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jump threading can have bugs, who knew? ;-) git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61983 91177308-0d34-0410-b5e6-96231b3b80d8
934 lines
35 KiB
C++
934 lines
35 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/Pass.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/Analysis/ConstantFolding.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/Target/TargetData.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/SmallPtrSet.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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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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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 VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
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TargetData *TD;
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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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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<TargetData>();
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}
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bool runOnFunction(Function &F);
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bool ProcessBlock(BasicBlock *BB);
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void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
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BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal);
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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 ProcessJumpOnPHI(PHINode *PN);
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bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
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bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
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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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DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
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TD = &getAnalysis<TargetData>();
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bool AnotherIteration = true, EverChanged = false;
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while (AnotherIteration) {
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AnotherIteration = false;
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bool 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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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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DOUT << " JT: Deleting dead block '" << BB->getNameStart()
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<< "' with terminator: " << *BB->getTerminator();
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DeleteDeadBlock(BB);
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Changed = true;
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}
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}
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AnotherIteration = Changed;
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EverChanged |= Changed;
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}
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return EverChanged;
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}
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/// FactorCommonPHIPreds - If there are multiple preds with the same incoming
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/// value for the PHI, factor them together so we get one block to thread for
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/// the whole group.
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/// This is important for things like "phi i1 [true, true, false, true, x]"
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/// where we only need to clone the block for the true blocks once.
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///
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BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) {
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SmallVector<BasicBlock*, 16> CommonPreds;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == CstVal)
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CommonPreds.push_back(PN->getIncomingBlock(i));
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if (CommonPreds.size() == 1)
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return CommonPreds[0];
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DOUT << " Factoring out " << CommonPreds.size()
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<< " common predecessors.\n";
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return SplitBlockPredecessors(PN->getParent(),
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&CommonPreds[0], CommonPreds.size(),
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".thr_comm", this);
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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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// 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) && isa<PointerType>(I->getType()))
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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 (isa<VectorType>(CI->getType()))
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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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/// 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 this block has a single predecessor, and if that pred has a single
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// successor, merge the blocks. This encourages recursive jump threading
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// because now the condition in this block can be threaded through
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// predecessors of our predecessor block.
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if (BasicBlock *SinglePred = BB->getSinglePredecessor())
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if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
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SinglePred != BB) {
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// Remember if SinglePred was the entry block of the function. If so, we
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// will need to move BB back to the entry position.
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bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
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MergeBasicBlockIntoOnlyPred(BB);
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if (isEntry && BB != &BB->getParent()->getEntryBlock())
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BB->moveBefore(&BB->getParent()->getEntryBlock());
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return true;
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}
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// See if this block ends with a branch or switch. If so, see if the
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// condition is a phi node. If so, and if an entry of the phi node is a
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// constant, we can thread the block.
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Value *Condition;
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if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
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// Can't thread an unconditional jump.
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if (BI->isUnconditional()) return false;
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Condition = BI->getCondition();
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
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Condition = SI->getCondition();
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else
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return false; // Must be an invoke.
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// If the terminator of this block is branching on a constant, simplify the
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// terminator to an unconditional branch. This can occur due to threading in
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// other blocks.
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if (isa<ConstantInt>(Condition)) {
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DOUT << " In block '" << BB->getNameStart()
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<< "' folding terminator: " << *BB->getTerminator();
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++NumFolds;
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ConstantFoldTerminator(BB);
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return true;
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}
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// If the terminator is branching on an undef, we can pick any of the
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// successors to branch to. Since this is arbitrary, we pick the successor
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// with the fewest predecessors. This should reduce the in-degree of the
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// others.
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if (isa<UndefValue>(Condition)) {
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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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// Fold the branch/switch.
