LLVM API Documentation

LoadValueNumbering.cpp

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00001 //===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements a value numbering pass that value numbers load and call
00011 // instructions.  To do this, it finds lexically identical load instructions,
00012 // and uses alias analysis to determine which loads are guaranteed to produce
00013 // the same value.  To value number call instructions, it looks for calls to
00014 // functions that do not write to memory which do not have intervening
00015 // instructions that clobber the memory that is read from.
00016 //
00017 // This pass builds off of another value numbering pass to implement value
00018 // numbering for non-load and non-call instructions.  It uses Alias Analysis so
00019 // that it can disambiguate the load instructions.  The more powerful these base
00020 // analyses are, the more powerful the resultant value numbering will be.
00021 //
00022 //===----------------------------------------------------------------------===//
00023 
00024 #include "llvm/Analysis/LoadValueNumbering.h"
00025 #include "llvm/Constants.h"
00026 #include "llvm/Function.h"
00027 #include "llvm/Instructions.h"
00028 #include "llvm/Pass.h"
00029 #include "llvm/Type.h"
00030 #include "llvm/Analysis/ValueNumbering.h"
00031 #include "llvm/Analysis/AliasAnalysis.h"
00032 #include "llvm/Analysis/Dominators.h"
00033 #include "llvm/Support/CFG.h"
00034 #include "llvm/Target/TargetData.h"
00035 #include <set>
00036 #include <algorithm>
00037 using namespace llvm;
00038 
00039 namespace {
00040   // FIXME: This should not be a FunctionPass.
00041   struct LoadVN : public FunctionPass, public ValueNumbering {
00042 
00043     /// Pass Implementation stuff.  This doesn't do any analysis.
00044     ///
00045     bool runOnFunction(Function &) { return false; }
00046 
00047     /// getAnalysisUsage - Does not modify anything.  It uses Value Numbering
00048     /// and Alias Analysis.
00049     ///
00050     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
00051 
00052     /// getEqualNumberNodes - Return nodes with the same value number as the
00053     /// specified Value.  This fills in the argument vector with any equal
00054     /// values.
00055     ///
00056     virtual void getEqualNumberNodes(Value *V1,
00057                                      std::vector<Value*> &RetVals) const;
00058 
00059     /// deleteValue - This method should be called whenever an LLVM Value is
00060     /// deleted from the program, for example when an instruction is found to be
00061     /// redundant and is eliminated.
00062     ///
00063     virtual void deleteValue(Value *V) {
00064       getAnalysis<AliasAnalysis>().deleteValue(V);
00065     }
00066 
00067     /// copyValue - This method should be used whenever a preexisting value in
00068     /// the program is copied or cloned, introducing a new value.  Note that
00069     /// analysis implementations should tolerate clients that use this method to
00070     /// introduce the same value multiple times: if the analysis already knows
00071     /// about a value, it should ignore the request.
00072     ///
00073     virtual void copyValue(Value *From, Value *To) {
00074       getAnalysis<AliasAnalysis>().copyValue(From, To);
00075     }
00076 
00077     /// getCallEqualNumberNodes - Given a call instruction, find other calls
00078     /// that have the same value number.
00079     void getCallEqualNumberNodes(CallInst *CI,
00080                                  std::vector<Value*> &RetVals) const;
00081   };
00082 
00083   // Register this pass...
00084   RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
00085 
00086   // Declare that we implement the ValueNumbering interface
00087   RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
00088 }
00089 
00090 FunctionPass *llvm::createLoadValueNumberingPass() { return new LoadVN(); }
00091 
00092 
00093 /// getAnalysisUsage - Does not modify anything.  It uses Value Numbering and
00094 /// Alias Analysis.
00095 ///
00096 void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
00097   AU.setPreservesAll();
00098   AU.addRequiredTransitive<AliasAnalysis>();
00099   AU.addRequired<ValueNumbering>();
00100   AU.addRequiredTransitive<DominatorSet>();
00101   AU.addRequiredTransitive<TargetData>();
00102 }
00103 
00104 static bool isPathTransparentTo(BasicBlock *CurBlock, BasicBlock *Dom,
00105                                 Value *Ptr, unsigned Size, AliasAnalysis &AA,
00106                                 std::set<BasicBlock*> &Visited,
00107                                 std::map<BasicBlock*, bool> &TransparentBlocks){
00108   // If we have already checked out this path, or if we reached our destination,
00109   // stop searching, returning success.
