forked from OSchip/llvm-project
1535 lines
59 KiB
C++
1535 lines
59 KiB
C++
//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the Jump Threading pass.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "jump-threading"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/Pass.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LazyValueInfo.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ValueHandle.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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STATISTIC(NumThreads, "Number of jumps threaded");
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STATISTIC(NumFolds, "Number of terminators folded");
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STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
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static cl::opt<unsigned>
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Threshold("jump-threading-threshold",
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cl::desc("Max block size to duplicate for jump threading"),
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cl::init(6), cl::Hidden);
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// Turn on use of LazyValueInfo.
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static cl::opt<bool>
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EnableLVI("enable-jump-threading-lvi", cl::ReallyHidden);
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namespace {
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/// This pass performs 'jump threading', which looks at blocks that have
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/// multiple predecessors and multiple successors. If one or more of the
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/// predecessors of the block can be proven to always jump to one of the
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/// successors, we forward the edge from the predecessor to the successor by
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/// duplicating the contents of this block.
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///
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/// An example of when this can occur is code like this:
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///
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/// if () { ...
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/// X = 4;
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/// }
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/// if (X < 3) {
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///
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/// In this case, the unconditional branch at the end of the first if can be
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/// revectored to the false side of the second if.
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///
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class JumpThreading : public FunctionPass {
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TargetData *TD;
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LazyValueInfo *LVI;
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#ifdef NDEBUG
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SmallPtrSet<BasicBlock*, 16> LoopHeaders;
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#else
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SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
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#endif
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public:
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static char ID; // Pass identification
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JumpThreading() : FunctionPass(&ID) {}
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bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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if (EnableLVI)
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AU.addRequired<LazyValueInfo>();
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}
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void FindLoopHeaders(Function &F);
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bool ProcessBlock(BasicBlock *BB);
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bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
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BasicBlock *SuccBB);
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bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock *> &PredBBs);
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typedef SmallVectorImpl<std::pair<ConstantInt*,
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BasicBlock*> > PredValueInfo;
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bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
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PredValueInfo &Result);
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bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
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bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
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bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
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bool ProcessBranchOnPHI(PHINode *PN);
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bool ProcessBranchOnXOR(BinaryOperator *BO);
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bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
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};
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}
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char JumpThreading::ID = 0;
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static RegisterPass<JumpThreading>
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X("jump-threading", "Jump Threading");
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// Public interface to the Jump Threading pass
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FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
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/// runOnFunction - Top level algorithm.
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///
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bool JumpThreading::runOnFunction(Function &F) {
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DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
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TD = getAnalysisIfAvailable<TargetData>();
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LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
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FindLoopHeaders(F);
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bool Changed, EverChanged = false;
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do {
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Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
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BasicBlock *BB = I;
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// Thread all of the branches we can over this block.
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while (ProcessBlock(BB))
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Changed = true;
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++I;
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// If the block is trivially dead, zap it. This eliminates the successor
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// edges which simplifies the CFG.
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if (pred_begin(BB) == pred_end(BB) &&
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BB != &BB->getParent()->getEntryBlock()) {
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DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
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<< "' with terminator: " << *BB->getTerminator() << '\n');
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LoopHeaders.erase(BB);
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DeleteDeadBlock(BB);
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Changed = true;
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} else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
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// Can't thread an unconditional jump, but if the block is "almost
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// empty", we can replace uses of it with uses of the successor and make
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// this dead.
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if (BI->isUnconditional() &&
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BB != &BB->getParent()->getEntryBlock()) {
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BasicBlock::iterator BBI = BB->getFirstNonPHI();
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// Ignore dbg intrinsics.
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while (isa<DbgInfoIntrinsic>(BBI))
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++BBI;
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// If the terminator is the only non-phi instruction, try to nuke it.
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if (BBI->isTerminator()) {
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// Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
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// block, we have to make sure it isn't in the LoopHeaders set. We
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// reinsert afterward if needed.
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bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
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BasicBlock *Succ = BI->getSuccessor(0);
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if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
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Changed = true;
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// If we deleted BB and BB was the header of a loop, then the
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// successor is now the header of the loop.
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BB = Succ;
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}
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if (ErasedFromLoopHeaders)
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LoopHeaders.insert(BB);
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}
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}
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}
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}
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EverChanged |= Changed;
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} while (Changed);
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LoopHeaders.clear();
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return EverChanged;
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}
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/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
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/// thread across it.
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static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
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/// Ignore PHI nodes, these will be flattened when duplication happens.
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BasicBlock::const_iterator I = BB->getFirstNonPHI();
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// FIXME: THREADING will delete values that are just used to compute the
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// branch, so they shouldn't count against the duplication cost.
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// Sum up the cost of each instruction until we get to the terminator. Don't
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// include the terminator because the copy won't include it.
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unsigned Size = 0;
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for (; !isa<TerminatorInst>(I); ++I) {
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// Debugger intrinsics don't incur code size.
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if (isa<DbgInfoIntrinsic>(I)) continue;
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// If this is a pointer->pointer bitcast, it is free.
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if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
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continue;
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// All other instructions count for at least one unit.
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++Size;
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// Calls are more expensive. If they are non-intrinsic calls, we model them
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// as having cost of 4. If they are a non-vector intrinsic, we model them
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// as having cost of 2 total, and if they are a vector intrinsic, we model
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// them as having cost 1.
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if (const CallInst *CI = dyn_cast<CallInst>(I)) {
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if (!isa<IntrinsicInst>(CI))
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Size += 3;
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else if (!CI->getType()->isVectorTy())
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Size += 1;
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}
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}
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// Threading through a switch statement is particularly profitable. If this
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// block ends in a switch, decrease its cost to make it more likely to happen.
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if (isa<SwitchInst>(I))
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Size = Size > 6 ? Size-6 : 0;
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return Size;
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}
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/// FindLoopHeaders - We do not want jump threading to turn proper loop
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/// structures into irreducible loops. Doing this breaks up the loop nesting
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/// hierarchy and pessimizes later transformations. To prevent this from
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/// happening, we first have to find the loop headers. Here we approximate this
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/// by finding targets of backedges in the CFG.
