1 //===- EarlyCSE.cpp - Simple and fast CSE pass ----------------------------===//
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 pass performs a simple dominator tree walk that eliminates trivially
11 // redundant instructions.
13 //===----------------------------------------------------------------------===//
15 #define DEBUG_TYPE "early-cse"
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/Instructions.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/Dominators.h"
20 #include "llvm/Analysis/InstructionSimplify.h"
21 #include "llvm/Target/TargetData.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/RecyclingAllocator.h"
25 #include "llvm/ADT/ScopedHashTable.h"
26 #include "llvm/ADT/Statistic.h"
29 STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd");
30 STATISTIC(NumCSE, "Number of instructions CSE'd");
31 STATISTIC(NumCSELoad, "Number of load instructions CSE'd");
32 STATISTIC(NumCSECall, "Number of call instructions CSE'd");
33 STATISTIC(NumDSE, "Number of trivial dead stores removed");
35 static unsigned getHash(const void *V) {
36 return DenseMapInfo<const void*>::getHashValue(V);
39 //===----------------------------------------------------------------------===//
41 //===----------------------------------------------------------------------===//
44 /// SimpleValue - Instances of this struct represent available values in the
45 /// scoped hash table.
49 SimpleValue(Instruction *I) : Inst(I) {
50 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
53 bool isSentinel() const {
54 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() ||
55 Inst == DenseMapInfo<Instruction*>::getTombstoneKey();
58 static bool canHandle(Instruction *Inst) {
59 return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) ||
60 isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) ||
61 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) ||
62 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) ||
63 isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst);
69 // SimpleValue is POD.
70 template<> struct isPodLike<SimpleValue> {
71 static const bool value = true;
74 template<> struct DenseMapInfo<SimpleValue> {
75 static inline SimpleValue getEmptyKey() {
76 return DenseMapInfo<Instruction*>::getEmptyKey();
78 static inline SimpleValue getTombstoneKey() {
79 return DenseMapInfo<Instruction*>::getTombstoneKey();
81 static unsigned getHashValue(SimpleValue Val);
82 static bool isEqual(SimpleValue LHS, SimpleValue RHS);
86 unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
87 Instruction *Inst = Val.Inst;
89 // Hash in all of the operands as pointers.
91 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i)
92 Res ^= getHash(Inst->getOperand(i)) << i;
94 if (CastInst *CI = dyn_cast<CastInst>(Inst))
95 Res ^= getHash(CI->getType());
96 else if (CmpInst *CI = dyn_cast<CmpInst>(Inst))
97 Res ^= CI->getPredicate();
98 else if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst)) {
99 for (ExtractValueInst::idx_iterator I = EVI->idx_begin(),
100 E = EVI->idx_end(); I != E; ++I)
102 } else if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst)) {
103 for (InsertValueInst::idx_iterator I = IVI->idx_begin(),
104 E = IVI->idx_end(); I != E; ++I)
107 // nothing extra to hash in.
108 assert((isa<BinaryOperator>(Inst) || isa<GetElementPtrInst>(Inst) ||
109 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) ||
110 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst)) &&
111 "Invalid/unknown instruction");
114 // Mix in the opcode.
115 return (Res << 1) ^ Inst->getOpcode();
118 bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) {
119 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
121 if (LHS.isSentinel() || RHS.isSentinel())
124 if (LHSI->getOpcode() != RHSI->getOpcode()) return false;
125 return LHSI->isIdenticalTo(RHSI);
128 //===----------------------------------------------------------------------===//
130 //===----------------------------------------------------------------------===//
133 /// CallValue - Instances of this struct represent available call values in
134 /// the scoped hash table.
138 CallValue(Instruction *I) : Inst(I) {
139 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
142 bool isSentinel() const {
143 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() ||
144 Inst == DenseMapInfo<Instruction*>::getTombstoneKey();
147 static bool canHandle(Instruction *Inst) {
148 CallInst *CI = dyn_cast<CallInst>(Inst);
149 if (CI == 0 || !CI->onlyReadsMemory())
152 // Check that there are no metadata operands.
153 for (unsigned i = 0, e = CI->getNumOperands(); i != e; ++i)
154 if (CI->getOperand(i)->getType()->isMetadataTy())
163 template<> struct isPodLike<CallValue> {
164 static const bool value = true;
167 template<> struct DenseMapInfo<CallValue> {
168 static inline CallValue getEmptyKey() {
169 return DenseMapInfo<Instruction*>::getEmptyKey();
171 static inline CallValue getTombstoneKey() {
172 return DenseMapInfo<Instruction*>::getTombstoneKey();
174 static unsigned getHashValue(CallValue Val);
175 static bool isEqual(CallValue LHS, CallValue RHS);
178 unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) {
179 Instruction *Inst = Val.Inst;
180 // Hash in all of the operands as pointers.