00110   if (CurBlock == Dom || !Visited.insert(CurBlock).second)
00111     return true;
00112 
00113   // Check whether this block is known transparent or not.
00114   std::map<BasicBlock*, bool>::iterator TBI =
00115     TransparentBlocks.lower_bound(CurBlock);
00116 
00117   if (TBI == TransparentBlocks.end() || TBI->first != CurBlock) {
00118     // If this basic block can modify the memory location, then the path is not
00119     // transparent!
00120     if (AA.canBasicBlockModify(*CurBlock, Ptr, Size)) {
00121       TransparentBlocks.insert(TBI, std::make_pair(CurBlock, false));
00122       return false;
00123     }
00124     TransparentBlocks.insert(TBI, std::make_pair(CurBlock, true));
00125   } else if (!TBI->second)
00126     // This block is known non-transparent, so that path can't be either.
00127     return false;
00128 
00129   // The current block is known to be transparent.  The entire path is
00130   // transparent if all of the predecessors paths to the parent is also
00131   // transparent to the memory location.
00132   for (pred_iterator PI = pred_begin(CurBlock), E = pred_end(CurBlock);
00133        PI != E; ++PI)
00134     if (!isPathTransparentTo(*PI, Dom, Ptr, Size, AA, Visited,
00135                              TransparentBlocks))
00136       return false;
00137   return true;
00138 }
00139 
00140 /// getCallEqualNumberNodes - Given a call instruction, find other calls that
00141 /// have the same value number.
00142 void LoadVN::getCallEqualNumberNodes(CallInst *CI,
00143                                      std::vector<Value*> &RetVals) const {
00144   Function *CF = CI->getCalledFunction();
00145   if (CF == 0) return;  // Indirect call.
00146   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
00147   AliasAnalysis::ModRefBehavior MRB = AA.getModRefBehavior(CF, CI);
00148   if (MRB != AliasAnalysis::DoesNotAccessMemory &&
00149       MRB != AliasAnalysis::OnlyReadsMemory)
00150     return;  // Nothing we can do for now.
00151 
00152   // Scan all of the arguments of the function, looking for one that is not
00153   // global.  In particular, we would prefer to have an argument or instruction
00154   // operand to chase the def-use chains of.
00155   Value *Op = CF;
00156   for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
00157     if (isa<Argument>(CI->getOperand(i)) ||
00158         isa<Instruction>(CI->getOperand(i))) {
00159       Op = CI->getOperand(i);
00160       break;
00161     }
00162 
00163   // Identify all lexically identical calls in this function.
00164   std::vector<CallInst*> IdenticalCalls;
00165 
00166   Function *CIFunc = CI->getParent()->getParent();
00167   for (Value::use_iterator UI = Op->use_begin(), E = Op->use_end(); UI != E;
00168        ++UI)
00169     if (CallInst *C = dyn_cast<CallInst>(*UI))
00170       if (C->getNumOperands() == CI->getNumOperands() &&
00171           C->getOperand(0) == CI->getOperand(0) &&
00172           C->getParent()->getParent() == CIFunc && C != CI) {
00173         bool AllOperandsEqual = true;
00174         for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
00175           if (C->getOperand(i) != CI->getOperand(i)) {
00176             AllOperandsEqual = false;
00177             break;
00178           }
00179 
00180         if (AllOperandsEqual)
00181           IdenticalCalls.push_back(C);
00182       }
00183 
00184   if (IdenticalCalls.empty()) return;
00185 
00186   // Eliminate duplicates, which could occur if we chose a value that is passed
00187   // into a call site multiple times.
00188   std::sort(IdenticalCalls.begin(), IdenticalCalls.end());
00189   IdenticalCalls.erase(std::unique(IdenticalCalls.begin(),IdenticalCalls.end()),
00190                        IdenticalCalls.end());
00191 
00192   // If the call reads memory, we must make sure that there are no stores
00193   // between the calls in question.
00194   //
00195   // FIXME: This should use mod/ref information.  What we really care about it
00196   // whether an intervening instruction could modify memory that is read, not
00197   // ANY memory.
00198   //
00199   if (MRB == AliasAnalysis::OnlyReadsMemory) {
00200     DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
00201     BasicBlock *CIBB = CI->getParent();
00202     for (unsigned i = 0; i != IdenticalCalls.size(); ++i) {
00203       CallInst *C = IdenticalCalls[i];
00204       bool CantEqual = false;
00205 
00206       if (DomSetInfo.dominates(CIBB, C->getParent())) {
00207         // FIXME: we currently only handle the case where both calls are in the
00208         // same basic block.