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for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
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if (i == MinSucc) continue;
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BBTerm->getSuccessor(i)->removePredecessor(BB);
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}
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DOUT << " In block '" << BB->getNameStart()
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<< "' folding undef terminator: " << *BBTerm;
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BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm);
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BBTerm->eraseFromParent();
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return true;
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}
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Instruction *CondInst = dyn_cast<Instruction>(Condition);
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// If the condition is an instruction defined in another block, see if a
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// predecessor has the same condition:
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// br COND, BBX, BBY
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// BBX:
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// br COND, BBZ, BBW
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if (!Condition->hasOneUse() && // Multiple uses.
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(CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
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pred_iterator PI = pred_begin(BB), E = pred_end(BB);
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if (isa<BranchInst>(BB->getTerminator())) {
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for (; PI != E; ++PI)
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if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
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if (PBI->isConditional() && PBI->getCondition() == Condition &&
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ProcessBranchOnDuplicateCond(*PI, BB))
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return true;
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} else {
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assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
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for (; PI != E; ++PI)
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if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
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if (PSI->getCondition() == Condition &&
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ProcessSwitchOnDuplicateCond(*PI, BB))
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return true;
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}
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}
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// If there is only a single predecessor of this block, nothing to fold.
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if (BB->getSinglePredecessor())
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return false;
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// All the rest of our checks depend on the condition being an instruction.
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if (CondInst == 0)
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return false;
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// See if this is a phi node in the current block.
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if (PHINode *PN = dyn_cast<PHINode>(CondInst))
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if (PN->getParent() == BB)
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return ProcessJumpOnPHI(PN);
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// If this is a conditional branch whose condition is and/or of a phi, try to
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// simplify it.
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if ((CondInst->getOpcode() == Instruction::And ||
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CondInst->getOpcode() == Instruction::Or) &&
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isa<BranchInst>(BB->getTerminator()) &&
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ProcessBranchOnLogical(CondInst, BB,
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CondInst->getOpcode() == Instruction::And))
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return true;
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// If we have "br (phi != 42)" and the phi node has any constant values as
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// operands, we can thread through this block.
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if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst))
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if (isa<PHINode>(CondCmp->getOperand(0)) &&
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isa<Constant>(CondCmp->getOperand(1)) &&
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ProcessBranchOnCompare(CondCmp, BB))
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return true;
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// Check for some cases that are worth simplifying. Right now we want to look
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// for loads that are used by a switch or by the condition for the branch. If
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// we see one, check to see if it's partially redundant. If so, insert a PHI
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// which can then be used to thread the values.
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//
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// This is particularly important because reg2mem inserts loads and stores all
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// over the place, and this blocks jump threading if we don't zap them.
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Value *SimplifyValue = CondInst;
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if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
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if (isa<Constant>(CondCmp->getOperand(1)))
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SimplifyValue = CondCmp->getOperand(0);
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if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
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if (SimplifyPartiallyRedundantLoad(LI))
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return true;
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// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
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// "(X == 4)" thread through this block.
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return false;
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}
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/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
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/// block that jump on exactly the same condition. This means that we almost
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/// always know the direction of the edge in the DESTBB:
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/// PREDBB:
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/// br COND, DESTBB, BBY
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/// DESTBB:
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/// br COND, BBZ, BBW
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///
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/// If DESTBB has multiple predecessors, we can't just constant fold the branch
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/// in DESTBB, we have to thread over it.
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bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
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BasicBlock *BB) {
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BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
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// If both successors of PredBB go to DESTBB, we don't know anything. We can
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// fold the branch to an unconditional one, which allows other recursive
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// simplifications.
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bool BranchDir;
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if (PredBI->getSuccessor(1) != BB)
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BranchDir = true;
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else if (PredBI->getSuccessor(0) != BB)
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BranchDir = false;
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else {
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DOUT << " In block '" << PredBB->getNameStart()
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<< "' folding terminator: " << *PredBB->getTerminator();
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++NumFolds;
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ConstantFoldTerminator(PredBB);
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return true;
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}
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BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
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// If the dest block has one predecessor, just fix the branch condition to a
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// constant and fold it.