182 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i)
183 Res ^= getHash(Inst->getOperand(i)) << i;
184 // Mix in the opcode.
185 return (Res << 1) ^ Inst->getOpcode();
188 bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) {
189 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
190 if (LHS.isSentinel() || RHS.isSentinel())
192 return LHSI->isIdenticalTo(RHSI);
196 //===----------------------------------------------------------------------===//
198 //===----------------------------------------------------------------------===//
202 /// EarlyCSE - This pass does a simple depth-first walk over the dominator
203 /// tree, eliminating trivially redundant instructions and using instsimplify
204 /// to canonicalize things as it goes. It is intended to be fast and catch
205 /// obvious cases so that instcombine and other passes are more effective. It
206 /// is expected that a later pass of GVN will catch the interesting/hard
208 class EarlyCSE : public FunctionPass {
210 const TargetData *TD;
212 typedef RecyclingAllocator<BumpPtrAllocator,
213 ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy;
214 typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>,
215 AllocatorTy> ScopedHTType;
217 /// AvailableValues - This scoped hash table contains the current values of
218 /// all of our simple scalar expressions. As we walk down the domtree, we
219 /// look to see if instructions are in this: if so, we replace them with what
220 /// we find, otherwise we insert them so that dominated values can succeed in
222 ScopedHTType *AvailableValues;
224 /// AvailableLoads - This scoped hash table contains the current values
225 /// of loads. This allows us to get efficient access to dominating loads when
226 /// we have a fully redundant load. In addition to the most recent load, we
227 /// keep track of a generation count of the read, which is compared against
228 /// the current generation count. The current generation count is
229 /// incremented after every possibly writing memory operation, which ensures
230 /// that we only CSE loads with other loads that have no intervening store.
231 typedef RecyclingAllocator<BumpPtrAllocator,
232 ScopedHashTableVal<Value*, std::pair<Value*, unsigned> > > LoadMapAllocator;
233 typedef ScopedHashTable<Value*, std::pair<Value*, unsigned>,
234 DenseMapInfo<Value*>, LoadMapAllocator> LoadHTType;
235 LoadHTType *AvailableLoads;
237 /// AvailableCalls - This scoped hash table contains the current values
238 /// of read-only call values. It uses the same generation count as loads.
239 typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType;
240 CallHTType *AvailableCalls;
242 /// CurrentGeneration - This is the current generation of the memory value.
243 unsigned CurrentGeneration;
246 explicit EarlyCSE() : FunctionPass(ID) {
247 initializeEarlyCSEPass(*PassRegistry::getPassRegistry());
250 bool runOnFunction(Function &F);
254 bool processNode(DomTreeNode *Node);
256 // This transformation requires dominator postdominator info
257 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
258 AU.addRequired<DominatorTree>();
259 AU.setPreservesCFG();
264 char EarlyCSE::ID = 0;
266 // createEarlyCSEPass - The public interface to this file.
267 FunctionPass *llvm::createEarlyCSEPass() {
268 return new EarlyCSE();
271 INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false)
272 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
273 INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false)
275 bool EarlyCSE::processNode(DomTreeNode *Node) {
276 // Define a scope in the scoped hash table. When we are done processing this
277 // domtree node and recurse back up to our parent domtree node, this will pop
278 // off all the values we install.
279 ScopedHTType::ScopeTy Scope(*AvailableValues);
281 // Define a scope for the load values so that anything we add will get
282 // popped when we recurse back up to our parent domtree node.
283 LoadHTType::ScopeTy LoadScope(*AvailableLoads);
285 // Define a scope for the call values so that anything we add will get
286 // popped when we recurse back up to our parent domtree node.
287 CallHTType::ScopeTy CallScope(*AvailableCalls);
289 BasicBlock *BB = Node->getBlock();
291 // If this block has a single predecessor, then the predecessor is the parent
292 // of the domtree node and all of the live out memory values are still current
293 // in this block. If this block has multiple predecessors, then they could
294 // have invalidated the live-out memory values of our parent value. For now,
295 // just be conservative and invalidate memory if this block has multiple
297 if (BB->getSinglePredecessor() == 0)
300 /// LastStore - Keep track of the last non-volatile store that we saw... for
301 /// as long as there in no instruction that reads memory. If we see a store
302 /// to the same location, we delete the dead store. This zaps trivial dead
303 /// stores which can occur in bitfield code among other things.
