1 //===-- GCSE.cpp - SSA based Global Common Subexpr Elimination ------------===//
3 // This pass is designed to be a very quick global transformation that
4 // eliminates global common subexpressions from a function. It does this by
5 // examining the SSA value graph of the function, instead of doing slow, dense,
6 // bit-vector computations.
8 // This pass works best if it is proceeded with a simple constant propogation
9 // pass and an instruction combination pass because this pass does not do any
10 // value numbering (in order to be speedy).
12 // This pass does not attempt to CSE load instructions, because it does not use
13 // pointer analysis to determine when it is safe.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/InstrTypes.h"
19 #include "llvm/iMemory.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Support/InstVisitor.h"
22 #include "llvm/Support/InstIterator.h"
23 #include "llvm/Support/CFG.h"
24 #include "Support/StatisticReporter.h"
30 static Statistic<> NumInstRemoved("gcse\t\t- Number of instructions removed");
31 static Statistic<> NumLoadRemoved("gcse\t\t- Number of loads removed");
34 class GCSE : public FunctionPass, public InstVisitor<GCSE, bool> {
35 set<Instruction*> WorkList;
36 DominatorSet *DomSetInfo;
37 ImmediateDominators *ImmDominator;
39 // BBContainsStore - Contains a value that indicates whether a basic block
40 // has a store or call instruction in it. This map is demand populated, so
41 // not having an entry means that the basic block has not been scanned yet.
43 map<BasicBlock*, bool> BBContainsStore;
45 virtual bool runOnFunction(Function &F);
47 // Visitation methods, these are invoked depending on the type of
48 // instruction being checked. They should return true if a common
49 // subexpression was folded.
51 bool visitBinaryOperator(Instruction &I);
52 bool visitGetElementPtrInst(GetElementPtrInst &I);
53 bool visitCastInst(CastInst &I);
54 bool visitShiftInst(ShiftInst &I) {
55 return visitBinaryOperator((Instruction&)I);
57 bool visitLoadInst(LoadInst &LI);
58 bool visitInstruction(Instruction &) { return false; }
61 void ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI);
62 void CommonSubExpressionFound(Instruction *I, Instruction *Other);
64 // TryToRemoveALoad - Try to remove one of L1 or L2. The problem with
65 // removing loads is that intervening stores might make otherwise identical
66 // load's yield different values. To ensure that this is not the case, we
67 // check that there are no intervening stores or calls between the
70 bool TryToRemoveALoad(LoadInst *L1, LoadInst *L2);
72 // CheckForInvalidatingInst - Return true if BB or any of the predecessors
73 // of BB (until DestBB) contain a store (or other invalidating) instruction.
75 bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
76 set<BasicBlock*> &VisitedSet);
78 // This transformation requires dominator and immediate dominator info
79 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81 AU.addRequired<DominatorSet>();
82 AU.addRequired<ImmediateDominators>();
86 RegisterOpt<GCSE> X("gcse", "Global Common Subexpression Elimination");
89 // createGCSEPass - The public interface to this file...
90 Pass *createGCSEPass() { return new GCSE(); }
93 // GCSE::runOnFunction - This is the main transformation entry point for a
96 bool GCSE::runOnFunction(Function &F) {
99 DomSetInfo = &getAnalysis<DominatorSet>();
100 ImmDominator = &getAnalysis<ImmediateDominators>();
102 // Step #1: Add all instructions in the function to the worklist for
103 // processing. All of the instructions are considered to be our
104 // subexpressions to eliminate if possible.
106 WorkList.insert(inst_begin(F), inst_end(F));
108 // Step #2: WorkList processing. Iterate through all of the instructions,
109 // checking to see if there are any additionally defined subexpressions in the
110 // program. If so, eliminate them!
112 while (!WorkList.empty()) {
113 Instruction &I = **WorkList.begin(); // Get an instruction from the worklist
114 WorkList.erase(WorkList.begin());
116 // Visit the instruction, dispatching to the correct visit function based on
117 // the instruction type. This does the checking.
122 // Clear out data structure so that next function starts fresh
123 BBContainsStore.clear();
125 // When the worklist is empty, return whether or not we changed anything...
