| //===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements a value numbering pass that value numbers load and call |
| // instructions. To do this, it finds lexically identical load instructions, |
| // and uses alias analysis to determine which loads are guaranteed to produce |
| // the same value. To value number call instructions, it looks for calls to |
| // functions that do not write to memory which do not have intervening |
| // instructions that clobber the memory that is read from. |
| // |
| // This pass builds off of another value numbering pass to implement value |
| // numbering for non-load and non-call instructions. It uses Alias Analysis so |
| // that it can disambiguate the load instructions. The more powerful these base |
| // analyses are, the more powerful the resultant value numbering will be. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Analysis/LoadValueNumbering.h" |
| #include "llvm/Constants.h" |
| #include "llvm/Function.h" |
| #include "llvm/Instructions.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Type.h" |
| #include "llvm/Analysis/ValueNumbering.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/Dominators.h" |
| #include "llvm/Support/CFG.h" |
| #include "llvm/Support/Compiler.h" |
| #include "llvm/Target/TargetData.h" |
| #include <set> |
| #include <algorithm> |
| using namespace llvm; |
| |
| namespace { |
| // FIXME: This should not be a FunctionPass. |
| struct VISIBILITY_HIDDEN LoadVN : public FunctionPass, public ValueNumbering { |
| static char ID; // Class identification, replacement for typeinfo |
| LoadVN() : FunctionPass((intptr_t)&ID) {} |
| |
| /// Pass Implementation stuff. This doesn't do any analysis. |
| /// |
| bool runOnFunction(Function &) { return false; } |
| |
| /// getAnalysisUsage - Does not modify anything. It uses Value Numbering |
| /// and Alias Analysis. |
| /// |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const; |
| |
| /// getEqualNumberNodes - Return nodes with the same value number as the |
| /// specified Value. This fills in the argument vector with any equal |
| /// values. |
| /// |
| virtual void getEqualNumberNodes(Value *V1, |
| std::vector<Value*> &RetVals) const; |
| |
| /// deleteValue - This method should be called whenever an LLVM Value is |
| /// deleted from the program, for example when an instruction is found to be |
| /// redundant and is eliminated. |
| /// |
| virtual void deleteValue(Value *V) { |
| getAnalysis<AliasAnalysis>().deleteValue(V); |
| } |
| |
| /// copyValue - This method should be used whenever a preexisting value in |
| /// the program is copied or cloned, introducing a new value. Note that |
| /// analysis implementations should tolerate clients that use this method to |
| /// introduce the same value multiple times: if the analysis already knows |
| /// about a value, it should ignore the request. |
| /// |
| virtual void copyValue(Value *From, Value *To) { |
| getAnalysis<AliasAnalysis>().copyValue(From, To); |
| } |
| |
| /// getCallEqualNumberNodes - Given a call instruction, find other calls |
| /// that have the same value number. |
| void getCallEqualNumberNodes(CallInst *CI, |
| std::vector<Value*> &RetVals) const; |
| }; |
| |
| char LoadVN::ID = 0; |
| // Register this pass... |
| RegisterPass<LoadVN> X("load-vn", "Load Value Numbering", false, true); |
| |
| // Declare that we implement the ValueNumbering interface |
| RegisterAnalysisGroup<ValueNumbering> Y(X); |
| } |
| |
| FunctionPass *llvm::createLoadValueNumberingPass() { return new LoadVN(); } |
| |
| |
| /// getAnalysisUsage - Does not modify anything. It uses Value Numbering and |
| /// Alias Analysis. |
| /// |
| void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.setPreservesAll(); |
| AU.addRequiredTransitive<AliasAnalysis>(); |
| AU.addRequired<ValueNumbering>(); |
| AU.addRequiredTransitive<DominatorTree>(); |
| AU.addRequiredTransitive<TargetData>(); |
| } |
| |
| static bool isPathTransparentTo(BasicBlock *CurBlock, BasicBlock *Dom, |