304 StoreInst *LastStore = 0;
306 bool Changed = false;
308 // See if any instructions in the block can be eliminated. If so, do it. If
309 // not, add them to AvailableValues.
310 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
311 Instruction *Inst = I++;
313 // Dead instructions should just be removed.
314 if (isInstructionTriviallyDead(Inst)) {
315 DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n');
316 Inst->eraseFromParent();
322 // If the instruction can be simplified (e.g. X+0 = X) then replace it with
323 // its simpler value.
324 if (Value *V = SimplifyInstruction(Inst, TD, DT)) {
325 DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n');
326 Inst->replaceAllUsesWith(V);
327 Inst->eraseFromParent();
333 // If this is a simple instruction that we can value number, process it.
334 if (SimpleValue::canHandle(Inst)) {
335 // See if the instruction has an available value. If so, use it.
336 if (Value *V = AvailableValues->lookup(Inst)) {
337 DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n');
338 Inst->replaceAllUsesWith(V);
339 Inst->eraseFromParent();
345 // Otherwise, just remember that this value is available.
346 AvailableValues->insert(Inst, Inst);
350 // If this is a non-volatile load, process it.
351 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
352 // Ignore volatile loads.
353 if (LI->isVolatile()) {
358 // If we have an available version of this load, and if it is the right
359 // generation, replace this instruction.
360 std::pair<Value*, unsigned> InVal =
361 AvailableLoads->lookup(Inst->getOperand(0));
362 if (InVal.first != 0 && InVal.second == CurrentGeneration) {
363 DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << " to: "
364 << *InVal.first << '\n');
365 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first);
366 Inst->eraseFromParent();
372 // Otherwise, remember that we have this instruction.
373 AvailableLoads->insert(Inst->getOperand(0),
374 std::pair<Value*, unsigned>(Inst, CurrentGeneration));
379 // If this instruction may read from memory, forget LastStore.
380 if (Inst->mayReadFromMemory())
383 // If this is a read-only call, process it.
384 if (CallValue::canHandle(Inst)) {
385 // If we have an available version of this call, and if it is the right
386 // generation, replace this instruction.
387 std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst);
388 if (InVal.first != 0 && InVal.second == CurrentGeneration) {
389 DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << " to: "
390 << *InVal.first << '\n');
391 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first);
392 Inst->eraseFromParent();
398 // Otherwise, remember that we have this instruction.
399 AvailableCalls->insert(Inst,
400 std::pair<Value*, unsigned>(Inst, CurrentGeneration));
404 // Okay, this isn't something we can CSE at all. Check to see if it is
405 // something that could modify memory. If so, our available memory values
406 // cannot be used so bump the generation count.
407 if (Inst->mayWriteToMemory()) {
410 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
411 // We do a trivial form of DSE if there are two stores to the same
412 // location with no intervening loads. Delete the earlier store.
414 LastStore->getPointerOperand() == SI->getPointerOperand()) {
415 DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << " due to: "
417 LastStore->eraseFromParent();
424 // Okay, we just invalidated anything we knew about loaded values. Try
425 // to salvage *something* by remembering that the stored value is a live
426 // version of the pointer. It is safe to forward from volatile stores
427 // to non-volatile loads, so we don't have to check for volatility of
429 AvailableLoads->insert(SI->getPointerOperand(),
430 std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration));
432 // Remember that this was the last store we saw for DSE.
433 if (!SI->isVolatile())
439 unsigned LiveOutGeneration = CurrentGeneration;
440 for (DomTreeNode::iterator I = Node->begin(), E = Node->end(); I != E; ++I) {
441 Changed |= processNode(*I);
442 // Pop any generation changes off the stack from the recursive walk.
443 CurrentGeneration = LiveOutGeneration;
449 bool EarlyCSE::runOnFunction(Function &F) {
450 TD = getAnalysisIfAvailable<TargetData>();
451 DT = &getAnalysis<DominatorTree>();
453 // Tables that the pass uses when walking the domtree.
454 ScopedHTType AVTable;
455 AvailableValues = &AVTable;
456 LoadHTType LoadTable;
457 AvailableLoads = &LoadTable;
458 CallHTType CallTable;
459 AvailableCalls = &CallTable;
461 CurrentGeneration = 0;
462 return processNode(DT->getRootNode());