00209         if (CIBB != C->getParent()) {
00210           CantEqual = true;
00211         } else {
00212           Instruction *First = CI, *Second = C;
00213           if (!DomSetInfo.dominates(CI, C))
00214             std::swap(First, Second);
00215 
00216           // Scan the instructions between the calls, checking for stores or
00217           // calls to dangerous functions.
00218           BasicBlock::iterator I = First;
00219           for (++First; I != BasicBlock::iterator(Second); ++I) {
00220             if (isa<StoreInst>(I)) {
00221               // FIXME: We could use mod/ref information to make this much
00222               // better!
00223               CantEqual = true;
00224               break;
00225             } else if (CallInst *CI = dyn_cast<CallInst>(I)) {
00226               if (CI->getCalledFunction() == 0 ||
00227                   !AA.onlyReadsMemory(CI->getCalledFunction())) {
00228                 CantEqual = true;
00229                 break;
00230               }
00231             } else if (I->mayWriteToMemory()) {
00232               CantEqual = true;
00233               break;
00234             }
00235           }
00236         }
00237 
00238       } else if (DomSetInfo.dominates(C->getParent(), CIBB)) {
00239         // FIXME: We could implement this, but we don't for now.
00240         CantEqual = true;
00241       } else {
00242         // FIXME: if one doesn't dominate the other, we can't tell yet.
00243         CantEqual = true;
00244       }
00245 
00246 
00247       if (CantEqual) {
00248         // This call does not produce the same value as the one in the query.
00249         std::swap(IdenticalCalls[i--], IdenticalCalls.back());
00250         IdenticalCalls.pop_back();
00251       }
00252     }
00253   }
00254 
00255   // Any calls that are identical and not destroyed will produce equal values!
00256   for (unsigned i = 0, e = IdenticalCalls.size(); i != e; ++i)
00257     RetVals.push_back(IdenticalCalls[i]);
00258 }
00259 
00260 // getEqualNumberNodes - Return nodes with the same value number as the
00261 // specified Value.  This fills in the argument vector with any equal values.
00262 //
00263 void LoadVN::getEqualNumberNodes(Value *V,
00264                                  std::vector<Value*> &RetVals) const {
00265   // If the alias analysis has any must alias information to share with us, we
00266   // can definitely use it.
00267   if (isa<PointerType>(V->getType()))
00268     getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals);
00269 
00270   if (!isa<LoadInst>(V)) {
00271     if (CallInst *CI = dyn_cast<CallInst>(V))
00272       getCallEqualNumberNodes(CI, RetVals);
00273 
00274     // Not a load instruction?  Just chain to the base value numbering
00275     // implementation to satisfy the request...
00276     assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
00277            "getAnalysis() returned this!");
00278 
00279     return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
00280   }
00281 
00282   // Volatile loads cannot be replaced with the value of other loads.
00283   LoadInst *LI = cast<LoadInst>(V);
00284   if (LI->isVolatile())
00285     return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
00286 
00287   Value *LoadPtr = LI->getOperand(0);
00288   BasicBlock *LoadBB = LI->getParent();
00289   Function *F = LoadBB->getParent();
00290 
00291   // Find out how many bytes of memory are loaded by the load instruction...
00292   unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LI->getType());
00293   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
00294 
00295   // Figure out if the load is invalidated from the entry of the block it is in
00296   // until the actual instruction.  This scans the block backwards from LI.  If
00297   // we see any candidate load or store instructions, then we know that the
00298   // candidates have the same value # as LI.
00299   bool LoadInvalidatedInBBBefore = false;
00300   for (BasicBlock::iterator I = LI; I != LoadBB->begin(); ) {
00301     --I;
00302     if (I == LoadPtr) {
00303       // If we run into an allocation of the value being loaded, then the
00304       // contents are not initialized.
00305       if (isa<AllocationInst>(I))
00306         RetVals.push_back(UndefValue::get(LI->getType()));
00307 
00308       // Otherwise, since this is the definition of what we are loading, this
00309       // loaded value cannot occur before this block.
00310       LoadInvalidatedInBBBefore = true;
00311       break;
00312     } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
00313       // If this instruction is a candidate load before LI, we know there are no
00314       // invalidating instructions between it and LI, so they have the same
00315       // value number.