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if (BB->getSinglePredecessor()) {
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DOUT << " In block '" << BB->getNameStart()
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<< "' folding condition to '" << BranchDir << "': "
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<< *BB->getTerminator();
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++NumFolds;
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DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir));
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ConstantFoldTerminator(BB);
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return true;
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}
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// Otherwise we need to thread from PredBB to DestBB's successor which
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// involves code duplication. Check to see if it is worth it.
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unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
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if (JumpThreadCost > Threshold) {
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DOUT << " Not threading BB '" << BB->getNameStart()
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<< "' - Cost is too high: " << JumpThreadCost << "\n";
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return false;
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}
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// Next, figure out which successor we are threading to.
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BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
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// If threading to the same block as we come from, we would infinite loop.
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if (SuccBB == BB) {
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DOUT << " Not threading BB '" << BB->getNameStart()
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<< "' - would thread to self!\n";
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return false;
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}
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// And finally, do it!
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DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
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<< SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
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<< ", across block:\n "
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<< *BB << "\n";
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ThreadEdge(BB, PredBB, SuccBB);
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++NumThreads;
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return true;
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}
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/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
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/// block that switch on exactly the same condition. This means that we almost
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/// always know the direction of the edge in the DESTBB:
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/// PREDBB:
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/// switch COND [... DESTBB, BBY ... ]
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/// DESTBB:
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/// switch COND [... BBZ, BBW ]
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///
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/// Optimizing switches like this is very important, because simplifycfg builds
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/// switches out of repeated 'if' conditions.
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bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
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BasicBlock *DestBB) {
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SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
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SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
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// There are a variety of optimizations that we can potentially do on these
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// blocks: we order them from most to least preferable.
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// If DESTBB *just* contains the switch, then we can forward edges from PREDBB
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// directly to their destination. This does not introduce *any* code size
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// growth.
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// FIXME: Thread if it just contains a PHI.
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if (isa<SwitchInst>(DestBB->begin())) {
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bool MadeChange = false;
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// Ignore the default edge for now.
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for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
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ConstantInt *DestVal = DestSI->getCaseValue(i);
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BasicBlock *DestSucc = DestSI->getSuccessor(i);
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// Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
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// PredSI has an explicit case for it. If so, forward. If it is covered
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// by the default case, we can't update PredSI.
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unsigned PredCase = PredSI->findCaseValue(DestVal);
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if (PredCase == 0) continue;
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// If PredSI doesn't go to DestBB on this value, then it won't reach the
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// case on this condition.
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if (PredSI->getSuccessor(PredCase) != DestBB &&
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DestSI->getSuccessor(i) != DestBB)
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continue;
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// Otherwise, we're safe to make the change. Make sure that the edge from
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// DestSI to DestSucc is not critical and has no PHI nodes.
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DOUT << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI;
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DOUT << "THROUGH: " << *DestSI;
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// If the destination has PHI nodes, just split the edge for updating
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// simplicity.
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if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
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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.
|
|
// FIXME: Could do 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-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",
|
|
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;
|
|
}
|
|
|
|
|
|
/// ProcessJumpOnPHI - We have a conditional branch of switch 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::ProcessJumpOnPHI(PHINode *PN) {
|
|
// See if the phi node has any constant values. If so, we can determine where
|
|
// the corresponding predecessor will branch.
|
|
ConstantInt *PredCst = 0;
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
|
|
if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
|
|
break;
|
|
|
|
// If no incoming value has a constant, we don't know the destination of any
|
|
// predecessors.
|
|
if (PredCst == 0)
|
|
return false;
|
|
|
|
// See if the cost of duplicating this block is low enough.
|
|
BasicBlock *BB = PN->getParent();
|
|
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
|
|
if (JumpThreadCost > Threshold) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - Cost is too high: " << JumpThreadCost << "\n";
|
|
return false;
|
|
}
|
|
|
|
// If so, we can actually do this threading. Merge any common predecessors
|
|
// that will act the same.