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///
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/// Note that there definitely are cases when we want to allow threading of
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/// edges across a loop header. For example, threading a jump from outside the
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/// loop (the preheader) to an exit block of the loop is definitely profitable.
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/// It is also almost always profitable to thread backedges from within the loop
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/// to exit blocks, and is often profitable to thread backedges to other blocks
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/// within the loop (forming a nested loop). This simple analysis is not rich
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/// enough to track all of these properties and keep it up-to-date as the CFG
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/// mutates, so we don't allow any of these transformations.
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///
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void JumpThreading::FindLoopHeaders(Function &F) {
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SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
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FindFunctionBackedges(F, Edges);
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for (unsigned i = 0, e = Edges.size(); i != e; ++i)
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LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
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}
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/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
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/// if we can infer that the value is a known ConstantInt in any of our
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/// predecessors. If so, return the known list of value and pred BB in the
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/// result vector. If a value is known to be undef, it is returned as null.
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///
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/// This returns true if there were any known values.
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///
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bool JumpThreading::
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ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
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// If V is a constantint, then it is known in all predecessors.
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if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
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ConstantInt *CI = dyn_cast<ConstantInt>(V);
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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Result.push_back(std::make_pair(CI, *PI));
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return true;
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}
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// If V is a non-instruction value, or an instruction in a different block,
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// then it can't be derived from a PHI.
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Instruction *I = dyn_cast<Instruction>(V);
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if (I == 0 || I->getParent() != BB) {
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// Okay, if this is a live-in value, see if it has a known value at the end
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// of any of our predecessors.
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//
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// FIXME: This should be an edge property, not a block end property.
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/// TODO: Per PR2563, we could infer value range information about a
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/// predecessor based on its terminator.
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//
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if (LVI) {
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// FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
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// "I" is a non-local compare-with-a-constant instruction. This would be
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// able to handle value inequalities better, for example if the compare is
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// "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
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// Perhaps getConstantOnEdge should be smart enough to do this?
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
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// If the value is known by LazyValueInfo to be a constant in a
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// predecessor, use that information to try to thread this block.
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Constant *PredCst = LVI->getConstantOnEdge(V, *PI, BB);
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if (PredCst == 0 ||
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(!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
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continue;
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Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), *PI));
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}
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return !Result.empty();
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}
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return false;
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}
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/// If I is a PHI node, then we know the incoming values for any constants.
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if (PHINode *PN = dyn_cast<PHINode>(I)) {
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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Value *InVal = PN->getIncomingValue(i);
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if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
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ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
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Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
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}
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}
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return !Result.empty();
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}
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SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
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// Handle some boolean conditions.
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if (I->getType()->getPrimitiveSizeInBits() == 1) {
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// X | true -> true
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// X & false -> false
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if (I->getOpcode() == Instruction::Or ||
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I->getOpcode() == Instruction::And) {
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ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
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ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
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if (LHSVals.empty() && RHSVals.empty())
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return false;
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ConstantInt *InterestingVal;
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if (I->getOpcode() == Instruction::Or)
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InterestingVal = ConstantInt::getTrue(I->getContext());
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else
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InterestingVal = ConstantInt::getFalse(I->getContext());
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// Scan for the sentinel. If we find an undef, force it to the
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// interesting value: x|undef -> true and x&undef -> false.
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for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
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if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
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Result.push_back(LHSVals[i]);
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Result.back().first = InterestingVal;
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}
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for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
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if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
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Result.push_back(RHSVals[i]);
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Result.back().first = InterestingVal;
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}
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return !Result.empty();
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}
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// Handle the NOT form of XOR.
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if (I->getOpcode() == Instruction::Xor &&
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isa<ConstantInt>(I->getOperand(1)) &&
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cast<ConstantInt>(I->getOperand(1))->isOne()) {
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ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
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if (Result.empty())
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return false;
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// Invert the known values.
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for (unsigned i = 0, e = Result.size(); i != e; ++i)
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if (Result[i].first)
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Result[i].first =
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cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
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return true;
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}
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}
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// Handle compare with phi operand, where the PHI is defined in this block.
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if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
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PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
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if (PN && PN->getParent() == BB) {
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// We can do this simplification if any comparisons fold to true or false.
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// See if any do.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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BasicBlock *PredBB = PN->getIncomingBlock(i);
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Value *LHS = PN->getIncomingValue(i);
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Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
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Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
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if (Res == 0) {
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if (!LVI || !isa<Constant>(RHS))
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continue;
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LazyValueInfo::Tristate
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ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
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cast<Constant>(RHS), PredBB, BB);
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if (ResT == LazyValueInfo::Unknown)
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continue;
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Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
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}
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if (isa<UndefValue>(Res))
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Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
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else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
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Result.push_back(std::make_pair(CI, PredBB));
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}
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return !Result.empty();
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}
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// If comparing a live-in value against a constant, see if we know the
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// live-in value on any predecessors.
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if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
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Cmp->getType()->isIntegerTy() && // Not vector compare.
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(!isa<Instruction>(Cmp->getOperand(0)) ||
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cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
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Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
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// If the value is known by LazyValueInfo to be a constant in a
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// predecessor, use that information to try to thread this block.
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LazyValueInfo::Tristate
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Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
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RHSCst, *PI, BB);
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if (Res == LazyValueInfo::Unknown)
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continue;
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Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
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Result.push_back(std::make_pair(cast<ConstantInt>(ResC), *PI));
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}
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return !Result.empty();
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}
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}
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return false;
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}
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/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
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/// in an undefined jump, decide which block is best to revector to.
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///
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/// Since we can pick an arbitrary destination, we pick the successor with the
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/// fewest predecessors. This should reduce the in-degree of the others.
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///
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static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
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TerminatorInst *BBTerm = BB->getTerminator();
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unsigned MinSucc = 0;
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BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
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// Compute the successor with the minimum number of predecessors.
