1 //===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a value numbering pass that value numbers load and call
11 // instructions. To do this, it finds lexically identical load instructions,
12 // and uses alias analysis to determine which loads are guaranteed to produce
13 // the same value. To value number call instructions, it looks for calls to
14 // functions that do not write to memory which do not have intervening
15 // instructions that clobber the memory that is read from.
17 // This pass builds off of another value numbering pass to implement value
18 // numbering for non-load and non-call instructions. It uses Alias Analysis so
19 // that it can disambiguate the load instructions. The more powerful these base
20 // analyses are, the more powerful the resultant value numbering will be.
22 //===----------------------------------------------------------------------===//
24 #include "llvm/Analysis/LoadValueNumbering.h"
25 #include "llvm/Constants.h"
26 #include "llvm/Function.h"
27 #include "llvm/Instructions.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Type.h"
30 #include "llvm/Analysis/ValueNumbering.h"
31 #include "llvm/Analysis/AliasAnalysis.h"
32 #include "llvm/Analysis/Dominators.h"
33 #include "llvm/Support/CFG.h"
34 #include "llvm/Support/Compiler.h"
35 #include "llvm/Target/TargetData.h"
41 // FIXME: This should not be a FunctionPass.
42 struct VISIBILITY_HIDDEN LoadVN : public FunctionPass, public ValueNumbering {
43 static char ID; // Class identification, replacement for typeinfo
44 LoadVN() : FunctionPass((intptr_t)&ID) {}
46 /// Pass Implementation stuff. This doesn't do any analysis.
48 bool runOnFunction(Function &) { return false; }
50 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering
51 /// and Alias Analysis.
53 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
55 /// getEqualNumberNodes - Return nodes with the same value number as the
56 /// specified Value. This fills in the argument vector with any equal
59 virtual void getEqualNumberNodes(Value *V1,
60 std::vector<Value*> &RetVals) const;
62 /// deleteValue - This method should be called whenever an LLVM Value is
63 /// deleted from the program, for example when an instruction is found to be
64 /// redundant and is eliminated.
66 virtual void deleteValue(Value *V) {
67 getAnalysis<AliasAnalysis>().deleteValue(V);
70 /// copyValue - This method should be used whenever a preexisting value in
71 /// the program is copied or cloned, introducing a new value. Note that
72 /// analysis implementations should tolerate clients that use this method to
73 /// introduce the same value multiple times: if the analysis already knows
74 /// about a value, it should ignore the request.
76 virtual void copyValue(Value *From, Value *To) {
77 getAnalysis<AliasAnalysis>().copyValue(From, To);
80 /// getCallEqualNumberNodes - Given a call instruction, find other calls
81 /// that have the same value number.
82 void getCallEqualNumberNodes(CallInst *CI,
83 std::vector<Value*> &RetVals) const;
87 // Register this pass...
88 RegisterPass<LoadVN> X("load-vn", "Load Value Numbering");
90 // Declare that we implement the ValueNumbering interface
91 RegisterAnalysisGroup<ValueNumbering> Y(X);
94 FunctionPass *llvm::createLoadValueNumberingPass() { return new LoadVN(); }
97 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
100 void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
101 AU.setPreservesAll();
102 AU.addRequiredTransitive<AliasAnalysis>();
103 AU.addRequired<ValueNumbering>();
104 AU.addRequiredTransitive<DominatorTree>();
105 AU.addRequiredTransitive<TargetData>();
108 static bool isPathTransparentTo(BasicBlock *CurBlock, BasicBlock *Dom,
109 Value *Ptr, unsigned Size, AliasAnalysis &AA,
110 std::set<BasicBlock*> &Visited,
111 std::map<BasicBlock*, bool> &TransparentBlocks){
112 // If we have already checked out this path, or if we reached our destination,
113 // stop searching, returning success.
114 if (CurBlock == Dom || !Visited.insert(CurBlock).second)
117 // Check whether this block is known transparent or not.
118 std::map<BasicBlock*, bool>::iterator TBI =
119 TransparentBlocks.lower_bound(CurBlock);
121 if (TBI == TransparentBlocks.end() || TBI->first != CurBlock) {
122 // If this basic block can modify the memory location, then the path is not
124 if (AA.canBasicBlockModify(*CurBlock, Ptr, Size)) {
125 TransparentBlocks.insert(TBI, std::make_pair(CurBlock, false));
128 TransparentBlocks.insert(TBI, std::make_pair(CurBlock, true));
129 } else if (!TBI->second)
130 // This block is known non-transparent, so that path can't be either.