| Value *Ptr, unsigned Size, AliasAnalysis &AA, |
| std::set<BasicBlock*> &Visited, |
| std::map<BasicBlock*, bool> &TransparentBlocks){ |
| // If we have already checked out this path, or if we reached our destination, |
| // stop searching, returning success. |
| if (CurBlock == Dom || !Visited.insert(CurBlock).second) |
| return true; |
| |
| // Check whether this block is known transparent or not. |
| std::map<BasicBlock*, bool>::iterator TBI = |
| TransparentBlocks.lower_bound(CurBlock); |
| |
| if (TBI == TransparentBlocks.end() || TBI->first != CurBlock) { |
| // If this basic block can modify the memory location, then the path is not |
| // transparent! |
| if (AA.canBasicBlockModify(*CurBlock, Ptr, Size)) { |
| TransparentBlocks.insert(TBI, std::make_pair(CurBlock, false)); |
| return false; |
| } |
| TransparentBlocks.insert(TBI, std::make_pair(CurBlock, true)); |
| } else if (!TBI->second) |
| // This block is known non-transparent, so that path can't be either. |
| return false; |
| |
| // The current block is known to be transparent. The entire path is |
| // transparent if all of the predecessors paths to the parent is also |
| // transparent to the memory location. |
| for (pred_iterator PI = pred_begin(CurBlock), E = pred_end(CurBlock); |
| PI != E; ++PI) |
| if (!isPathTransparentTo(*PI, Dom, Ptr, Size, AA, Visited, |
| TransparentBlocks)) |
| return false; |
| return true; |
| } |
| |
| /// getCallEqualNumberNodes - Given a call instruction, find other calls that |
| /// have the same value number. |
| void LoadVN::getCallEqualNumberNodes(CallInst *CI, |
| std::vector<Value*> &RetVals) const { |
| Function *CF = CI->getCalledFunction(); |
| if (CF == 0) return; // Indirect call. |
| AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); |
| AliasAnalysis::ModRefBehavior MRB = AA.getModRefBehavior(CI); |
| if (MRB != AliasAnalysis::DoesNotAccessMemory && |
| MRB != AliasAnalysis::OnlyReadsMemory) |
| return; // Nothing we can do for now. |
| |
| // Scan all of the arguments of the function, looking for one that is not |
| // global. In particular, we would prefer to have an argument or instruction |
| // operand to chase the def-use chains of. |
| Value *Op = CF; |
| for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i) |
| if (isa<Argument>(CI->getOperand(i)) || |
| isa<Instruction>(CI->getOperand(i))) { |
| Op = CI->getOperand(i); |
| break; |
| } |
| |
| // Identify all lexically identical calls in this function. |
| std::vector<CallInst*> IdenticalCalls; |
| |
| Function *CIFunc = CI->getParent()->getParent(); |
| for (Value::use_iterator UI = Op->use_begin(), E = Op->use_end(); UI != E; |
| ++UI) |
| if (CallInst *C = dyn_cast<CallInst>(*UI)) |
| if (C->getNumOperands() == CI->getNumOperands() && |
| C->getOperand(0) == CI->getOperand(0) && |
| C->getParent()->getParent() == CIFunc && C != CI) { |
| bool AllOperandsEqual = true; |
| for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i) |
| if (C->getOperand(i) != CI->getOperand(i)) { |
| AllOperandsEqual = false; |
| break; |
| } |
| |
| if (AllOperandsEqual) |
| IdenticalCalls.push_back(C); |
| } |
| |
| if (IdenticalCalls.empty()) return; |
| |
| // Eliminate duplicates, which could occur if we chose a value that is passed |
| // into a call site multiple times. |
| std::sort(IdenticalCalls.begin(), IdenticalCalls.end()); |
| IdenticalCalls.erase(std::unique(IdenticalCalls.begin(),IdenticalCalls.end()), |
| IdenticalCalls.end()); |
| |
| // If the call reads memory, we must make sure that there are no stores |
| // between the calls in question. |
| // |
| // FIXME: This should use mod/ref information. What we really care about it |
| // whether an intervening instruction could modify memory that is read, not |
| // ANY memory. |
| // |
| if (MRB == AliasAnalysis::OnlyReadsMemory) { |
| DominatorTree &DT = getAnalysis<DominatorTree>(); |
| BasicBlock *CIBB = CI->getParent(); |
| for (unsigned i = 0; i != IdenticalCalls.size(); ++i) { |
| CallInst *C = IdenticalCalls[i]; |
| bool CantEqual = false; |