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unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
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for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
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TestBB = BBTerm->getSuccessor(i);
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unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
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if (NumPreds < MinNumPreds)
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MinSucc = i;
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}
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return MinSucc;
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}
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/// ProcessBlock - If there are any predecessors whose control can be threaded
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/// through to a successor, transform them now.
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bool JumpThreading::ProcessBlock(BasicBlock *BB) {
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// If the block is trivially dead, just return and let the caller nuke it.
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// This simplifies other transformations.
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if (pred_begin(BB) == pred_end(BB) &&
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BB != &BB->getParent()->getEntryBlock())
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return false;
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// If this block has a single predecessor, and if that pred has a single
|
|
// 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 (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
|
|
SinglePred != BB) {
|
|
// If SinglePred was a loop header, BB becomes one.
|
|
if (LoopHeaders.erase(SinglePred))
|
|
LoopHeaders.insert(BB);
|
|
|
|
// Remember if SinglePred was the entry block of the function. If so, we
|
|
// will need to move BB back to the entry position.
|
|
bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
|
|
MergeBasicBlockIntoOnlyPred(BB);
|
|
|
|
if (isEntry && BB != &BB->getParent()->getEntryBlock())
|
|
BB->moveBefore(&BB->getParent()->getEntryBlock());
|
|
return true;
|
|
}
|
|
}
|
|
|
|
// 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<BranchInst>(BB->getTerminator())) {
|
|
// Can't thread an unconditional jump.
|
|
if (BI->isUnconditional()) return false;
|
|
Condition = BI->getCondition();
|
|
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
|
|
Condition = SI->getCondition();
|
|
else
|
|
return false; // Must be an invoke.
|
|
|
|
// If the terminator of this block is branching on a constant, simplify the
|
|
// terminator to an unconditional branch. This can occur due to threading in
|
|
// other blocks.
|
|
if (isa<ConstantInt>(Condition)) {
|
|
DEBUG(dbgs() << " In block '" << BB->getName()
|
|
<< "' folding terminator: " << *BB->getTerminator() << '\n');
|
|
++NumFolds;
|
|
ConstantFoldTerminator(BB);
|
|
return true;
|
|
}
|
|
|
|
// If the terminator is branching on an undef, we can pick any of the
|
|
// successors to branch to. Let GetBestDestForJumpOnUndef decide.
|
|
if (isa<UndefValue>(Condition)) {
|
|
unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
|
|
|
|
// Fold the branch/switch.
|
|
TerminatorInst *BBTerm = BB->getTerminator();
|
|
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
|
|
if (i == BestSucc) continue;
|
|
RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
|
|
}
|
|
|
|
DEBUG(dbgs() << " In block '" << BB->getName()
|
|
<< "' folding undef terminator: " << *BBTerm << '\n');
|
|
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
|
|
BBTerm->eraseFromParent();
|
|
return true;
|
|
}
|
|
|
|
Instruction *CondInst = dyn_cast<Instruction>(Condition);
|
|
|
|
// If the condition is an instruction defined in another block, see if a
|
|
// predecessor has the same condition:
|
|
// br COND, BBX, BBY
|
|
// BBX:
|
|
// br COND, BBZ, BBW
|
|
if (!LVI &&
|
|
!Condition->hasOneUse() && // Multiple uses.
|
|
(CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
|
|
pred_iterator PI = pred_begin(BB), E = pred_end(BB);
|
|
if (isa<BranchInst>(BB->getTerminator())) {
|
|
for (; PI != E; ++PI)
|
|
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
|
|
if (PBI->isConditional() && PBI->getCondition() == Condition &&
|
|
ProcessBranchOnDuplicateCond(*PI, BB))
|
|
return true;
|
|
} else {
|
|
assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
|
|
for (; PI != E; ++PI)
|
|
if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
|
|
if (PSI->getCondition() == Condition &&
|
|
ProcessSwitchOnDuplicateCond(*PI, BB))
|
|
return true;
|
|
}
|
|
}
|
|
|
|
// All the rest of our checks depend on the condition being an instruction.
|
|
if (CondInst == 0) {
|
|
// FIXME: Unify this with code below.
|
|
if (LVI && ProcessThreadableEdges(Condition, BB))
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
|
|
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
|
|
if (!LVI &&
|
|
(!isa<PHINode>(CondCmp->getOperand(0)) ||
|
|
cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
|
|
// If we have a comparison, loop over the predecessors to see if there is
|
|
// 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<BranchInst>((*PI)->getTerminator()))
|
|
if (PBI->isConditional() && *PI != BB) {
|
|
if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
|
|
if (CI->getOperand(0) == CondCmp->getOperand(0) &&
|
|
CI->getOperand(1) == CondCmp->getOperand(1) &&
|
|
CI->getPredicate() == CondCmp->getPredicate()) {
|
|
// TODO: Could handle things like (x != 4) --> (x == 17)
|
|
if (ProcessBranchOnDuplicateCond(*PI, BB))
|
|
return true;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
// Check for some cases that are worth simplifying. Right now we want to look
|
|
// for loads that are used by a switch or by the condition for the branch. If
|
|
// we see one, check to see if it's partially redundant. If so, insert a PHI
|
|
// which can then be used to thread the values.
|
|
//
|
|
Value *SimplifyValue = CondInst;
|
|
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
|
|
if (isa<Constant>(CondCmp->getOperand(1)))
|
|
SimplifyValue = CondCmp->getOperand(0);
|
|
|
|
// TODO: There are other places where load PRE would be profitable, such as
|
|
// more complex comparisons.
|
|
if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
|
|
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.
|
|
//
|
|
if (ProcessThreadableEdges(CondInst, BB))
|
|
return true;
|
|
|
|
// If this is an otherwise-unfoldable branch on a phi node in the current
|
|
// block, see if we can simplify.
|
|
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
|
|
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
|
|
return ProcessBranchOnPHI(PN);
|
|
|
|
|
|
// If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
|
|
if (CondInst->getOpcode() == Instruction::Xor &&
|
|
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
|
|
return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
|
|
|
|
|
|
// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
|
|
// "(X == 4)", thread through this block.