133 // The current block is known to be transparent. The entire path is
134 // transparent if all of the predecessors paths to the parent is also
135 // transparent to the memory location.
136 for (pred_iterator PI = pred_begin(CurBlock), E = pred_end(CurBlock);
138 if (!isPathTransparentTo(*PI, Dom, Ptr, Size, AA, Visited,
144 /// getCallEqualNumberNodes - Given a call instruction, find other calls that
145 /// have the same value number.
146 void LoadVN::getCallEqualNumberNodes(CallInst *CI,
147 std::vector<Value*> &RetVals) const {
148 Function *CF = CI->getCalledFunction();
149 if (CF == 0) return; // Indirect call.
150 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
151 AliasAnalysis::ModRefBehavior MRB = AA.getModRefBehavior(CF, CI);
152 if (MRB != AliasAnalysis::DoesNotAccessMemory &&
153 MRB != AliasAnalysis::OnlyReadsMemory)
154 return; // Nothing we can do for now.
156 // Scan all of the arguments of the function, looking for one that is not
157 // global. In particular, we would prefer to have an argument or instruction
158 // operand to chase the def-use chains of.
160 for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
161 if (isa<Argument>(CI->getOperand(i)) ||
162 isa<Instruction>(CI->getOperand(i))) {
163 Op = CI->getOperand(i);
167 // Identify all lexically identical calls in this function.
168 std::vector<CallInst*> IdenticalCalls;
170 Function *CIFunc = CI->getParent()->getParent();
171 for (Value::use_iterator UI = Op->use_begin(), E = Op->use_end(); UI != E;
173 if (CallInst *C = dyn_cast<CallInst>(*UI))
174 if (C->getNumOperands() == CI->getNumOperands() &&
175 C->getOperand(0) == CI->getOperand(0) &&
176 C->getParent()->getParent() == CIFunc && C != CI) {
177 bool AllOperandsEqual = true;
178 for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
179 if (C->getOperand(i) != CI->getOperand(i)) {
180 AllOperandsEqual = false;
184 if (AllOperandsEqual)
185 IdenticalCalls.push_back(C);
188 if (IdenticalCalls.empty()) return;
190 // Eliminate duplicates, which could occur if we chose a value that is passed
191 // into a call site multiple times.
192 std::sort(IdenticalCalls.begin(), IdenticalCalls.end());
193 IdenticalCalls.erase(std::unique(IdenticalCalls.begin(),IdenticalCalls.end()),
194 IdenticalCalls.end());
196 // If the call reads memory, we must make sure that there are no stores
197 // between the calls in question.
199 // FIXME: This should use mod/ref information. What we really care about it
200 // whether an intervening instruction could modify memory that is read, not
203 if (MRB == AliasAnalysis::OnlyReadsMemory) {
204 DominatorTree &DT = getAnalysis<DominatorTree>();
205 BasicBlock *CIBB = CI->getParent();
206 for (unsigned i = 0; i != IdenticalCalls.size(); ++i) {
207 CallInst *C = IdenticalCalls[i];
208 bool CantEqual = false;
210 if (DT.dominates(CIBB, C->getParent())) {
211 // FIXME: we currently only handle the case where both calls are in the
213 if (CIBB != C->getParent()) {
216 Instruction *First = CI, *Second = C;
217 if (!DT.dominates(CI, C))
218 std::swap(First, Second);
220 // Scan the instructions between the calls, checking for stores or
221 // calls to dangerous functions.
222 BasicBlock::iterator I = First;
223 for (++First; I != BasicBlock::iterator(Second); ++I) {
224 if (isa<StoreInst>(I)) {
225 // FIXME: We could use mod/ref information to make this much
229 } else if (CallInst *CI = dyn_cast<CallInst>(I)) {
230 if (CI->getCalledFunction() == 0 ||
231 !AA.onlyReadsMemory(CI->getCalledFunction())) {
235 } else if (I->mayWriteToMemory()) {
242 } else if (DT.dominates(C->getParent(), CIBB)) {
243 // FIXME: We could implement this, but we don't for now.
246 // FIXME: if one doesn't dominate the other, we can't tell yet.
252 // This call does not produce the same value as the one in the query.
253 std::swap(IdenticalCalls[i--], IdenticalCalls.back());
254 IdenticalCalls.pop_back();
259 // Any calls that are identical and not destroyed will produce equal values!
260 for (unsigned i = 0, e = IdenticalCalls.size(); i != e; ++i)
261 RetVals.push_back(IdenticalCalls[i]);
264 // getEqualNumberNodes - Return nodes with the same value number as the
265 // specified Value. This fills in the argument vector with any equal values.