130 // ReplaceInstWithInst - Destroy the instruction pointed to by SI, making all
131 // uses of the instruction use First now instead.
133 void GCSE::ReplaceInstWithInst(Instruction *First, BasicBlock::iterator SI) {
134 Instruction &Second = *SI;
136 //cerr << "DEL " << (void*)Second << Second;
138 // Add the first instruction back to the worklist
139 WorkList.insert(First);
141 // Add all uses of the second instruction to the worklist
142 for (Value::use_iterator UI = Second.use_begin(), UE = Second.use_end();
144 WorkList.insert(cast<Instruction>(*UI));
146 // Make all users of 'Second' now use 'First'
147 Second.replaceAllUsesWith(First);
149 // Erase the second instruction from the program
150 Second.getParent()->getInstList().erase(SI);
153 // CommonSubExpressionFound - The two instruction I & Other have been found to
154 // be common subexpressions. This function is responsible for eliminating one
155 // of them, and for fixing the worklist to be correct.
157 void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
161 WorkList.erase(Other); // Other may not actually be on the worklist anymore...
163 ++NumInstRemoved; // Keep track of number of instructions eliminated
165 // Handle the easy case, where both instructions are in the same basic block
166 BasicBlock *BB1 = I->getParent(), *BB2 = Other->getParent();
168 // Eliminate the second occuring instruction. Add all uses of the second
169 // instruction to the worklist.
171 // Scan the basic block looking for the "first" instruction
172 BasicBlock::iterator BI = BB1->begin();
173 while (&*BI != I && &*BI != Other) {
175 assert(BI != BB1->end() && "Instructions not found in parent BB!");
178 // Keep track of which instructions occurred first & second
179 Instruction *First = BI;
180 Instruction *Second = I != First ? I : Other; // Get iterator to second inst
183 // Destroy Second, using First instead.
184 ReplaceInstWithInst(First, BI);
186 // Otherwise, the two instructions are in different basic blocks. If one
187 // dominates the other instruction, we can simply use it
189 } else if (DomSetInfo->dominates(BB1, BB2)) { // I dom Other?
190 ReplaceInstWithInst(I, Other);
191 } else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
192 ReplaceInstWithInst(Other, I);
194 // This code is disabled because it has several problems:
195 // One, the actual assumption is wrong, as shown by this code:
196 // int "test"(int %X, int %Y) {
197 // %Z = add int %X, %Y
200 // %Q = add int %X, %Y
204 // Here there are no shared dominators. Additionally, this had the habit of
205 // moving computations where they were not always computed. For example, in
214 // In thiscase, the expression would be hoisted to outside the 'if' stmt,
215 // causing the expression to be evaluated, even for the if (d) path, which
216 // could cause problems, if, for example, it caused a divide by zero. In
217 // general the problem this case is trying to solve is better addressed with
222 // Handle the most general case now. In this case, neither I dom Other nor
223 // Other dom I. Because we are in SSA form, we are guaranteed that the
224 // operands of the two instructions both dominate the uses, so we _know_
225 // that there must exist a block that dominates both instructions (if the
226 // operands of the instructions are globals or constants, worst case we
227 // would get the entry node of the function). Search for this block now.
230 // Search up the immediate dominator chain of BB1 for the shared dominator
231 BasicBlock *SharedDom = (*ImmDominator)[BB1];
232 while (!DomSetInfo->dominates(SharedDom, BB2))
233 SharedDom = (*ImmDominator)[SharedDom];
235 // At this point, shared dom must dominate BOTH BB1 and BB2...
236 assert(SharedDom && DomSetInfo->dominates(SharedDom, BB1) &&
237 DomSetInfo->dominates(SharedDom, BB2) && "Dominators broken!");
239 // Rip 'I' out of BB1, and move it to the end of SharedDom.
240 BB1->getInstList().remove(I);
241 SharedDom->getInstList().insert(--SharedDom->end(), I);
243 // Eliminate 'Other' now.