|
|
|
|
return false;
|
|
}
|
|
|
|
/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
|
|
/// block that jump on exactly the same condition. This means that we almost
|
|
/// always know the direction of the edge in the DESTBB:
|
|
/// PREDBB:
|
|
/// br COND, DESTBB, BBY
|
|
/// DESTBB:
|
|
/// br COND, BBZ, BBW
|
|
///
|
|
/// If DESTBB has multiple predecessors, we can't just constant fold the branch
|
|
/// in DESTBB, we have to thread over it.
|
|
bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
|
|
BasicBlock *BB) {
|
|
BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
|
|
|
|
// If both successors of PredBB go to DESTBB, we don't know anything. We can
|
|
// fold the branch to an unconditional one, which allows other recursive
|
|
// simplifications.
|
|
bool BranchDir;
|
|
if (PredBI->getSuccessor(1) != BB)
|
|
BranchDir = true;
|
|
else if (PredBI->getSuccessor(0) != BB)
|
|
BranchDir = false;
|
|
else {
|
|
DEBUG(dbgs() << " In block '" << PredBB->getName()
|
|
<< "' folding terminator: " << *PredBB->getTerminator() << '\n');
|
|
++NumFolds;
|
|
ConstantFoldTerminator(PredBB);
|
|
return true;
|
|
}
|
|
|
|
BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
|
|
|
|
// If the dest block has one predecessor, just fix the branch condition to a
|
|
// constant and fold it.
|
|
if (BB->getSinglePredecessor()) {
|
|
DEBUG(dbgs() << " In block '" << BB->getName()
|
|
<< "' folding condition to '" << BranchDir << "': "
|
|
<< *BB->getTerminator() << '\n');
|
|
++NumFolds;
|
|
Value *OldCond = DestBI->getCondition();
|
|
DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
|
|
BranchDir));
|
|
// Delete dead instructions before we fold the branch. Folding the branch
|
|
// can eliminate edges from the CFG which can end up deleting OldCond.
|
|
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
|
|
ConstantFoldTerminator(BB);
|
|
return true;
|
|
}
|
|
|
|
|
|
// Next, figure out which successor we are threading to.
|
|
BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
|
|
|
|
SmallVector<BasicBlock*, 2> Preds;
|
|
Preds.push_back(PredBB);
|
|
|
|
// Ok, try to thread it!
|
|
return ThreadEdge(BB, Preds, SuccBB);
|
|
}
|
|
|
|
/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
|
|
/// block that switch on exactly the same condition. This means that we almost
|
|
/// always know the direction of the edge in the DESTBB:
|
|
/// PREDBB:
|
|
/// switch COND [... DESTBB, BBY ... ]
|
|
/// DESTBB:
|
|
/// switch COND [... BBZ, BBW ]
|
|
///
|
|
/// Optimizing switches like this is very important, because simplifycfg builds
|
|
/// switches out of repeated 'if' conditions.
|
|
bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
|
|
BasicBlock *DestBB) {
|
|
// Can't thread edge to self.
|
|
if (PredBB == DestBB)
|
|
return false;
|
|
|
|
SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
|
|
SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
|
|
|
|
// There are a variety of optimizations that we can potentially do on these
|
|
// blocks: we order them from most to least preferable.
|
|
|
|
// If DESTBB *just* contains the switch, then we can forward edges from PREDBB
|
|
// directly to their destination. This does not introduce *any* code size
|
|
// growth. Skip debug info first.
|
|
BasicBlock::iterator BBI = DestBB->begin();
|
|
while (isa<DbgInfoIntrinsic>(BBI))
|
|
BBI++;
|
|
|
|
// FIXME: Thread if it just contains a PHI.
|
|
if (isa<SwitchInst>(BBI)) {
|
|
bool MadeChange = false;
|
|
// Ignore the default edge for now.
|
|
for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
|
|
ConstantInt *DestVal = DestSI->getCaseValue(i);
|
|
BasicBlock *DestSucc = DestSI->getSuccessor(i);
|
|
|
|
// Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
|
|
// PredSI has an explicit case for it. If so, forward. If it is covered
|
|
// by the default case, we can't update PredSI.
|
|
unsigned PredCase = PredSI->findCaseValue(DestVal);
|
|
if (PredCase == 0) continue;
|
|
|
|
// If PredSI doesn't go to DestBB on this value, then it won't reach the
|
|
// case on this condition.
|
|
if (PredSI->getSuccessor(PredCase) != DestBB &&
|
|
DestSI->getSuccessor(i) != DestBB)
|
|
continue;
|
|
|
|
// Do not forward this if it already goes to this destination, this would
|
|
// be an infinite loop.
|
|
if (PredSI->getSuccessor(PredCase) == DestSucc)
|
|
continue;
|
|
|
|
// Otherwise, we're safe to make the change. Make sure that the edge from
|
|
// DestSI to DestSucc is not critical and has no PHI nodes.
|
|
DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
|
|
DEBUG(dbgs() << "THROUGH: " << *DestSI);
|
|
|
|
// If the destination has PHI nodes, just split the edge for updating
|
|
// simplicity.
|
|
if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
|
|
SplitCriticalEdge(DestSI, i, this);
|
|
DestSucc = DestSI->getSuccessor(i);
|
|
}
|
|
FoldSingleEntryPHINodes(DestSucc);
|
|
PredSI->setSuccessor(PredCase, DestSucc);
|
|
MadeChange = true;
|
|
}
|
|
|
|
if (MadeChange)
|
|
return true;
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
|
|
/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
|
|
/// load instruction, eliminate it by replacing it with a PHI node. This is an
|
|
/// important optimization that encourages jump threading, and needs to be run
|
|
/// interlaced with other jump threading tasks.
|
|
bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
|
|
// Don't hack volatile loads.
|
|
if (LI->isVolatile()) return false;
|
|
|
|
// If the load is defined in a block with exactly one predecessor, it can't be
|
|
// partially redundant.