267 void LoadVN::getEqualNumberNodes(Value *V,
268 std::vector<Value*> &RetVals) const {
269 // If the alias analysis has any must alias information to share with us, we
270 // can definitely use it.
271 if (isa<PointerType>(V->getType()))
272 getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals);
274 if (!isa<LoadInst>(V)) {
275 if (CallInst *CI = dyn_cast<CallInst>(V))
276 getCallEqualNumberNodes(CI, RetVals);
278 // Not a load instruction? Just chain to the base value numbering
279 // implementation to satisfy the request...
280 assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
281 "getAnalysis() returned this!");
283 return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
286 // Volatile loads cannot be replaced with the value of other loads.
287 LoadInst *LI = cast<LoadInst>(V);
288 if (LI->isVolatile())
289 return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
291 Value *LoadPtr = LI->getOperand(0);
292 BasicBlock *LoadBB = LI->getParent();
293 Function *F = LoadBB->getParent();
295 // Find out how many bytes of memory are loaded by the load instruction...
296 unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LI->getType());
297 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
299 // Figure out if the load is invalidated from the entry of the block it is in
300 // until the actual instruction. This scans the block backwards from LI. If
301 // we see any candidate load or store instructions, then we know that the
302 // candidates have the same value # as LI.
303 bool LoadInvalidatedInBBBefore = false;
304 for (BasicBlock::iterator I = LI; I != LoadBB->begin(); ) {
307 // If we run into an allocation of the value being loaded, then the
308 // contents are not initialized.
309 if (isa<AllocationInst>(I))
310 RetVals.push_back(UndefValue::get(LI->getType()));
312 // Otherwise, since this is the definition of what we are loading, this
313 // loaded value cannot occur before this block.
314 LoadInvalidatedInBBBefore = true;
316 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
317 // If this instruction is a candidate load before LI, we know there are no
318 // invalidating instructions between it and LI, so they have the same
320 if (LI->getOperand(0) == LoadPtr && !LI->isVolatile())
321 RetVals.push_back(I);
324 if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
325 // If the invalidating instruction is a store, and its in our candidate
326 // set, then we can do store-load forwarding: the load has the same value
327 // # as the stored value.
328 if (StoreInst *SI = dyn_cast<StoreInst>(I))
329 if (SI->getOperand(1) == LoadPtr)
330 RetVals.push_back(I->getOperand(0));
332 LoadInvalidatedInBBBefore = true;
337 // Figure out if the load is invalidated between the load and the exit of the
338 // block it is defined in. While we are scanning the current basic block, if
339 // we see any candidate loads, then we know they have the same value # as LI.
341 bool LoadInvalidatedInBBAfter = false;
343 BasicBlock::iterator I = LI;
344 for (++I; I != LoadBB->end(); ++I) {
345 // If this instruction is a load, then this instruction returns the same
347 if (isa<LoadInst>(I) && cast<LoadInst>(I)->getOperand(0) == LoadPtr)
348 RetVals.push_back(I);
350 if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
351 LoadInvalidatedInBBAfter = true;
357 // If the pointer is clobbered on entry and on exit to the function, there is
358 // no need to do any global analysis at all.
359 if (LoadInvalidatedInBBBefore && LoadInvalidatedInBBAfter)
362 // Now that we know the value is not neccesarily killed on entry or exit to
363 // the BB, find out how many load and store instructions (to this location)
364 // live in each BB in the function.
366 std::map<BasicBlock*, unsigned> CandidateLoads;
367 std::set<BasicBlock*> CandidateStores;
369 for (Value::use_iterator UI = LoadPtr->use_begin(), UE = LoadPtr->use_end();
371 if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source?
372 if (Cand->getParent()->getParent() == F && // In the same function?
373 // Not in LI's block?
374 Cand->getParent() != LoadBB && !Cand->isVolatile())
375 ++CandidateLoads[Cand->getParent()]; // Got one.
376 } else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
377 if (Cand->getParent()->getParent() == F && !Cand->isVolatile() &&
378 Cand->getOperand(1) == LoadPtr) // It's a store THROUGH the ptr.
379 CandidateStores.insert(Cand->getParent());
383 DominatorTree &DT = getAnalysis<DominatorTree>();
385 // TransparentBlocks - For each basic block the load/store is alive across,
386 // figure out if the pointer is invalidated or not. If it is invalidated, the
387 // boolean is set to false, if it's not it is set to true. If we don't know
388 // yet, the entry is not in the map.
