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Diffstat (limited to 'lib/Analysis/LoadValueNumbering.cpp')
| -rw-r--r-- | lib/Analysis/LoadValueNumbering.cpp | 522 |
1 files changed, 522 insertions, 0 deletions
diff --git a/lib/Analysis/LoadValueNumbering.cpp b/lib/Analysis/LoadValueNumbering.cpp new file mode 100644 index 0000000000..16e79a1286 --- /dev/null +++ b/lib/Analysis/LoadValueNumbering.cpp @@ -0,0 +1,522 @@ +//===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===// +// +// The LLVM Compiler Infrastructure +// +// This file was developed by the LLVM research group and 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/Target/TargetData.h" +#include <set> +#include <algorithm> +using namespace llvm; + +namespace { + // FIXME: This should not be a FunctionPass. + struct LoadVN : public FunctionPass, public ValueNumbering { + + /// 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; + }; + + // Register this pass... + RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering"); + + // Declare that we implement the ValueNumbering interface + RegisterAnalysisGroup<ValueNumbering, LoadVN> Y; +} + +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.addRequired<AliasAnalysis>(); + AU.addRequired<ValueNumbering>(); + AU.addRequired<DominatorSet>(); + AU.addRequired<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(CF, 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) { + DominatorSet &DomSetInfo = getAnalysis<DominatorSet>(); + BasicBlock *CIBB = CI->getParent(); + for (unsigned i = 0; i != IdenticalCalls.size(); ++i) { + CallInst *C = IdenticalCalls[i]; + bool CantEqual = false; + + if (DomSetInfo.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 (!DomSetInfo.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 (CI->getCalledFunction() == 0 || + !AA.onlyReadsMemory(CI->getCalledFunction())) { + CantEqual = true; + break; + } + } else if (I->mayWriteToMemory()) { + CantEqual = true; + break; + } + } + } + + } else if (DomSetInfo.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>().getTypeSize(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; + for (BasicBlock::iterator I = LI->getNext(); 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. + DominatorSet &DomSetInfo = getAnalysis<DominatorSet>(); + + // 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 (DomSetInfo.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 (DomSetInfo.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 (DomSetInfo.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; + } + } + } + } +} |