00316       if (LI->getOperand(0) == LoadPtr && !LI->isVolatile())
00317         RetVals.push_back(I);
00318     }
00319 
00320     if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
00321       // If the invalidating instruction is a store, and its in our candidate
00322       // set, then we can do store-load forwarding: the load has the same value
00323       // # as the stored value.
00324       if (StoreInst *SI = dyn_cast<StoreInst>(I))
00325         if (SI->getOperand(1) == LoadPtr)
00326           RetVals.push_back(I->getOperand(0));
00327 
00328       LoadInvalidatedInBBBefore = true;
00329       break;
00330     }
00331   }
00332 
00333   // Figure out if the load is invalidated between the load and the exit of the
00334   // block it is defined in.  While we are scanning the current basic block, if
00335   // we see any candidate loads, then we know they have the same value # as LI.
00336   //
00337   bool LoadInvalidatedInBBAfter = false;
00338   for (BasicBlock::iterator I = LI->getNext(); I != LoadBB->end(); ++I) {
00339     // If this instruction is a load, then this instruction returns the same
00340     // value as LI.
00341     if (isa<LoadInst>(I) && cast<LoadInst>(I)->getOperand(0) == LoadPtr)
00342       RetVals.push_back(I);
00343 
00344     if (AA.getModRefInfo(I, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
00345       LoadInvalidatedInBBAfter = true;
00346       break;
00347     }
00348   }
00349 
00350   // If the pointer is clobbered on entry and on exit to the function, there is
00351   // no need to do any global analysis at all.
00352   if (LoadInvalidatedInBBBefore && LoadInvalidatedInBBAfter)
00353     return;
00354 
00355   // Now that we know the value is not neccesarily killed on entry or exit to
00356   // the BB, find out how many load and store instructions (to this location)
00357   // live in each BB in the function.
00358   //
00359   std::map<BasicBlock*, unsigned>  CandidateLoads;
00360   std::set<BasicBlock*> CandidateStores;
00361 
00362   for (Value::use_iterator UI = LoadPtr->use_begin(), UE = LoadPtr->use_end();
00363        UI != UE; ++UI)
00364     if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source?
00365       if (Cand->getParent()->getParent() == F &&   // In the same function?
00366           // Not in LI's block?
00367           Cand->getParent() != LoadBB && !Cand->isVolatile())
00368         ++CandidateLoads[Cand->getParent()];       // Got one.
00369     } else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
00370       if (Cand->getParent()->getParent() == F && !Cand->isVolatile() &&
00371           Cand->getOperand(1) == LoadPtr) // It's a store THROUGH the ptr.
00372         CandidateStores.insert(Cand->getParent());
00373     }
00374 
00375   // Get dominators.
00376   DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
00377 
00378   // TransparentBlocks - For each basic block the load/store is alive across,
00379   // figure out if the pointer is invalidated or not.  If it is invalidated, the
00380   // boolean is set to false, if it's not it is set to true.  If we don't know
00381   // yet, the entry is not in the map.
00382   std::map<BasicBlock*, bool> TransparentBlocks;
00383 
00384   // Loop over all of the basic blocks that also load the value.  If the value
00385   // is live across the CFG from the source to destination blocks, and if the
00386   // value is not invalidated in either the source or destination blocks, add it
00387   // to the equivalence sets.
00388   for (std::map<BasicBlock*, unsigned>::iterator
00389          I = CandidateLoads.begin(), E = CandidateLoads.end(); I != E; ++I) {
00390     bool CantEqual = false;
00391 
00392     // Right now we only can handle cases where one load dominates the other.
00393     // FIXME: generalize this!
00394     BasicBlock *BB1 = I->first, *BB2 = LoadBB;
00395     if (DomSetInfo.dominates(BB1, BB2)) {
00396       // The other load dominates LI.  If the loaded value is killed entering
00397       // the LoadBB block, we know the load is not live.
00398       if (LoadInvalidatedInBBBefore)
00399         CantEqual = true;
00400     } else if (DomSetInfo.dominates(BB2, BB1)) {
00401       std::swap(BB1, BB2);          // Canonicalize
00402       // LI dominates the other load.  If the loaded value is killed exiting
00403       // the LoadBB block, we know the load is not live.
00404       if (LoadInvalidatedInBBAfter)
00405         CantEqual = true;
00406     } else {
00407       // None of these loads can VN the same.
00408       CantEqual = true;
00409     }
00410 
00411     if (!CantEqual) {
00412       // Ok, at this point, we know that BB1 dominates BB2, and that there is
00413       // nothing in the LI block that kills the loaded value.  Check to see if
00414       // the value is live across the CFG.