|
|
BasicBlock *LoadBB = LI->getParent();
|
|
if (LoadBB->getSinglePredecessor())
|
|
return false;
|
|
|
|
Value *LoadedPtr = LI->getOperand(0);
|
|
|
|
// If the loaded operand is defined in the LoadBB, it can't be available.
|
|
// TODO: Could do simple PHI translation, that would be fun :)
|
|
if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
|
|
if (PtrOp->getParent() == LoadBB)
|
|
return false;
|
|
|
|
// Scan a few instructions up from the load, to see if it is obviously live at
|
|
// the entry to its block.
|
|
BasicBlock::iterator BBIt = LI;
|
|
|
|
if (Value *AvailableVal =
|
|
FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
|
|
// If the value if the load is locally available within the block, just use
|
|
// it. This frequently occurs for reg2mem'd allocas.
|
|
//cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
|
|
|
|
// If the returned value is the load itself, replace with an undef. This can
|
|
// only happen in dead loops.
|
|
if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
|
|
LI->replaceAllUsesWith(AvailableVal);
|
|
LI->eraseFromParent();
|
|
return true;
|
|
}
|
|
|
|
// Otherwise, if we scanned the whole block and got to the top of the block,
|
|
// we know the block is locally transparent to the load. If not, something
|
|
// might clobber its value.
|
|
if (BBIt != LoadBB->begin())
|
|
return false;
|
|
|
|
|
|
SmallPtrSet<BasicBlock*, 8> PredsScanned;
|
|
typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
|
|
AvailablePredsTy AvailablePreds;
|
|
BasicBlock *OneUnavailablePred = 0;
|
|
|
|
// If we got here, the loaded value is transparent through to the start of the
|
|
// block. Check to see if it is available in any of the predecessor blocks.
|
|
for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
|
|
PI != PE; ++PI) {
|
|
BasicBlock *PredBB = *PI;
|
|
|
|
// If we already scanned this predecessor, skip it.
|
|
if (!PredsScanned.insert(PredBB))
|
|
continue;
|
|
|
|
// Scan the predecessor to see if the value is available in the pred.
|
|
BBIt = PredBB->end();
|
|
Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
|
|
if (!PredAvailable) {
|
|
OneUnavailablePred = PredBB;
|
|
continue;
|
|
}
|
|
|
|
// If so, this load is partially redundant. Remember this info so that we
|
|
// can create a PHI node.
|
|
AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
|
|
}
|
|
|
|
// If the loaded value isn't available in any predecessor, it isn't partially
|
|
// redundant.
|
|
if (AvailablePreds.empty()) return false;
|
|
|
|
// Okay, the loaded value is available in at least one (and maybe all!)
|
|
// predecessors. If the value is unavailable in more than one unique
|
|
// predecessor, we want to insert a merge block for those common predecessors.
|
|
// This ensures that we only have to insert one reload, thus not increasing
|
|
// code size.
|
|
BasicBlock *UnavailablePred = 0;
|
|
|
|
// If there is exactly one predecessor where the value is unavailable, the
|
|
// already computed 'OneUnavailablePred' block is it. If it ends in an
|
|
// unconditional branch, we know that it isn't a critical edge.
|
|
if (PredsScanned.size() == AvailablePreds.size()+1 &&
|
|
OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
|
|
UnavailablePred = OneUnavailablePred;
|
|
} else if (PredsScanned.size() != AvailablePreds.size()) {
|
|
// Otherwise, we had multiple unavailable predecessors or we had a critical
|
|
// edge from the one.
|
|
SmallVector<BasicBlock*, 8> PredsToSplit;
|
|
SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
|
|
|
|
for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
|
|
AvailablePredSet.insert(AvailablePreds[i].first);
|
|
|
|
// Add all the unavailable predecessors to the PredsToSplit list.
|
|
for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
|
|
PI != PE; ++PI)
|
|
if (!AvailablePredSet.count(*PI))
|
|
PredsToSplit.push_back(*PI);
|
|
|
|
// Split them out to their own block.
|
|
UnavailablePred =
|
|
SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
|
|
"thread-pre-split", this);
|
|
}
|
|
|
|
// If the value isn't available in all predecessors, then there will be
|
|
// exactly one where it isn't available. Insert a load on that edge and add
|
|
// it to the AvailablePreds list.
|
|
if (UnavailablePred) {
|
|
assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
|
|
"Can't handle critical edge here!");
|
|
Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
|
|
LI->getAlignment(),
|
|
UnavailablePred->getTerminator());
|
|
AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
|
|
}
|
|
|
|
// Now we know that each predecessor of this block has a value in
|
|
// AvailablePreds, sort them for efficient access as we're walking the preds.
|
|
array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
|
|
|
|
// Create a PHI node at the start of the block for the PRE'd load value.
|
|
PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
|
|
PN->takeName(LI);
|
|
|
|
// Insert new entries into the PHI for each predecessor. A single block may
|
|
// have multiple entries here.
|
|
for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
|
|
++PI) {
|
|
AvailablePredsTy::iterator I =
|
|
std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
|
|
std::make_pair(*PI, (Value*)0));
|
|
|
|
assert(I != AvailablePreds.end() && I->first == *PI &&
|
|
"Didn't find entry for predecessor!");
|
|
|
|
PN->addIncoming(I->second, I->first);
|
|
}
|
|
|
|
//cerr << "PRE: " << *LI << *PN << "\n";
|
|
|
|
LI->replaceAllUsesWith(PN);
|
|
LI->eraseFromParent();
|
|
|
|
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<std::pair<BasicBlock*,
|
|
BasicBlock*> > &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<BasicBlock*, unsigned> 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<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
|
|
BasicBlock *MostPopularDest = DPI->first;
|
|
unsigned Popularity = DPI->second;
|
|
SmallVector<BasicBlock*, 4> 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(Value *Cond, 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<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
|
|
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
|
|
return false;
|
|
assert(!PredValues.empty() &&
|
|
"ComputeValueKnownInPredecessors returned true with no values");
|
|
|
|
DEBUG(dbgs() << "IN BB: " << *BB;
|
|
for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
|
|
dbgs() << " BB '" << BB->getName() << "': FOUND condition = ";
|
|
if (PredValues[i].first)
|
|
dbgs() << *PredValues[i].first;
|
|
else
|
|
dbgs() << "UNDEF";
|
|
dbgs() << " 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<BasicBlock*, 16> SeenPreds;
|
|
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 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<IndirectBrInst>(Pred->getTerminator()))
|
|
continue;
|
|
|
|
ConstantInt *Val = PredValues[i].first;
|
|
|
|
BasicBlock *DestBB;
|
|
if (Val == 0) // Undef.
