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https://github.com/RPCS3/llvm-mirror.git
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9e7d74961b
llvm-svn: 31102
558 lines
20 KiB
C++
558 lines
20 KiB
C++
//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass forwards branches to unconditional branches to make them branch
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// directly to the target block. This pass often results in dead MBB's, which
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// it then removes.
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//
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// Note that this pass must be run after register allocation, it cannot handle
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// SSA form.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineDebugInfo.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineJumpTableInfo.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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using namespace llvm;
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static Statistic<> NumDeadBlocks("branchfold", "Number of dead blocks removed");
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static Statistic<> NumBranchOpts("branchfold", "Number of branches optimized");
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static Statistic<> NumTailMerge ("branchfold", "Number of block tails merged");
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static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::init(false));
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namespace {
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struct BranchFolder : public MachineFunctionPass {
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virtual bool runOnMachineFunction(MachineFunction &MF);
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virtual const char *getPassName() const { return "Control Flow Optimizer"; }
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const TargetInstrInfo *TII;
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MachineDebugInfo *MDI;
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bool MadeChange;
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private:
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// Tail Merging.
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bool TailMergeBlocks(MachineFunction &MF);
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void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
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MachineBasicBlock *NewDest);
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// Branch optzn.
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bool OptimizeBranches(MachineFunction &MF);
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void OptimizeBlock(MachineFunction::iterator MBB);
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void RemoveDeadBlock(MachineBasicBlock *MBB);
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};
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}
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FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
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/// RemoveDeadBlock - Remove the specified dead machine basic block from the
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/// function, updating the CFG.
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void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
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assert(MBB->pred_empty() && "MBB must be dead!");
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MachineFunction *MF = MBB->getParent();
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// drop all successors.
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while (!MBB->succ_empty())
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MBB->removeSuccessor(MBB->succ_end()-1);
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// If there is DWARF info to active, check to see if there are any DWARF_LABEL
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// records in the basic block. If so, unregister them from MachineDebugInfo.
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if (MDI && !MBB->empty()) {
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unsigned DWARF_LABELOpc = TII->getDWARF_LABELOpcode();
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assert(DWARF_LABELOpc &&
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"Target supports dwarf but didn't implement getDWARF_LABELOpcode!");
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for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
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I != E; ++I) {
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if ((unsigned)I->getOpcode() == DWARF_LABELOpc) {
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// The label ID # is always operand #0, an immediate.
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MDI->RemoveLabelInfo(I->getOperand(0).getImm());
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}
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}
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}
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// Remove the block.
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MF->getBasicBlockList().erase(MBB);
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}
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bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
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TII = MF.getTarget().getInstrInfo();
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if (!TII) return false;
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MDI = getAnalysisToUpdate<MachineDebugInfo>();
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bool EverMadeChange = false;
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bool MadeChangeThisIteration = true;
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while (MadeChangeThisIteration) {
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MadeChangeThisIteration = false;
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MadeChangeThisIteration |= TailMergeBlocks(MF);
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MadeChangeThisIteration |= OptimizeBranches(MF);
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EverMadeChange |= MadeChangeThisIteration;
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}
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return EverMadeChange;
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}
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//===----------------------------------------------------------------------===//
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// Tail Merging of Blocks
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//===----------------------------------------------------------------------===//
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/// HashMachineInstr - Compute a hash value for MI and its operands.
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static unsigned HashMachineInstr(const MachineInstr *MI) {
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unsigned Hash = MI->getOpcode();
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &Op = MI->getOperand(i);
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// Merge in bits from the operand if easy.
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unsigned OperandHash = 0;
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switch (Op.getType()) {
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case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
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case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
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case MachineOperand::MO_MachineBasicBlock:
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OperandHash = Op.getMachineBasicBlock()->getNumber();
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break;
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case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break;
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case MachineOperand::MO_ConstantPoolIndex:
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OperandHash = Op.getConstantPoolIndex();
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break;
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case MachineOperand::MO_JumpTableIndex:
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OperandHash = Op.getJumpTableIndex();
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break;
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case MachineOperand::MO_GlobalAddress:
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case MachineOperand::MO_ExternalSymbol:
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// Global address / external symbol are too hard, don't bother, but do
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// pull in the offset.
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OperandHash = Op.getOffset();
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break;
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default: break;
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}
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Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
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}
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return Hash;
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}
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/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two
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/// instructions, because cross-jumping only saves code when at least two
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/// instructions are removed (since a branch must be inserted).
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static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
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MachineBasicBlock::const_iterator I = MBB->end();
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if (I == MBB->begin())
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return 0; // Empty MBB.
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--I;
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unsigned Hash = HashMachineInstr(I);
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if (I == MBB->begin())
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return Hash; // Single instr MBB.
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--I;
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// Hash in the second-to-last instruction.
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Hash ^= HashMachineInstr(I) << 2;
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return Hash;
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}
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/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
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/// of instructions they actually have in common together at their end. Return
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/// iterators for the first shared instruction in each block.