00415       std::set<BasicBlock*> Visited;
00416       for (pred_iterator PI = pred_begin(BB2), E = pred_end(BB2); PI!=E; ++PI)
00417         if (!isPathTransparentTo(*PI, BB1, LoadPtr, LoadSize, AA,
00418                                  Visited, TransparentBlocks)) {
00419           // None of these loads can VN the same.
00420           CantEqual = true;
00421           break;
00422         }
00423     }
00424 
00425     // If the loads can equal so far, scan the basic block that contains the
00426     // loads under consideration to see if they are invalidated in the block.
00427     // For any loads that are not invalidated, add them to the equivalence
00428     // set!
00429     if (!CantEqual) {
00430       unsigned NumLoads = I->second;
00431       if (BB1 == LoadBB) {
00432         // If LI dominates the block in question, check to see if any of the
00433         // loads in this block are invalidated before they are reached.
00434         for (BasicBlock::iterator BBI = I->first->begin(); ; ++BBI) {
00435           if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
00436             if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) {
00437               // The load is in the set!
00438               RetVals.push_back(BBI);
00439               if (--NumLoads == 0) break;  // Found last load to check.
00440             }
00441           } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
00442                                 & AliasAnalysis::Mod) {
00443             // If there is a modifying instruction, nothing below it will value
00444             // # the same.
00445             break;
00446           }
00447         }
00448       } else {
00449         // If the block dominates LI, make sure that the loads in the block are
00450         // not invalidated before the block ends.
00451         BasicBlock::iterator BBI = I->first->end();
00452         while (1) {
00453           --BBI;
00454           if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
00455             if (LI->getOperand(0) == LoadPtr && !LI->isVolatile()) {
00456               // The load is the same as this load!
00457               RetVals.push_back(BBI);
00458               if (--NumLoads == 0) break;  // Found all of the laods.
00459             }
00460           } else if (AA.getModRefInfo(BBI, LoadPtr, LoadSize)
00461                              & AliasAnalysis::Mod) {
00462             // If there is a modifying instruction, nothing above it will value
00463             // # the same.
00464             break;
00465           }
00466         }
00467       }
00468     }
00469   }
00470 
00471   // Handle candidate stores.  If the loaded location is clobbered on entrance
00472   // to the LoadBB, no store outside of the LoadBB can value number equal, so
00473   // quick exit.
00474   if (LoadInvalidatedInBBBefore)
00475     return;
00476 
00477   // Stores in the load-bb are handled above.
00478   CandidateStores.erase(LoadBB);
00479 
00480   for (std::set<BasicBlock*>::iterator I = CandidateStores.begin(),
00481          E = CandidateStores.end(); I != E; ++I)
00482     if (DomSetInfo.dominates(*I, LoadBB)) {
00483       BasicBlock *StoreBB = *I;
00484 
00485       // Check to see if the path from the store to the load is transparent
00486       // w.r.t. the memory location.
00487       bool CantEqual = false;
00488       std::set<BasicBlock*> Visited;
00489       for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
00490            PI != E; ++PI)
00491         if (!isPathTransparentTo(*PI, StoreBB, LoadPtr, LoadSize, AA,
00492                                  Visited, TransparentBlocks)) {
00493           // None of these stores can VN the same.
00494           CantEqual = true;
00495           break;
00496         }
00497       Visited.clear();
00498       if (!CantEqual) {
00499         // Okay, the path from the store block to the load block is clear, and
00500         // we know that there are no invalidating instructions from the start
00501         // of the load block to the load itself.  Now we just scan the store
00502         // block.
00503 
00504         BasicBlock::iterator BBI = StoreBB->end();
00505         while (1) {
00506           assert(BBI != StoreBB->begin() &&
00507                  "There is a store in this block of the pointer, but the store"
00508                  " doesn't mod the address being stored to??  Must be a bug in"
00509                  " the alias analysis implementation!");
00510           --BBI;
00511           if (AA.getModRefInfo(BBI, LoadPtr, LoadSize) & AliasAnalysis::Mod) {
00512             // If the invalidating instruction is one of the candidates,
00513             // then it provides the value the load loads.
00514             if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
00515               if (SI->getOperand(1) == LoadPtr)
00516                 RetVals.push_back(SI->getOperand(0));
00517             break;
00518           }
00519         }
00520       }
00521     }
00522 }