|
|
DestBB = 0;
|
|
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
|
|
DestBB = BI->getSuccessor(Val->isZero());
|
|
else {
|
|
SwitchInst *SI = cast<SwitchInst>(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<BasicBlock*, 16> PredsToFactor;
|
|
for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
|
|
if (PredToDestList[i].second == MostPopularDest) {
|
|
BasicBlock *Pred = PredToDestList[i].first;
|
|
|
|
// This predecessor may be a switch or something else that has multiple
|
|
// edges to the block. Factor each of these edges by listing them
|
|
// according to # occurrences in PredsToFactor.
|
|
TerminatorInst *PredTI = Pred->getTerminator();
|
|
for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
|
|
if (PredTI->getSuccessor(i) == BB)
|
|
PredsToFactor.push_back(Pred);
|
|
}
|
|
|
|
// 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, PredsToFactor, MostPopularDest);
|
|
}
|
|
|
|
/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
|
|
/// a PHI node in the current block. See if there are any simplifications we
|
|
/// can do based on inputs to the phi node.
|
|
///
|
|
bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
|
|
BasicBlock *BB = PN->getParent();
|
|
|
|
// TODO: We could make use of this to do it once for blocks with common PHI
|
|
// values.
|
|
SmallVector<BasicBlock*, 1> PredBBs;
|
|
PredBBs.resize(1);
|
|
|
|
// If any of the predecessor blocks end in an unconditional branch, we can
|
|
// *duplicate* the conditional branch into that block in order to further
|
|
// encourage jump threading and to eliminate cases where we have branch on a
|
|
// phi of an icmp (branch on icmp is much better).
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
BasicBlock *PredBB = PN->getIncomingBlock(i);
|
|
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
|
|
if (PredBr->isUnconditional()) {
|
|
PredBBs[0] = PredBB;
|
|
// Try to duplicate BB into PredBB.
|
|
if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
|
|
return true;
|
|
}
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
|
|
/// a xor instruction in the current block. See if there are any
|
|
/// simplifications we can do based on inputs to the xor.
|
|
///
|
|
bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
|
|
BasicBlock *BB = BO->getParent();
|
|
|
|
// If either the LHS or RHS of the xor is a constant, don't do this
|
|
// optimization.
|
|
if (isa<ConstantInt>(BO->getOperand(0)) ||
|
|
isa<ConstantInt>(BO->getOperand(1)))
|
|
return false;
|
|
|
|
// If the first instruction in BB isn't a phi, we won't be able to infer
|
|
// anything special about any particular predecessor.
|
|
if (!isa<PHINode>(BB->front()))
|
|
return false;
|
|
|
|
// If we have a xor as the branch input to this block, and we know that the
|
|
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
|
|
// the condition into the predecessor and fix that value to true, saving some
|
|
// logical ops on that path and encouraging other paths to simplify.
|
|
//
|
|
// This copies something like this:
|
|
//
|
|
// BB:
|
|
// %X = phi i1 [1], [%X']
|
|
// %Y = icmp eq i32 %A, %B
|
|
// %Z = xor i1 %X, %Y
|
|
// br i1 %Z, ...
|
|
//
|
|
// Into:
|
|
// BB':
|
|
// %Y = icmp ne i32 %A, %B
|
|
// br i1 %Z, ...
|
|
|
|
SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
|
|
bool isLHS = true;
|
|
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
|
|
assert(XorOpValues.empty());
|
|
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
|
|
return false;
|
|
isLHS = false;
|
|
}
|
|
|
|
assert(!XorOpValues.empty() &&
|
|
"ComputeValueKnownInPredecessors returned true with no values");
|
|
|
|
// Scan the information to see which is most popular: true or false. The
|
|
// predecessors can be of the set true, false, or undef.
|
|
unsigned NumTrue = 0, NumFalse = 0;
|
|
for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
|
|
if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
|
|
if (XorOpValues[i].first->isZero())
|
|
++NumFalse;
|
|
else
|
|
++NumTrue;
|
|
}
|
|
|
|
// Determine which value to split on, true, false, or undef if neither.
|
|
ConstantInt *SplitVal = 0;
|
|
if (NumTrue > NumFalse)
|
|
SplitVal = ConstantInt::getTrue(BB->getContext());
|
|
else if (NumTrue != 0 || NumFalse != 0)
|
|
SplitVal = ConstantInt::getFalse(BB->getContext());
|
|
|
|
// Collect all of the blocks that this can be folded into so that we can
|
|
// factor this once and clone it once.
|
|
SmallVector<BasicBlock*, 8> BlocksToFoldInto;
|
|
for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
|
|
if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
|
|
|
|
BlocksToFoldInto.push_back(XorOpValues[i].second);
|
|
}
|
|
|
|
// If we inferred a value for all of the predecessors, then duplication won't
|
|
// help us. However, we can just replace the LHS or RHS with the constant.
|
|
if (BlocksToFoldInto.size() ==
|
|
cast<PHINode>(BB->front()).getNumIncomingValues()) {
|
|
if (SplitVal == 0) {
|
|
// If all preds provide undef, just nuke the xor, because it is undef too.
|
|
BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
|
|
BO->eraseFromParent();
|
|
} else if (SplitVal->isZero()) {
|
|
// If all preds provide 0, replace the xor with the other input.
|
|
BO->replaceAllUsesWith(BO->getOperand(isLHS));
|
|
BO->eraseFromParent();
|
|
} else {
|
|
// If all preds provide 1, set the computed value to 1.