389 std::map<BasicBlock*, bool> TransparentBlocks;
391 // Loop over all of the basic blocks that also load the value. If the value
392 // is live across the CFG from the source to destination blocks, and if the
393 // value is not invalidated in either the source or destination blocks, add it
394 // to the equivalence sets.
395 for (std::map<BasicBlock*, unsigned>::iterator
396 I = CandidateLoads.begin(), E = CandidateLoads.end(); I != E; ++I) {
397 bool CantEqual = false;
399 // Right now we only can handle cases where one load dominates the other.
400 // FIXME: generalize this!
401 BasicBlock *BB1 = I->first, *BB2 = LoadBB;
402 if (DT.dominates(BB1, BB2)) {
403 // The other load dominates LI. If the loaded value is killed entering
404 // the LoadBB block, we know the load is not live.
405 if (LoadInvalidatedInBBBefore)
407 } else if (DT.dominates(BB2, BB1)) {
408 std::swap(BB1, BB2); // Canonicalize
409 // LI dominates the other load. If the loaded value is killed exiting
410 // the LoadBB block, we know the load is not live.
411 if (LoadInvalidatedInBBAfter)
414 // None of these loads can VN the same.
419 // Ok, at this point, we know that BB1 dominates BB2, and that there is
420 // nothing in the LI block that kills the loaded value. Check to see if
421 // the value is live across the CFG.
422 std::set<BasicBlock*> Visited;
423 for (pred_iterator PI = pred_begin(BB2), E = pred_end(BB2); PI!=E; ++PI)
424 if (!isPathTransparentTo(*PI, BB1, LoadPtr, LoadSize, AA,
425 Visited, TransparentBlocks)) {
426 // None of these loads can VN the same.
432 // If the loads can equal so far, scan the basic block that contains the
433 // loads under consideration to see if they are invalidated in the block.
434 // For any loads that are not invalidated, add them to the equivalence
437 unsigned NumLoads = I->second;
439 // If LI dominates the block in question, check to see if any of the
440 // loads in this block are invalidated before they are reached.
441 for (BasicBlock::iterator BBI = I->first->begin(); ; ++BBI) {
442 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
443 if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) {
444 // The load is in the set!
445 RetVals.push_back(BBI);
446 if (--NumLoads == 0) break; // Found last load to check.
448 } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
449 & AliasAnalysis::Mod) {
450 // If there is a modifying instruction, nothing below it will value
456 // If the block dominates LI, make sure that the loads in the block are
457 // not invalidated before the block ends.
458 BasicBlock::iterator BBI = I->first->end();
461 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
462 if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) {
463 // The load is the same as this load!
464 RetVals.push_back(BBI);
465 if (--NumLoads == 0) break; // Found all of the laods.
467 } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
468 & AliasAnalysis::Mod) {
469 // If there is a modifying instruction, nothing above it will value
478 // Handle candidate stores. If the loaded location is clobbered on entrance
479 // to the LoadBB, no store outside of the LoadBB can value number equal, so
481 if (LoadInvalidatedInBBBefore)
484 // Stores in the load-bb are handled above.
485 CandidateStores.erase(LoadBB);
487 for (std::set<BasicBlock*>::iterator I = CandidateStores.begin(),
488 E = CandidateStores.end(); I != E; ++I)
489 if (DT.dominates(*I, LoadBB)) {
490 BasicBlock *StoreBB = *I;
492 // Check to see if the path from the store to the load is transparent
493 // w.r.t. the memory location.
494 bool CantEqual = false;
495 std::set<BasicBlock*> Visited;
496 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
498 if (!isPathTransparentTo(*PI, StoreBB, LoadPtr, LoadSize, AA,
499 Visited, TransparentBlocks)) {
500 // None of these stores can VN the same.
506 // Okay, the path from the store block to the load block is clear, and
507 // we know that there are no invalidating instructions from the start
508 // of the load block to the load itself. Now we just scan the store
511 BasicBlock::iterator BBI = StoreBB->end();
513 assert(BBI != StoreBB->begin() &&
514 "There is a store in this block of the pointer, but the store"
515 " doesn't mod the address being stored to?? Must be a bug in"
516 " the alias analysis implementation!");
518 if (AA.getModRefInfo(BBI, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
519 // If the invalidating instruction is one of the candidates,
520 // then it provides the value the load loads.
521 if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
522 if (SI->getOperand(1) == LoadPtr)
523 RetVals.push_back(SI->getOperand(0));