mirror of
https://github.com/c64scene-ar/llvm-6502.git
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51591 91177308-0d34-0410-b5e6-96231b3b80d8
516 lines
19 KiB
C++
516 lines
19 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/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.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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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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public:
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static char ID; // Pass identification
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JumpThreading() : FunctionPass((intptr_t)&ID) {}
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bool runOnFunction(Function &F);
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bool ThreadBlock(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 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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};
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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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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; ++I)
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while (ThreadBlock(I))
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Changed = true;
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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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/// ThreadBlock - 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::ThreadBlock(BasicBlock *BB) {
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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 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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// See if this is a phi node in the current block.
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PHINode *PN = dyn_cast<PHINode>(Condition);
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if (PN && 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 (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) {
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if ((CondI->getOpcode() == Instruction::And ||
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CondI->getOpcode() == Instruction::Or) &&
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isa<BranchInst>(BB->getTerminator()) &&
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ProcessBranchOnLogical(CondI, BB,
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CondI->getOpcode() == Instruction::And))
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return true;
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}
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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>(Condition))
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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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return false;
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}
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/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in
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/// the current block. See if there are any simplifications we can do based on
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/// inputs to the phi node.
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///
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bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
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// See if the phi node has any constant values. If so, we can determine where
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// the corresponding predecessor will branch.
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ConstantInt *PredCst = 0;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
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break;
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// If no incoming value has a constant, we don't know the destination of any
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// predecessors.
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if (PredCst == 0)
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return false;
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// See if the cost of duplicating this block is low enough.
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BasicBlock *BB = PN->getParent();
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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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// If so, we can actually do this threading. Merge any common predecessors
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// that will act the same.
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BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
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// Next, figure out which successor we are threading to.
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BasicBlock *SuccBB;
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if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
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SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse());
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else {
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SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
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SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
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}
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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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/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch
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/// whose condition is an AND/OR where one side is PN. If PN has constant
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/// operands that permit us to evaluate the condition for some operand, thread
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/// through the block. For example with:
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/// br (and X, phi(Y, Z, false))
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/// the predecessor corresponding to the 'false' will always jump to the false
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/// destination of the branch.
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///
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bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
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bool isAnd) {
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// If this is a binary operator tree of the same AND/OR opcode, check the
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// LHS/RHS.
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if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V))
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if ((isAnd && BO->getOpcode() == Instruction::And) ||
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(!isAnd && BO->getOpcode() == Instruction::Or)) {
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if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd))
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return true;
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if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd))
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return true;
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}
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// If this isn't a PHI node, we can't handle it.
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PHINode *PN = dyn_cast<PHINode>(V);
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if (!PN || PN->getParent() != BB) return false;
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// We can only do the simplification for phi nodes of 'false' with AND or
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// 'true' with OR. See if we have any entries in the phi for this.
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unsigned PredNo = ~0U;
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ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd);
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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if (PN->getIncomingValue(i) == PredCst) {
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PredNo = i;
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break;
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}
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}
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// If no match, bail out.
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if (PredNo == ~0U)
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return false;
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// See if the cost of duplicating this block is low enough.
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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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// If so, we can actually do this threading. Merge any common predecessors
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// that will act the same.
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BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
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// Next, figure out which successor we are threading to. If this was an AND,
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// the constant must be FALSE, and we must be targeting the 'false' block.
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// If this is an OR, the constant must be TRUE, and we must be targeting the
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// 'true' block.
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BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
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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 through bool from '" << PredBB->getNameStart()
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<< "' to '" << SuccBB->getNameStart() << "' with cost: "
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<< JumpThreadCost << ", 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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/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
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/// node and a constant. If the PHI node contains any constants as inputs, we
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/// can fold the compare for that edge and thread through it.
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bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
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PHINode *PN = cast<PHINode>(Cmp->getOperand(0));
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Constant *RHS = cast<Constant>(Cmp->getOperand(1));
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// If the phi isn't in the current block, an incoming edge to this block
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// doesn't control the destination.
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if (PN->getParent() != BB)
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return false;
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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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Constant *PredCst = 0;
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bool TrueDirection = false;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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PredCst = dyn_cast<Constant>(PN->getIncomingValue(i));
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if (PredCst == 0) continue;
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Constant *Res;
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if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp))
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Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS);
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else
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Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(),
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PredCst, RHS);
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// If this folded to a constant expr, we can't do anything.
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if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) {
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TrueDirection = ResC->getZExtValue();
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break;
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}
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// If this folded to undef, just go the false way.
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if (isa<UndefValue>(Res)) {
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TrueDirection = false;
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break;
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}
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// Otherwise, we can't fold this input.
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PredCst = 0;
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}
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// If no match, bail out.
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if (PredCst == 0)
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return false;
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// See if the cost of duplicating this block is low enough.
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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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// If so, we can actually do this threading. Merge any common predecessors
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// that will act the same.
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BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
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// Next, get our successor.
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BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
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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 through bool from '" << PredBB->getNameStart()
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<< "' to '" << SuccBB->getNameStart() << "' with cost: "
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<< JumpThreadCost << ", 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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/// ThreadEdge - We have decided that it is safe and profitable to thread an
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/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this
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/// change.
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void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
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BasicBlock *SuccBB) {
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// Jump Threading can not update SSA properties correctly if the values
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// defined in the duplicated block are used outside of the block itself. For
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// this reason, we spill all values that are used outside of BB to the stack.
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for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
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if (!I->isUsedOutsideOfBlock(BB))
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continue;
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// We found a use of I outside of BB. Create a new stack slot to
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// break this inter-block usage pattern.
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if (!isa<StructType>(I->getType())) {
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DemoteRegToStack(*I);
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continue;
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}
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// Alternatively, I must be a call or invoke that returns multiple retvals.
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// We can't use 'DemoteRegToStack' because that will create loads and
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// stores of aggregates which is not valid yet. If I is a call, we can just
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// pull all the getresult instructions up to this block. If I is an invoke,
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// we are out of luck.
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BasicBlock::iterator IP = I; ++IP;
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
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UI != E; ++UI)
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cast<GetResultInst>(UI)->moveBefore(IP);
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}
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// We are going to have to map operands from the original BB block to the new
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// copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
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// account for entry from PredBB.
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DenseMap<Instruction*, Value*> ValueMapping;
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BasicBlock *NewBB =
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BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB);
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NewBB->moveAfter(PredBB);
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BasicBlock::iterator BI = BB->begin();
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for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
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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);
|
|
}
|
|
|
|
// Finally, 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);
|
|
}
|
|
}
|