|
|
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
|
|
|
|
// Next, figure out which successor we are threading to.
|
|
BasicBlock *SuccBB;
|
|
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
|
|
SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse());
|
|
else {
|
|
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
|
|
SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
|
|
}
|
|
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - would thread to self!\n";
|
|
return false;
|
|
}
|
|
|
|
// And finally, do it!
|
|
DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
|
|
<< SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
|
|
<< ", across block:\n "
|
|
<< *BB << "\n";
|
|
|
|
ThreadEdge(BB, PredBB, SuccBB);
|
|
++NumThreads;
|
|
return true;
|
|
}
|
|
|
|
/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch
|
|
/// whose condition is an AND/OR where one side is PN. If PN has constant
|
|
/// operands that permit us to evaluate the condition for some operand, thread
|
|
/// through the block. For example with:
|
|
/// br (and X, phi(Y, Z, false))
|
|
/// the predecessor corresponding to the 'false' will always jump to the false
|
|
/// destination of the branch.
|
|
///
|
|
bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
|
|
bool isAnd) {
|
|
// If this is a binary operator tree of the same AND/OR opcode, check the
|
|
// LHS/RHS.
|
|
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V))
|
|
if ((isAnd && BO->getOpcode() == Instruction::And) ||
|
|
(!isAnd && BO->getOpcode() == Instruction::Or)) {
|
|
if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd))
|
|
return true;
|
|
if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd))
|
|
return true;
|
|
}
|
|
|
|
// If this isn't a PHI node, we can't handle it.
|
|
PHINode *PN = dyn_cast<PHINode>(V);
|
|
if (!PN || PN->getParent() != BB) return false;
|
|
|
|
// We can only do the simplification for phi nodes of 'false' with AND or
|
|
// 'true' with OR. See if we have any entries in the phi for this.
|
|
unsigned PredNo = ~0U;
|
|
ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd);
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
if (PN->getIncomingValue(i) == PredCst) {
|
|
PredNo = i;
|
|
break;
|
|
}
|
|
}
|
|
|
|
// If no match, bail out.
|
|
if (PredNo == ~0U)
|
|
return false;
|
|
|
|
// See if the cost of duplicating this block is low enough.
|
|
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
|
|
if (JumpThreadCost > Threshold) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - Cost is too high: " << JumpThreadCost << "\n";
|
|
return false;
|
|
}
|
|
|
|
// If so, we can actually do this threading. Merge any common predecessors
|
|
// that will act the same.
|
|
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
|
|
|
|
// Next, figure out which successor we are threading to. If this was an AND,
|
|
// the constant must be FALSE, and we must be targeting the 'false' block.
|
|
// If this is an OR, the constant must be TRUE, and we must be targeting the
|
|
// 'true' block.
|
|
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
|
|
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - would thread to self!\n";
|
|
return false;
|
|
}
|
|
|
|
// And finally, do it!
|
|
DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
|
|
<< "' to '" << SuccBB->getNameStart() << "' with cost: "
|
|
<< JumpThreadCost << ", across block:\n "
|
|
<< *BB << "\n";
|
|
|
|
ThreadEdge(BB, PredBB, SuccBB);
|
|
++NumThreads;
|
|
return true;
|
|
}
|
|
|
|
/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
|
|
/// node and a constant. If the PHI node contains any constants as inputs, we
|
|
/// can fold the compare for that edge and thread through it.
|
|
bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
|
|
PHINode *PN = cast<PHINode>(Cmp->getOperand(0));
|
|
Constant *RHS = cast<Constant>(Cmp->getOperand(1));
|
|
|
|
// If the phi isn't in the current block, an incoming edge to this block
|
|
// doesn't control the destination.
|
|
if (PN->getParent() != BB)
|
|
return false;
|
|
|
|
// We can do this simplification if any comparisons fold to true or false.
|
|
// See if any do.