| |
| if (DT.dominates(CIBB, C->getParent())) { |
| // FIXME: we currently only handle the case where both calls are in the |
| // same basic block. |
| if (CIBB != C->getParent()) { |
| CantEqual = true; |
| } else { |
| Instruction *First = CI, *Second = C; |
| if (!DT.dominates(CI, C)) |
| std::swap(First, Second); |
| |
| // Scan the instructions between the calls, checking for stores or |
| // calls to dangerous functions. |
| BasicBlock::iterator I = First; |
| for (++First; I != BasicBlock::iterator(Second); ++I) { |
| if (isa<StoreInst>(I)) { |
| // FIXME: We could use mod/ref information to make this much |
| // better! |
| CantEqual = true; |
| break; |
| } else if (CallInst *CI = dyn_cast<CallInst>(I)) { |
| if (!AA.onlyReadsMemory(CI)) { |
| CantEqual = true; |
| break; |
| } |
| } else if (I->mayWriteToMemory()) { |
| CantEqual = true; |
| break; |
| } |
| } |
| } |
| |
| } else if (DT.dominates(C->getParent(), CIBB)) { |
| // FIXME: We could implement this, but we don't for now. |
| CantEqual = true; |
| } else { |
| // FIXME: if one doesn't dominate the other, we can't tell yet. |
| CantEqual = true; |
| } |
| |
| |
| if (CantEqual) { |
| // This call does not produce the same value as the one in the query. |
| std::swap(IdenticalCalls[i--], IdenticalCalls.back()); |
| IdenticalCalls.pop_back(); |
| } |
| } |
| } |
| |
| // Any calls that are identical and not destroyed will produce equal values! |
| for (unsigned i = 0, e = IdenticalCalls.size(); i != e; ++i) |
| RetVals.push_back(IdenticalCalls[i]); |
| } |
| |
| // getEqualNumberNodes - Return nodes with the same value number as the |
| // specified Value. This fills in the argument vector with any equal values. |
| // |
| void LoadVN::getEqualNumberNodes(Value *V, |
| std::vector<Value*> &RetVals) const { |
| // If the alias analysis has any must alias information to share with us, we |
| // can definitely use it. |
| if (isa<PointerType>(V->getType())) |
| getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals); |
| |
| if (!isa<LoadInst>(V)) { |
| if (CallInst *CI = dyn_cast<CallInst>(V)) |
| getCallEqualNumberNodes(CI, RetVals); |
| |
| // Not a load instruction? Just chain to the base value numbering |
| // implementation to satisfy the request... |
| assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this && |
| "getAnalysis() returned this!"); |
| |
| return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals); |
| } |
| |
| // Volatile loads cannot be replaced with the value of other loads. |
| LoadInst *LI = cast<LoadInst>(V); |
| if (LI->isVolatile()) |
| return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals); |
| |
| Value *LoadPtr = LI->getOperand(0); |
| BasicBlock *LoadBB = LI->getParent(); |
| Function *F = LoadBB->getParent(); |
| |
| // Find out how many bytes of memory are loaded by the load instruction... |
| unsigned LoadSize = getAnalysis<TargetData>().getTypeStoreSize(LI->getType()); |
| AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); |
| |
| // Figure out if the load is invalidated from the entry of the block it is in |
| // until the actual instruction. This scans the block backwards from LI. If |
| // we see any candidate load or store instructions, then we know that the |
| // candidates have the same value # as LI. |
| bool LoadInvalidatedInBBBefore = false; |
| for (BasicBlock::iterator I = LI; I != LoadBB->begin(); ) { |
| --I; |
| if (I == LoadPtr) { |
| // If we run into an allocation of the value being loaded, then the |
| // contents are not initialized. |
| if (isa<AllocationInst>(I)) |
| RetVals.push_back(UndefValue::get(LI->getType())); |
| |
| // Otherwise, since this is the definition of what we are loading, this |
| // loaded value cannot occur before this block. |
| LoadInvalidatedInBBBefore = true; |
| break; |
| } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { |
| // If this instruction is a candidate load before LI, we know there are no |
| // invalidating instructions between it and LI, so they have the same |