|
|
BO->setOperand(!isLHS, SplitVal);
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
// Try to duplicate BB into PredBB.
|
|
return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
|
|
}
|
|
|
|
|
|
/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
|
|
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
|
|
/// NewPred using the entries from OldPred (suitably mapped).
|
|
static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
|
|
BasicBlock *OldPred,
|
|
BasicBlock *NewPred,
|
|
DenseMap<Instruction*, Value*> &ValueMap) {
|
|
for (BasicBlock::iterator PNI = PHIBB->begin();
|
|
PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
|
|
// Ok, we have a PHI node. Figure out what the incoming value was for the
|
|
// DestBlock.
|
|
Value *IV = PN->getIncomingValueForBlock(OldPred);
|
|
|
|
// Remap the value if necessary.
|
|
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
|
|
DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
|
|
if (I != ValueMap.end())
|
|
IV = I->second;
|
|
}
|
|
|
|
PN->addIncoming(IV, NewPred);
|
|
}
|
|
}
|
|
|
|
/// ThreadEdge - We have decided that it is safe and profitable to factor the
|
|
/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
|
|
/// across BB. Transform the IR to reflect this change.
|
|
bool JumpThreading::ThreadEdge(BasicBlock *BB,
|
|
const SmallVectorImpl<BasicBlock*> &PredBBs,
|
|
BasicBlock *SuccBB) {
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
|
|
<< "' - would thread to self!\n");
|
|
return false;
|
|
}
|
|
|
|
// If threading this would thread across a loop header, don't thread the edge.
|
|
// See the comments above FindLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB)) {
|
|
DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
|
|
<< "' to dest BB '" << SuccBB->getName()
|
|
<< "' - it might create an irreducible loop!\n");
|
|
return false;
|
|
}
|
|
|
|
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
|
|
if (JumpThreadCost > Threshold) {
|
|
DEBUG(dbgs() << " Not threading BB '" << BB->getName()
|
|
<< "' - Cost is too high: " << JumpThreadCost << "\n");
|
|
return false;
|
|
}
|
|
|
|
// And finally, do it! Start by factoring the predecessors is needed.
|
|
BasicBlock *PredBB;
|
|
if (PredBBs.size() == 1)
|
|
PredBB = PredBBs[0];
|
|
else {
|
|
DEBUG(dbgs() << " Factoring out " << PredBBs.size()
|
|
<< " common predecessors.\n");
|
|
PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
|
|
".thr_comm", this);
|
|
}
|
|
|
|
// And finally, do it!
|
|
DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
|
|
<< SuccBB->getName() << "' with cost: " << JumpThreadCost
|
|
<< ", across block:\n "
|
|
<< *BB << "\n");
|
|
|
|
// We are going to have to map operands from the original BB block to the new
|
|
// copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
|
|
// account for entry from PredBB.
|
|
DenseMap<Instruction*, Value*> ValueMapping;
|
|
|
|
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
|
|
BB->getName()+".thread",
|
|
BB->getParent(), BB);
|
|
NewBB->moveAfter(PredBB);
|
|
|
|
BasicBlock::iterator BI = BB->begin();
|
|
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
|
|
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
|
|
|
|
// Clone the non-phi instructions of BB into NewBB, keeping track of the
|
|
// mapping and using it to remap operands in the cloned instructions.
|
|
for (; !isa<TerminatorInst>(BI); ++BI) {
|
|
Instruction *New = BI->clone();
|
|
New->setName(BI->getName());
|
|
NewBB->getInstList().push_back(New);
|
|
ValueMapping[BI] = New;
|
|
|
|
// Remap operands to patch up intra-block references.
|
|
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
|
|
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
|
|
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
|
|
if (I != ValueMapping.end())
|
|
New->setOperand(i, I->second);
|
|
}
|
|
}
|
|
|
|
// We didn't copy the terminator from BB over to NewBB, because there is now
|
|
// an unconditional jump to SuccBB. Insert the unconditional jump.
|
|
BranchInst::Create(SuccBB, NewBB);
|
|
|
|
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
|
|
// PHI nodes for NewBB now.
|
|
AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
|
|
|
|
// If there were values defined in BB that are used outside the block, then we
|
|
// now have to update all uses of the value to use either the original value,
|
|
// the cloned value, or some PHI derived value. This can require arbitrary
|
|
// PHI insertion, of which we are prepared to do, clean these up now.
|
|
SSAUpdater SSAUpdate;
|
|
SmallVector<Use*, 16> UsesToRename;
|
|
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
|
|
// Scan all uses of this instruction to see if it is used outside of its
|
|
// block, and if so, record them in UsesToRename.
|
|
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
|
|
++UI) {
|
|
Instruction *User = cast<Instruction>(*UI);
|
|
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
|
|
if (UserPN->getIncomingBlock(UI) == BB)
|
|
continue;
|
|
} else if (User->getParent() == BB)
|
|
continue;
|
|
|
|
UsesToRename.push_back(&UI.getUse());
|
|
}
|
|
|
|
// If there are no uses outside the block, we're done with this instruction.
|
|
if (UsesToRename.empty())
|
|
continue;
|
|
|
|
DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
|
|
|
|
// We found a use of I outside of BB. Rename all uses of I that are outside
|
|
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
|
|
// with the two values we know.
|
|
SSAUpdate.Initialize(I);
|
|
SSAUpdate.AddAvailableValue(BB, I);
|
|
SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
|
|
|
|
while (!UsesToRename.empty())
|
|
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
|
|
DEBUG(dbgs() << "\n");
|
|
}
|
|
|
|
|
|
// Ok, NewBB is good to go. Update the terminator of PredBB to jump to
|
|
// NewBB instead of BB. This eliminates predecessors from BB, which requires
|
|
// us to simplify any PHI nodes in BB.
