From 68d2417e0526d1e88b95e232777893cd56c55cf7 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Fri, 6 Nov 2009 18:15:14 +0000 Subject: [PATCH] Extend jump threading to support much more general threading predicates. This allows us to jump thread things like: _ZN12StringSwitchI5ColorE4CaseILj7EEERS1_RAT__KcRKS0_.exit119: %tmp1.i24166 = phi i8 [ 1, %bb5.i117 ], [ %tmp1.i24165, %_Z....exit ], [ %tmp1.i24165, %bb4.i114 ] %toBoolnot.i87 = icmp eq i8 %tmp1.i24166, 0 ; [#uses=1] %tmp4.i90 = icmp eq i32 %tmp2.i, 6 ; [#uses=1] %or.cond173 = and i1 %toBoolnot.i87, %tmp4.i90 ; [#uses=1] br i1 %or.cond173, label %bb4.i96, label %_ZN12... Where it is "obvious" that when coming from %bb5.i117 that the 'and' is always false. This triggers a surprisingly high number of times in the testsuite, and gets us closer to generating good code for doug's strswitch testcase. This also make a bunch of other code in jump threading redundant, I'll rip out in the next patch. This survived an enable-checking llvm-gcc bootstrap. llvm-svn: 86264 --- llvm/lib/Transforms/Scalar/JumpThreading.cpp | 352 +++++++++++++++++-- llvm/test/Transforms/JumpThreading/basic.ll | 31 ++ 2 files changed, 356 insertions(+), 27 deletions(-) diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp index f134946019ce..7c8cf2db9fb3 100644 --- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp +++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp @@ -75,8 +75,16 @@ namespace { bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, BasicBlock *PredBB); - BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val); + + typedef SmallVectorImpl > PredValueInfo; + + bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, + PredValueInfo &Result); + bool ProcessThreadableEdges(Instruction *CondInst, BasicBlock *BB); + + bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); @@ -220,7 +228,133 @@ BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) { &CommonPreds[0], CommonPreds.size(), ".thr_comm", this); } + +/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right +/// hand sides of the compare instruction, try to determine the result. If the +/// result can not be determined, a null pointer is returned. +static Constant *GetResultOfComparison(CmpInst::Predicate pred, + Value *LHS, Value *RHS) { + if (Constant *CLHS = dyn_cast(LHS)) + if (Constant *CRHS = dyn_cast(RHS)) + return ConstantExpr::getCompare(pred, CLHS, CRHS); + if (LHS == RHS) + if (isa(LHS->getType()) || isa(LHS->getType())) + if (ICmpInst::isTrueWhenEqual(pred)) + return ConstantInt::getTrue(LHS->getContext()); + else + return ConstantInt::getFalse(LHS->getContext()); + return 0; +} + + +/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see +/// if we can infer that the value is a known ConstantInt in any of our +/// predecessors. If so, return the known the list of value and pred BB in the +/// result vector. If a value is known to be undef, it is returned as null. +/// +/// The BB basic block is known to start with a PHI node. +/// +/// This returns true if there were any known values. +/// +/// +/// TODO: Per PR2563, we could infer value range information about a predecessor +/// based on its terminator. +bool JumpThreading:: +ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){ + PHINode *TheFirstPHI = cast(BB->begin()); + + // If V is a constantint, then it is known in all predecessors. + if (isa(V) || isa(V)) { + ConstantInt *CI = dyn_cast(V); + Result.resize(TheFirstPHI->getNumIncomingValues()); + for (unsigned i = 0, e = Result.size(); i != e; ++i) + Result.push_back(std::make_pair(CI, TheFirstPHI->getIncomingBlock(i))); + return true; + } + + // If V is a non-instruction value, or an instruction in a different block, + // then it can't be derived from a PHI. + Instruction *I = dyn_cast(V); + if (I == 0 || I->getParent() != BB) + return false; + + /// If I is a PHI node, then we know the incoming values for any constants. + if (PHINode *PN = dyn_cast(I)) { + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + Value *InVal = PN->getIncomingValue(i); + if (isa(InVal) || isa(InVal)) { + ConstantInt *CI = dyn_cast(InVal); + Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i))); + } + } + return !Result.empty(); + } + + SmallVector, 8> LHSVals, RHSVals; + + // Handle some boolean conditions. + if (I->getType()->getPrimitiveSizeInBits() == 1) { + // X | true -> true + // X & false -> false + if (I->getOpcode() == Instruction::Or || + I->getOpcode() == Instruction::And) { + ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); + ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals); + + if (LHSVals.empty() && RHSVals.empty()) + return false; + + ConstantInt *InterestingVal; + if (I->getOpcode() == Instruction::Or) + InterestingVal = ConstantInt::getTrue(I->getContext()); + else + InterestingVal = ConstantInt::getFalse(I->getContext()); + + // Scan for the sentinel. + for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) + if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) + Result.push_back(LHSVals[i]); + for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) + if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) + Result.push_back(RHSVals[i]); + return !Result.empty(); + } + + // TODO: Should handle the NOT form of XOR. + + } + + // Handle compare with phi operand, where the PHI is defined in this block. + if (CmpInst *Cmp = dyn_cast(I)) { + PHINode *PN = dyn_cast(Cmp->getOperand(0)); + if (PN && PN->getParent() == BB) { + // We can do this simplification if any comparisons fold to true or false. + // See if any do. + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + BasicBlock *PredBB = PN->getIncomingBlock(i); + Value *LHS = PN->getIncomingValue(i); + Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); + + Constant *Res = GetResultOfComparison(Cmp->getPredicate(), LHS, RHS); + if (Res == 0) continue; + + if (isa(Res)) + Result.push_back(std::make_pair((ConstantInt*)0, PredBB)); + else if (ConstantInt *CI = dyn_cast(Res)) + Result.push_back(std::make_pair(CI, PredBB)); + } + + return !Result.empty(); + } + + // TODO: We could also recurse to see if we can determine constants another + // way. + } + return false; +} + + /// GetBestDestForBranchOnUndef - If we determine that the specified block ends /// in an undefined jump, decide which block is best to revector to. @@ -251,7 +385,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { // successor, merge the blocks. This encourages recursive jump threading // because now the condition in this block can be threaded through // predecessors of our predecessor block. - if (BasicBlock *SinglePred = BB->getSinglePredecessor()) + if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { if (SinglePred->getTerminator()->getNumSuccessors() == 1 && SinglePred != BB) { // If SinglePred was a loop header, BB becomes one. @@ -267,10 +401,10 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { BB->moveBefore(&BB->getParent()->getEntryBlock()); return true; } - - // See if this block ends with a branch or switch. If so, see if the - // condition is a phi node. If so, and if an entry of the phi node is a - // constant, we can thread the block. + } + + // Look to see if the terminator is a branch of switch, if not we can't thread + // it. Value *Condition; if (BranchInst *BI = dyn_cast(BB->getTerminator())) { // Can't thread an unconditional jump. @@ -369,7 +503,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { } // If we have a comparison, loop over the predecessors to see if there is - // a condition with the same value. + // a condition with a lexically identical value. pred_iterator PI = pred_begin(BB), E = pred_end(BB); for (; PI != E; ++PI) if (BranchInst *PBI = dyn_cast((*PI)->getTerminator())) @@ -402,6 +536,19 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { if (SimplifyPartiallyRedundantLoad(LI)) return true; + + // Handle a variety of cases where we are branching on something derived from + // a PHI node in the current block. If we can prove that any predecessors + // compute a predictable value based on a PHI node, thread those predecessors. + // + // We only bother doing this if the current block has a PHI node and if the + // conditional instruction lives in the current block. If either condition + // fail, this won't be a computable value anyway. + if (CondInst->getParent() == BB && isa(BB->front())) + if (ProcessThreadableEdges(CondInst, BB)) + return true; + + // TODO: If we have: "br (X > 0)" and we have a predecessor where we know // "(X == 4)" thread through this block. @@ -690,6 +837,176 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { return true; } +/// FindMostPopularDest - The specified list contains multiple possible +/// threadable destinations. Pick the one that occurs the most frequently in +/// the list. +static BasicBlock * +FindMostPopularDest(BasicBlock *BB, + const SmallVectorImpl > &PredToDestList) { + assert(!PredToDestList.empty()); + + // Determine popularity. If there are multiple possible destinations, we + // explicitly choose to ignore 'undef' destinations. We prefer to thread + // blocks with known and real destinations to threading undef. We'll handle + // them later if interesting. + DenseMap DestPopularity; + for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) + if (PredToDestList[i].second) + DestPopularity[PredToDestList[i].second]++; + + // Find the most popular dest. + DenseMap::iterator DPI = DestPopularity.begin(); + BasicBlock *MostPopularDest = DPI->first; + unsigned Popularity = DPI->second; + SmallVector SamePopularity; + + for (++DPI; DPI != DestPopularity.end(); ++DPI) { + // If the popularity of this entry isn't higher than the popularity we've + // seen so far, ignore it. + if (DPI->second < Popularity) + ; // ignore. + else if (DPI->second == Popularity) { + // If it is the same as what we've seen so far, keep track of it. + SamePopularity.push_back(DPI->first); + } else { + // If it is more popular, remember it. + SamePopularity.clear(); + MostPopularDest = DPI->first; + Popularity = DPI->second; + } + } + + // Okay, now we know the most popular destination. If there is more than + // destination, we need to determine one. This is arbitrary, but we need + // to make a deterministic decision. Pick the first one that appears in the + // successor list. + if (!SamePopularity.empty()) { + SamePopularity.push_back(MostPopularDest); + TerminatorInst *TI = BB->getTerminator(); + for (unsigned i = 0; ; ++i) { + assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); + + if (std::find(SamePopularity.begin(), SamePopularity.end(), + TI->getSuccessor(i)) == SamePopularity.end()) + continue; + + MostPopularDest = TI->getSuccessor(i); + break; + } + } + + // Okay, we have finally picked the most popular destination. + return MostPopularDest; +} + +bool JumpThreading::ProcessThreadableEdges(Instruction *CondInst, + BasicBlock *BB) { + // If threading this would thread across a loop header, don't even try to + // thread the edge. + if (LoopHeaders.count(BB)) + return false; + + + + SmallVector, 8> PredValues; + if (!ComputeValueKnownInPredecessors(CondInst, BB, PredValues)) + return false; + assert(!PredValues.empty() && + "ComputeValueKnownInPredecessors returned true with no values"); + + DEBUG(errs() << "IN BB: " << *BB; + for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { + errs() << " BB '" << BB->getName() << "': FOUND condition = "; + if (PredValues[i].first) + errs() << *PredValues[i].first; + else + errs() << "UNDEF"; + errs() << " for pred '" << PredValues[i].second->getName() + << "'.\n"; + }); + + // Decide what we want to thread through. Convert our list of known values to + // a list of known destinations for each pred. This also discards duplicate + // predecessors and keeps track of the undefined inputs (which are represented + // as a null dest in the PredToDestList. + SmallPtrSet SeenPreds; + SmallVector, 16> PredToDestList; + + BasicBlock *OnlyDest = 0; + BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; + + for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { + BasicBlock *Pred = PredValues[i].second; + if (!SeenPreds.insert(Pred)) + continue; // Duplicate predecessor entry. + + // If the predecessor ends with an indirect goto, we can't change its + // destination. + if (isa(Pred->getTerminator())) + continue; + + ConstantInt *Val = PredValues[i].first; + + BasicBlock *DestBB; + if (Val == 0) // Undef. + DestBB = 0; + else if (BranchInst *BI = dyn_cast(BB->getTerminator())) + DestBB = BI->getSuccessor(Val->isZero()); + else { + SwitchInst *SI = cast(BB->getTerminator()); + DestBB = SI->getSuccessor(SI->findCaseValue(Val)); + } + + // If we have exactly one destination, remember it for efficiency below. + if (i == 0) + OnlyDest = DestBB; + else if (OnlyDest != DestBB) + OnlyDest = MultipleDestSentinel; + + PredToDestList.push_back(std::make_pair(Pred, DestBB)); + } + + // If all edges were unthreadable, we fail. + if (PredToDestList.empty()) + return false; + + // Determine which is the most common successor. If we have many inputs and + // this block is a switch, we want to start by threading the batch that goes + // to the most popular destination first. If we only know about one + // threadable destination (the common case) we can avoid this. + BasicBlock *MostPopularDest = OnlyDest; + + if (MostPopularDest == MultipleDestSentinel) + MostPopularDest = FindMostPopularDest(BB, PredToDestList); + + // Now that we know what the most popular destination is, factor all + // predecessors that will jump to it into a single predecessor. + SmallVector PredsToFactor; + for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) + if (PredToDestList[i].second == MostPopularDest) + PredsToFactor.push_back(PredToDestList[i].first); + + BasicBlock *PredToThread; + if (PredsToFactor.size() == 1) + PredToThread = PredsToFactor[0]; + else { + DEBUG(errs() << " Factoring out " << PredsToFactor.size() + << " common predecessors.\n"); + PredToThread = SplitBlockPredecessors(BB, &PredsToFactor[0], + PredsToFactor.size(), + ".thr_comm", this); + } + + // If the threadable edges are branching on an undefined value, we get to pick + // the destination that these predecessors should get to. + if (MostPopularDest == 0) + MostPopularDest = BB->getTerminator()-> + getSuccessor(GetBestDestForJumpOnUndef(BB)); + + // Ok, try to thread it! + return ThreadEdge(BB, PredToThread, MostPopularDest); +} /// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in /// the current block. See if there are any simplifications we can do based on @@ -814,24 +1131,6 @@ bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, return ThreadEdge(BB, PredBB, SuccBB); } -/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right -/// hand sides of the compare instruction, try to determine the result. If the -/// result can not be determined, a null pointer is returned. -static Constant *GetResultOfComparison(CmpInst::Predicate pred, - Value *LHS, Value *RHS, - LLVMContext &Context) { - if (Constant *CLHS = dyn_cast(LHS)) - if (Constant *CRHS = dyn_cast(RHS)) - return ConstantExpr::getCompare(pred, CLHS, CRHS); - - if (LHS == RHS) - if (isa(LHS->getType()) || isa(LHS->getType())) - return ICmpInst::isTrueWhenEqual(pred) ? - ConstantInt::getTrue(Context) : ConstantInt::getFalse(Context); - - return 0; -} - /// ProcessBranchOnCompare - We found a branch on a comparison between a phi /// node and a value. If we can identify when the comparison is true between /// the phi inputs and the value, we can fold the compare for that edge and @@ -852,8 +1151,7 @@ bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { PredVal = PN->getIncomingValue(i); - Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, - RHS, Cmp->getContext()); + Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, RHS); if (!Res) { PredVal = 0; continue; diff --git a/llvm/test/Transforms/JumpThreading/basic.ll b/llvm/test/Transforms/JumpThreading/basic.ll index 3d936b8f30fc..7b444ad0a918 100644 --- a/llvm/test/Transforms/JumpThreading/basic.ll +++ b/llvm/test/Transforms/JumpThreading/basic.ll @@ -170,5 +170,36 @@ BB4: } +;; This tests that the branch in 'merge' can be cloned up into T1. +;; rdar://7367025 +define i32 @test7(i1 %cond, i1 %cond2) { +Entry: +; CHECK: @test7 + %v1 = call i32 @f1() + br i1 %cond, label %Merge, label %F1 +F1: + %v2 = call i32 @f2() + br label %Merge + +Merge: + %B = phi i32 [%v1, %Entry], [%v2, %F1] + %M = icmp ne i32 %B, %v1 + %N = icmp eq i32 %B, 47 + %O = and i1 %M, %N + br i1 %O, label %T2, label %F2 + +; CHECK: Merge: +; CHECK-NOT: phi +; CHECK-NEXT: %v2 = call i32 @f2() + +T2: + call void @f3() + ret i32 %B + +F2: + ret i32 %B +; CHECK: F2: +; CHECK-NEXT: phi i32 +}