| // value number. |
| if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) |
| RetVals.push_back(I); |
| } |
| |
| if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) { |
| // If the invalidating instruction is a store, and its in our candidate |
| // set, then we can do store-load forwarding: the load has the same value |
| // # as the stored value. |
| if (StoreInst *SI = dyn_cast<StoreInst>(I)) |
| if (SI->getOperand(1) == LoadPtr) |
| RetVals.push_back(I->getOperand(0)); |
| |
| LoadInvalidatedInBBBefore = true; |
| break; |
| } |
| } |
| |
| // Figure out if the load is invalidated between the load and the exit of the |
| // block it is defined in. While we are scanning the current basic block, if |
| // we see any candidate loads, then we know they have the same value # as LI. |
| // |
| bool LoadInvalidatedInBBAfter = false; |
| { |
| BasicBlock::iterator I = LI; |
| for (++I; I != LoadBB->end(); ++I) { |
| // If this instruction is a load, then this instruction returns the same |
| // value as LI. |
| if (isa<LoadInst>(I) && cast<LoadInst>(I)->getOperand(0) == LoadPtr) |
| RetVals.push_back(I); |
| |
| if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) { |
| LoadInvalidatedInBBAfter = true; |
| break; |
| } |
| } |
| } |
| |
| // If the pointer is clobbered on entry and on exit to the function, there is |
| // no need to do any global analysis at all. |
| if (LoadInvalidatedInBBBefore && LoadInvalidatedInBBAfter) |
| return; |
| |
| // Now that we know the value is not neccesarily killed on entry or exit to |
| // the BB, find out how many load and store instructions (to this location) |
| // live in each BB in the function. |
| // |
| std::map<BasicBlock*, unsigned> CandidateLoads; |
| std::set<BasicBlock*> CandidateStores; |
| |
| for (Value::use_iterator UI = LoadPtr->use_begin(), UE = LoadPtr->use_end(); |
| UI != UE; ++UI) |
| if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source? |
| if (Cand->getParent()->getParent() == F && // In the same function? |
| // Not in LI's block? |
| Cand->getParent() != LoadBB && !Cand->isVolatile()) |
| ++CandidateLoads[Cand->getParent()]; // Got one. |
| } else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) { |
| if (Cand->getParent()->getParent() == F && !Cand->isVolatile() && |
| Cand->getOperand(1) == LoadPtr) // It's a store THROUGH the ptr. |
| CandidateStores.insert(Cand->getParent()); |
| } |
| |
| // Get dominators. |
| DominatorTree &DT = getAnalysis<DominatorTree>(); |
| |
| // TransparentBlocks - For each basic block the load/store is alive across, |
| // figure out if the pointer is invalidated or not. If it is invalidated, the |
| // boolean is set to false, if it's not it is set to true. If we don't know |
| // yet, the entry is not in the map. |
| std::map<BasicBlock*, bool> TransparentBlocks; |
| |
| // Loop over all of the basic blocks that also load the value. If the value |
| // is live across the CFG from the source to destination blocks, and if the |
| // value is not invalidated in either the source or destination blocks, add it |
| // to the equivalence sets. |
| for (std::map<BasicBlock*, unsigned>::iterator |
| I = CandidateLoads.begin(), E = CandidateLoads.end(); I != E; ++I) { |
| bool CantEqual = false; |
| |
| // Right now we only can handle cases where one load dominates the other. |
| // FIXME: generalize this! |
| BasicBlock *BB1 = I->first, *BB2 = LoadBB; |
| if (DT.dominates(BB1, BB2)) { |
| // The other load dominates LI. If the loaded value is killed entering |
| // the LoadBB block, we know the load is not live. |
| if (LoadInvalidatedInBBBefore) |
| CantEqual = true; |
| } else if (DT.dominates(BB2, BB1)) { |
| std::swap(BB1, BB2); // Canonicalize |
| // LI dominates the other load. If the loaded value is killed exiting |
| // the LoadBB block, we know the load is not live. |
| if (LoadInvalidatedInBBAfter) |
| CantEqual = true; |
| } else { |
| // None of these loads can VN the same. |
| CantEqual = true; |
| } |
| |
| if (!CantEqual) { |