|
|
TerminatorInst *PredTerm = PredBB->getTerminator();
|
|
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
|
|
if (PredTerm->getSuccessor(i) == BB) {
|
|
RemovePredecessorAndSimplify(BB, PredBB, TD);
|
|
PredTerm->setSuccessor(i, NewBB);
|
|
}
|
|
|
|
// At this point, the IR is fully up to date and consistent. Do a quick scan
|
|
// over the new instructions and zap any that are constants or dead. This
|
|
// frequently happens because of phi translation.
|
|
SimplifyInstructionsInBlock(NewBB, TD);
|
|
|
|
// Threaded an edge!
|
|
++NumThreads;
|
|
return true;
|
|
}
|
|
|
|
/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
|
|
/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
|
|
/// If we can duplicate the contents of BB up into PredBB do so now, this
|
|
/// improves the odds that the branch will be on an analyzable instruction like
|
|
/// a compare.
|
|
bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
|
|
const SmallVectorImpl<BasicBlock *> &PredBBs) {
|
|
assert(!PredBBs.empty() && "Can't handle an empty set");
|
|
|
|
// If BB is a loop header, then duplicating this block outside the loop would
|
|
// cause us to transform this into an irreducible loop, don't do this.
|
|
// See the comments above FindLoopHeaders for justifications and caveats.
|
|
if (LoopHeaders.count(BB)) {
|
|
DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
|
|
<< "' into predecessor block '" << PredBBs[0]->getName()
|
|
<< "' - it might create an irreducible loop!\n");
|
|
return false;
|
|
}
|
|
|
|
unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
|
|
if (DuplicationCost > Threshold) {
|
|
DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
|
|
<< "' - Cost is too high: " << DuplicationCost << "\n");
|
|
return false;
|
|
}
|
|
|
|
// And finally, do it! Start by factoring the predecessors is needed.
|
|
BasicBlock *PredBB;
|
|
if (PredBBs.size() == 1)
|
|
PredBB = PredBBs[0];
|
|
else {
|
|
DEBUG(dbgs() << " Factoring out " << PredBBs.size()
|
|
<< " common predecessors.\n");
|
|
PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
|
|
".thr_comm", this);
|
|
}
|
|
|
|
// Okay, we decided to do this! Clone all the instructions in BB onto the end
|
|
// of PredBB.
|
|
DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
|
|
<< PredBB->getName() << "' to eliminate branch on phi. Cost: "
|
|
<< DuplicationCost << " block is:" << *BB << "\n");
|
|
|
|
// Unless PredBB ends with an unconditional branch, split the edge so that we
|
|
// can just clone the bits from BB into the end of the new PredBB.
|
|
BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
|
|
|
|
if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
|
|
PredBB = SplitEdge(PredBB, BB, this);
|
|
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
|
|
}
|
|
|
|
// We are going to have to map operands from the original BB block into the
|
|
// PredBB block. Evaluate PHI nodes in BB.
|
|
DenseMap<Instruction*, Value*> ValueMapping;
|
|
|
|
BasicBlock::iterator BI = BB->begin();
|
|
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
|
|
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
|
|
|
|
// Clone the non-phi instructions of BB into PredBB, keeping track of the
|
|
// mapping and using it to remap operands in the cloned instructions.
|
|
for (; BI != BB->end(); ++BI) {
|
|
Instruction *New = BI->clone();
|
|
|
|
// Remap operands to patch up intra-block references.
|
|
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
|
|
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
|
|
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
|
|
if (I != ValueMapping.end())
|
|
New->setOperand(i, I->second);
|
|
}
|
|
|
|
// If this instruction can be simplified after the operands are updated,
|
|
// just use the simplified value instead. This frequently happens due to
|
|
// phi translation.
|
|
if (Value *IV = SimplifyInstruction(New, TD)) {
|
|
delete New;
|
|
ValueMapping[BI] = IV;
|
|
} else {
|
|
// Otherwise, insert the new instruction into the block.
|
|
New->setName(BI->getName());
|
|
PredBB->getInstList().insert(OldPredBranch, New);
|
|
ValueMapping[BI] = New;
|
|
}
|
|
}
|
|
|
|
// Check to see if the targets of the branch had PHI nodes. If so, we need to
|
|
// add entries to the PHI nodes for branch from PredBB now.
|
|
BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
|
|
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
|
|
ValueMapping);
|
|
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
|
|
ValueMapping);
|
|
|
|
// If there were values defined in BB that are used outside the block, then we
|
|
// now have to update all uses of the value to use either the original value,
|
|
// the cloned value, or some PHI derived value. This can require arbitrary
|
|
// PHI insertion, of which we are prepared to do, clean these up now.
|
|
SSAUpdater SSAUpdate;
|
|
SmallVector<Use*, 16> UsesToRename;
|
|
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
|
|
// Scan all uses of this instruction to see if it is used outside of its
|
|
// block, and if so, record them in UsesToRename.
|
|
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
|
|
++UI) {
|
|
Instruction *User = cast<Instruction>(*UI);
|
|
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
|
|
if (UserPN->getIncomingBlock(UI) == BB)
|
|
continue;
|
|
} else if (User->getParent() == BB)
|
|
continue;
|
|
|
|
UsesToRename.push_back(&UI.getUse());
|
|
}
|
|
|
|
// If there are no uses outside the block, we're done with this instruction.
|
|
if (UsesToRename.empty())
|
|
continue;
|
|
|
|
DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
|
|
|
|
// We found a use of I outside of BB. Rename all uses of I that are outside
|
|
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
|
|
// with the two values we know.
|
|
SSAUpdate.Initialize(I);
|
|
SSAUpdate.AddAvailableValue(BB, I);
|
|
SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
|
|
|
|
while (!UsesToRename.empty())
|
|
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
|
|
DEBUG(dbgs() << "\n");
|
|
}
|
|
|
|
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
|
|
// that we nuked.
|
|
RemovePredecessorAndSimplify(BB, PredBB, TD);
|
|
|
|
// Remove the unconditional branch at the end of the PredBB block.
|
|
OldPredBranch->eraseFromParent();
|
|
|
|
++NumDupes;
|
|
return true;
|
|
}
|
|
|
|
|