blocks of the function forming chains where they don't
386 // violate any CFG structure.
387 for (MachineFunction::iterator BI = F.begin(), BE = F.end();
389 MachineBasicBlock *BB = BI;
390 BlockChain *Chain = BlockToChain[BB];
391 if (!Chain) Chain = CreateChain(BB);
392 mergeSuccessor(BB, Chain);
396 void MachineBlockPlacement::placeChainsTopologically(MachineFunction &F) {
397 MachineBasicBlock *EntryB = &F.front();
398 assert(BlockToChain[EntryB] && "Missing chain for entry block");
399 assert(*BlockToChain[EntryB]->begin() == EntryB &&
400 "Entry block is not the head of the entry block chain");
402 // Walk the blocks in RPO, and insert each block for a chain in order the
403 // first time we see that chain.
404 MachineFunction::iterator InsertPos = F.begin();
405 SmallPtrSet<BlockChain *, 16> VisitedChains;
406 ReversePostOrderTraversal<MachineBasicBlock *> RPOT(EntryB);
407 typedef ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
409 for (rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) {
410 BlockChain *Chain = BlockToChain[*I];
412 if(!VisitedChains.insert(Chain))
414 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); BI != BE;
416 DEBUG(dbgs() << (BI == Chain->begin() ? "Placing chain "
418 << getBlockName(*BI) << "\n");
419 if (InsertPos != MachineFunction::iterator(*BI))
420 F.splice(InsertPos, *BI);
426 // Now that every block is in its final position, update all of the
428 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
429 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
430 // FIXME: It would be awesome of updateTerminator would just return rather
431 // than assert when the branch cannot be analyzed in order to remove this
434 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
435 if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
436 FI->updateTerminator();
440 /// \brief Recursive helper to align a loop and any nested loops.
441 static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
442 // Recurse through nested loops.
443 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
444 AlignLoop(F, *I, Align);
446 L->getTopBlock()->setAlignment(Align);
449 /// \brief Align loop headers to target preferred alignments.
450 void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
451 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
454 unsigned Align = TLI->getPrefLoopAlignment();
456 return; // Don't care about loop alignment.
458 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
459 AlignLoop(F, *I, Align);
462 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
463 // Check for single-block functions and skip them.
464 if (llvm::next(F.begin()) == F.end())
467 MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
468 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
469 MLI = &getAnalysis<MachineLoopInfo>();
470 TII = F.getTarget().getInstrInfo();
471 TLI = F.getTarget().getTargetLowering();
472 assert(BlockToChain.empty());
475 placeChainsTopologically(F);
478 BlockToChain.clear();
480 // We always return true as we have no way to track whether the final order
481 // differs from the original order.