244 ReplaceInstWithInst(I, Other);
249 //===----------------------------------------------------------------------===//
251 // Visitation methods, these are invoked depending on the type of instruction
252 // being checked. They should return true if a common subexpression was folded.
254 //===----------------------------------------------------------------------===//
256 bool GCSE::visitCastInst(CastInst &CI) {
257 Instruction &I = (Instruction&)CI;
258 Value *Op = I.getOperand(0);
259 Function *F = I.getParent()->getParent();
261 for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
263 if (Instruction *Other = dyn_cast<Instruction>(*UI))
264 // Check to see if this new cast is not I, but has the same operand...
265 if (Other != &I && Other->getOpcode() == I.getOpcode() &&
266 Other->getOperand(0) == Op && // Is the operand the same?
267 // Is it embeded in the same function? (This could be false if LHS
268 // is a constant or global!)
269 Other->getParent()->getParent() == F &&
271 // Check that the types are the same, since this code handles casts...
272 Other->getType() == I.getType()) {
274 // These instructions are identical. Handle the situation.
275 CommonSubExpressionFound(&I, Other);
276 return true; // One instruction eliminated!
282 // isIdenticalBinaryInst - Return true if the two binary instructions are
285 static inline bool isIdenticalBinaryInst(const Instruction &I1,
286 const Instruction *I2) {
287 // Is it embeded in the same function? (This could be false if LHS
288 // is a constant or global!)
289 if (I1.getOpcode() != I2->getOpcode() ||
290 I1.getParent()->getParent() != I2->getParent()->getParent())
293 // They are identical if both operands are the same!
294 if (I1.getOperand(0) == I2->getOperand(0) &&
295 I1.getOperand(1) == I2->getOperand(1))
298 // If the instruction is commutative and associative, the instruction can
299 // match if the operands are swapped!
301 if ((I1.getOperand(0) == I2->getOperand(1) &&
302 I1.getOperand(1) == I2->getOperand(0)) &&
303 (I1.getOpcode() == Instruction::Add ||
304 I1.getOpcode() == Instruction::Mul ||
305 I1.getOpcode() == Instruction::And ||
306 I1.getOpcode() == Instruction::Or ||
307 I1.getOpcode() == Instruction::Xor))
313 bool GCSE::visitBinaryOperator(Instruction &I) {
314 Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
315 Function *F = I.getParent()->getParent();
317 for (Value::use_iterator UI = LHS->use_begin(), UE = LHS->use_end();
319 if (Instruction *Other = dyn_cast<Instruction>(*UI))
320 // Check to see if this new binary operator is not I, but same operand...
321 if (Other != &I && isIdenticalBinaryInst(I, Other)) {
322 // These instructions are identical. Handle the situation.
323 CommonSubExpressionFound(&I, Other);
324 return true; // One instruction eliminated!
330 // IdenticalComplexInst - Return true if the two instructions are the same, by
331 // using a brute force comparison.
333 static bool IdenticalComplexInst(const Instruction *I1, const Instruction *I2) {
334 assert(I1->getOpcode() == I2->getOpcode());
335 // Equal if they are in the same function...
336 return I1->getParent()->getParent() == I2->getParent()->getParent() &&
337 // And return the same type...
338 I1->getType() == I2->getType() &&
339 // And have the same number of operands...
340 I1->getNumOperands() == I2->getNumOperands() &&
341 // And all of the operands are equal.
342 std::equal(I1->op_begin(), I1->op_end(), I2->op_begin());
345 bool GCSE::visitGetElementPtrInst(GetElementPtrInst &I) {
346 Value *Op = I.getOperand(0);
347 Function *F = I.getParent()->getParent();
349 for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
351 if (GetElementPtrInst *Other = dyn_cast<GetElementPtrInst>(*UI))
352 // Check to see if this new getelementptr is not I, but same operand...
353 if (Other != &I && IdenticalComplexInst(&I, Other)) {
354 // These instructions are identical. Handle the situation.
355 CommonSubExpressionFound(&I, Other);
356 return true; // One instruction eliminated!