|
|
Constant *PredCst = 0;
|
|
bool TrueDirection = false;
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
PredCst = dyn_cast<Constant>(PN->getIncomingValue(i));
|
|
if (PredCst == 0) continue;
|
|
|
|
Constant *Res;
|
|
if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp))
|
|
Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS);
|
|
else
|
|
Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(),
|
|
PredCst, RHS);
|
|
// If this folded to a constant expr, we can't do anything.
|
|
if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) {
|
|
TrueDirection = ResC->getZExtValue();
|
|
break;
|
|
}
|
|
// If this folded to undef, just go the false way.
|
|
if (isa<UndefValue>(Res)) {
|
|
TrueDirection = false;
|
|
break;
|
|
}
|
|
|
|
// Otherwise, we can't fold this input.
|
|
PredCst = 0;
|
|
}
|
|
|
|
// If no match, bail out.
|
|
if (PredCst == 0)
|
|
return false;
|
|
|
|
// See if the cost of duplicating this block is low enough.
|
|
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
|
|
if (JumpThreadCost > Threshold) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - Cost is too high: " << JumpThreadCost << "\n";
|
|
return false;
|
|
}
|
|
|
|
// If so, we can actually do this threading. Merge any common predecessors
|
|
// that will act the same.
|
|
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
|
|
|
|
// Next, get our successor.
|
|
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
|
|
|
|
// If threading to the same block as we come from, we would infinite loop.
|
|
if (SuccBB == BB) {
|
|
DOUT << " Not threading BB '" << BB->getNameStart()
|
|
<< "' - would thread to self!\n";
|
|
return false;
|
|
}
|
|
|
|
|
|
// And finally, do it!
|
|
DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
|
|
<< "' to '" << SuccBB->getNameStart() << "' with cost: "
|
|
<< JumpThreadCost << ", across block:\n "
|
|
<< *BB << "\n";
|
|
|
|
ThreadEdge(BB, PredBB, SuccBB);
|
|
++NumThreads;
|
|
return true;
|
|
}
|
|
|
|
|
|
/// ThreadEdge - We have decided that it is safe and profitable to thread an
|
|
/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this
|
|
/// change.
|
|
void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
|
|
BasicBlock *SuccBB) {
|
|
|
|
// Jump Threading can not update SSA properties correctly if the values
|
|
// defined in the duplicated block are used outside of the block itself. For
|
|
// this reason, we spill all values that are used outside of BB to the stack.
|
|
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
|
|
if (!I->isUsedOutsideOfBlock(BB))
|
|
continue;
|
|
|
|
// We found a use of I outside of BB. Create a new stack slot to
|
|
// break this inter-block usage pattern.
|
|
DemoteRegToStack(*I);
|
|
}
|
|
|
|
// 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->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->getNameStart());
|
|
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)))
|
|
if (Value *Remapped = ValueMapping[Inst])
|
|
New->setOperand(i, Remapped);
|
|
}
|
|
|
|
// 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.
|
|
for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) {
|
|
PHINode *PN = cast<PHINode>(PNI);
|
|
// Ok, we have a PHI node. Figure out what the incoming value was for the
|
|
// DestBlock.
|
|
Value *IV = PN->getIncomingValueForBlock(BB);
|
|
|
|
// Remap the value if necessary.
|
|
if (Instruction *Inst = dyn_cast<Instruction>(IV))
|
|
if (Value *MappedIV = ValueMapping[Inst])
|
|
IV = MappedIV;
|
|
PN->addIncoming(IV, NewBB);
|
|
}
|
|
|
|
// 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) {
|
|
BB->removePredecessor(PredBB);
|
|
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.
|
|
BI = NewBB->begin();
|
|
for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
|
|
Instruction *Inst = BI++;
|
|
if (Constant *C = ConstantFoldInstruction(Inst, TD)) {
|
|
Inst->replaceAllUsesWith(C);
|
|
Inst->eraseFromParent();
|
|
continue;
|
|
}
|
|
|
|
RecursivelyDeleteTriviallyDeadInstructions(Inst);
|
|
}
|
|
}
|