| // Ok, at this point, we know that BB1 dominates BB2, and that there is |
| // nothing in the LI block that kills the loaded value. Check to see if |
| // the value is live across the CFG. |
| std::set<BasicBlock*> Visited; |
| for (pred_iterator PI = pred_begin(BB2), E = pred_end(BB2); PI!=E; ++PI) |
| if (!isPathTransparentTo(*PI, BB1, LoadPtr, LoadSize, AA, |
| Visited, TransparentBlocks)) { |
| // None of these loads can VN the same. |
| CantEqual = true; |
| break; |
| } |
| } |
| |
| // If the loads can equal so far, scan the basic block that contains the |
| // loads under consideration to see if they are invalidated in the block. |
| // For any loads that are not invalidated, add them to the equivalence |
| // set! |
| if (!CantEqual) { |
| unsigned NumLoads = I->second; |
| if (BB1 == LoadBB) { |
| // If LI dominates the block in question, check to see if any of the |
| // loads in this block are invalidated before they are reached. |
| for (BasicBlock::iterator BBI = I->first->begin(); ; ++BBI) { |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { |
| if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) { |
| // The load is in the set! |
| RetVals.push_back(BBI); |
| if (--NumLoads == 0) break; // Found last load to check. |
| } |
| } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize) |
| & AliasAnalysis::Mod) { |
| // If there is a modifying instruction, nothing below it will value |
| // # the same. |
| break; |
| } |
| } |
| } else { |
| // If the block dominates LI, make sure that the loads in the block are |
| // not invalidated before the block ends. |
| BasicBlock::iterator BBI = I->first->end(); |
| while (1) { |
| --BBI; |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { |
| if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) { |
| // The load is the same as this load! |
| RetVals.push_back(BBI); |
| if (--NumLoads == 0) break; // Found all of the laods. |
| } |
| } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize) |
| & AliasAnalysis::Mod) { |
| // If there is a modifying instruction, nothing above it will value |
| // # the same. |
| break; |
| } |
| } |
| } |
| } |
| } |
| |
| // Handle candidate stores. If the loaded location is clobbered on entrance |
| // to the LoadBB, no store outside of the LoadBB can value number equal, so |
| // quick exit. |
| if (LoadInvalidatedInBBBefore) |
| return; |
| |
| // Stores in the load-bb are handled above. |
| CandidateStores.erase(LoadBB); |
| |
| for (std::set<BasicBlock*>::iterator I = CandidateStores.begin(), |
| E = CandidateStores.end(); I != E; ++I) |
| if (DT.dominates(*I, LoadBB)) { |
| BasicBlock *StoreBB = *I; |
| |
| // Check to see if the path from the store to the load is transparent |
| // w.r.t. the memory location. |
| bool CantEqual = false; |
| std::set<BasicBlock*> Visited; |
| for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); |
| PI != E; ++PI) |
| if (!isPathTransparentTo(*PI, StoreBB, LoadPtr, LoadSize, AA, |
| Visited, TransparentBlocks)) { |
| // None of these stores can VN the same. |
| CantEqual = true; |
| break; |
| } |
| Visited.clear(); |
| if (!CantEqual) { |
| // Okay, the path from the store block to the load block is clear, and |
| // we know that there are no invalidating instructions from the start |
| // of the load block to the load itself. Now we just scan the store |
| // block. |
| |
| BasicBlock::iterator BBI = StoreBB->end(); |
| while (1) { |
| assert(BBI != StoreBB->begin() && |
| "There is a store in this block of the pointer, but the store" |
| " doesn't mod the address being stored to?? Must be a bug in" |
| " the alias analysis implementation!"); |
| --BBI; |
| if (AA.getModRefInfo(BBI, LoadPtr, LoadSize) & AliasAnalysis::Mod) { |
| // If the invalidating instruction is one of the candidates, |
| // then it provides the value the load loads. |
| if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) |
| if (SI->getOperand(1) == LoadPtr) |
| RetVals.push_back(SI->getOperand(0)); |
| break; |
| } |
| } |
| } |
| } |
| } |