362 bool GCSE::visitLoadInst(LoadInst &LI) {
363 Value *Op = LI.getOperand(0);
364 Function *F = LI.getParent()->getParent();
366 for (Value::use_iterator UI = Op->use_begin(), UE = Op->use_end();
368 if (LoadInst *Other = dyn_cast<LoadInst>(*UI))
369 // Check to see if this new load is not LI, but has the same operands...
370 if (Other != &LI && IdenticalComplexInst(&LI, Other) &&
371 TryToRemoveALoad(&LI, Other))
372 return true; // An instruction was eliminated!
377 static inline bool isInvalidatingInst(const Instruction &I) {
378 return I.getOpcode() == Instruction::Store ||
379 I.getOpcode() == Instruction::Call ||
380 I.getOpcode() == Instruction::Invoke;
383 // TryToRemoveALoad - Try to remove one of L1 or L2. The problem with removing
384 // loads is that intervening stores might make otherwise identical load's yield
385 // different values. To ensure that this is not the case, we check that there
386 // are no intervening stores or calls between the instructions.
388 bool GCSE::TryToRemoveALoad(LoadInst *L1, LoadInst *L2) {
389 // Figure out which load dominates the other one. If neither dominates the
390 // other we cannot eliminate one...
392 if (DomSetInfo->dominates(L2, L1))
393 std::swap(L1, L2); // Make L1 dominate L2
394 else if (!DomSetInfo->dominates(L1, L2))
395 return false; // Neither instruction dominates the other one...
397 BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
399 BasicBlock::iterator L1I = L1;
401 // L1 now dominates L2. Check to see if the intervening instructions between
402 // the two loads include a store or call...
404 if (BB1 == BB2) { // In same basic block?
405 // In this degenerate case, no checking of global basic blocks has to occur
406 // just check the instructions BETWEEN L1 & L2...
408 for (++L1I; &*L1I != L2; ++L1I)
409 if (isInvalidatingInst(*L1I))
410 return false; // Cannot eliminate load
413 CommonSubExpressionFound(L1, L2);
416 // Make sure that there are no store instructions between L1 and the end of
417 // it's basic block...
419 for (++L1I; L1I != BB1->end(); ++L1I)
420 if (isInvalidatingInst(*L1I)) {
421 BBContainsStore[BB1] = true;
422 return false; // Cannot eliminate load
425 // Make sure that there are no store instructions between the start of BB2
426 // and the second load instruction...
428 for (BasicBlock::iterator II = BB2->begin(); &*II != L2; ++II)
429 if (isInvalidatingInst(*II)) {
430 BBContainsStore[BB2] = true;
431 return false; // Cannot eliminate load
434 // Do a depth first traversal of the inverse CFG starting at L2's block,
435 // looking for L1's block. The inverse CFG is made up of the predecessor
436 // nodes of a block... so all of the edges in the graph are "backward".
438 set<BasicBlock*> VisitedSet;
439 for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
440 if (CheckForInvalidatingInst(*PI, BB1, VisitedSet))
444 CommonSubExpressionFound(L1, L2);
450 // CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
451 // (until DestBB) contain a store (or other invalidating) instruction.
453 bool GCSE::CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
454 set<BasicBlock*> &VisitedSet) {
455 // Found the termination point!
456 if (BB == DestBB || VisitedSet.count(BB)) return false;
458 // Avoid infinite recursion!
459 VisitedSet.insert(BB);
461 // Have we already checked this block?
462 map<BasicBlock*, bool>::iterator MI = BBContainsStore.find(BB);
464 if (MI != BBContainsStore.end()) {
465 // If this block is known to contain a store, exit the recursion early...
466 if (MI->second) return true;
467 // Otherwise continue checking predecessors...
469 // We don't know if this basic block contains an invalidating instruction.
471 bool HasStore = std::find_if(BB->begin(), BB->end(),
472 isInvalidatingInst) != BB->end();
473 if ((BBContainsStore[BB] = HasStore)) // Update map
474 return true; // Exit recursion early...
477 // Check all of our predecessor blocks...
478 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
479 if (CheckForInvalidatingInst(*PI, DestBB, VisitedSet))
482 // None of our predecessor blocks contain a store, and we don't either!