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static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
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MachineBasicBlock *MBB2,
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MachineBasicBlock::iterator &I1,
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MachineBasicBlock::iterator &I2) {
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I1 = MBB1->end();
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I2 = MBB2->end();
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unsigned TailLen = 0;
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while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
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--I1; --I2;
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if (!I1->isIdenticalTo(I2)) {
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++I1; ++I2;
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break;
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}
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++TailLen;
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}
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return TailLen;
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}
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/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
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/// after it, replacing it with an unconditional branch to NewDest. This
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/// returns true if OldInst's block is modified, false if NewDest is modified.
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void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
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MachineBasicBlock *NewDest) {
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MachineBasicBlock *OldBB = OldInst->getParent();
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// Remove all the old successors of OldBB from the CFG.
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while (!OldBB->succ_empty())
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OldBB->removeSuccessor(OldBB->succ_begin());
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// Remove all the dead instructions from the end of OldBB.
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OldBB->erase(OldInst, OldBB->end());
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// If OldBB isn't immediately before OldBB, insert a branch to it.
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if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
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TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>());
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OldBB->addSuccessor(NewDest);
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++NumTailMerge;
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}
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bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
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MadeChange = false;
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if (!EnableTailMerge)
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return false;
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// Find blocks with no successors.
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std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
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if (I->succ_empty())
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MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I));
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}
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// Sort by hash value so that blocks with identical end sequences sort
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// together.
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std::stable_sort(MergePotentials.begin(), MergePotentials.end());
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// Walk through equivalence sets looking for actual exact matches.
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while (MergePotentials.size() > 1) {
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unsigned CurHash = (MergePotentials.end()-1)->first;
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unsigned PrevHash = (MergePotentials.end()-2)->first;
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MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second;
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// If there is nothing that matches the hash of the current basic block,
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// give up.
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if (CurHash != PrevHash) {
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MergePotentials.pop_back();
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continue;
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}
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// Determine the actual length of the shared tail between these two basic
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// blocks. Because the hash can have collisions, it's possible that this is
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// less than 2.
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MachineBasicBlock::iterator BBI1, BBI2;
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unsigned CommonTailLen =
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ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second,
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BBI1, BBI2);
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// If the tails don't have at least two instructions in common, see if there
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// is anything else in the equivalence class that does match.
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if (CommonTailLen < 2) {
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unsigned FoundMatch = ~0U;
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for (int i = MergePotentials.size()-2;
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i != -1 && MergePotentials[i].first == CurHash; --i) {
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CommonTailLen = ComputeCommonTailLength(CurMBB,
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MergePotentials[i].second,
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BBI1, BBI2);
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if (CommonTailLen >= 2) {
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FoundMatch = i;
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break;
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}
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}
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// If we didn't find anything that has at least two instructions matching
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// this one, bail out.
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if (FoundMatch == ~0U) {
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MergePotentials.pop_back();
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continue;
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}
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// Otherwise, move the matching block to the right position.
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std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2));
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}
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// If either block is the entire common tail, make the longer one branch to
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// the shorter one.
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MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
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if (CurMBB->begin() == BBI1) {
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// Hack the end off MBB2, making it jump to CurMBB instead.
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ReplaceTailWithBranchTo(BBI2, CurMBB);
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// This modifies MBB2, so remove it from the worklist.
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MergePotentials.erase(MergePotentials.end()-2);
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MadeChange = true;
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continue;
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} else if (MBB2->begin() == BBI2) {
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// Hack the end off CurMBB, making it jump to MBBI@ instead.
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ReplaceTailWithBranchTo(BBI1, MBB2);
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// This modifies CurMBB, so remove it from the worklist.
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MergePotentials.pop_back();
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MadeChange = true;
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continue;
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}
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MergePotentials.pop_back();
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}
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return MadeChange;
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}
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//===----------------------------------------------------------------------===//
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// Branch Optimization
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//===----------------------------------------------------------------------===//
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bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
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MadeChange = false;
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for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
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MachineBasicBlock *MBB = I++;
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OptimizeBlock(MBB);
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// If it is dead, remove it.
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if (MBB->pred_empty()) {
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RemoveDeadBlock(MBB);
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MadeChange = true;
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++NumDeadBlocks;
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}
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}
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return MadeChange;
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}
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/// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the
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/// CFG to be inserted. If we have proven that MBB can only branch to DestA and
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/// DestB, remove any other MBB successors from the CFG. DestA and DestB can
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/// be null.
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static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB,
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MachineBasicBlock *DestA,
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MachineBasicBlock *DestB,
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bool isCond,
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MachineFunction::iterator FallThru) {
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bool MadeChange = false;
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bool AddedFallThrough = false;
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// If this block ends with a conditional branch that falls through to its
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// successor, set DestB as the successor.
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if (isCond) {
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if (DestB == 0 && FallThru != MBB.getParent()->end()) {
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DestB = FallThru;
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AddedFallThrough = true;
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}
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} else {
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// If this is an unconditional branch with no explicit dest, it must just be
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// a fallthrough into DestB.
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if (DestA == 0 && FallThru != MBB.getParent()->end()) {
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DestA = FallThru;
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AddedFallThrough = true;
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}
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}
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MachineBasicBlock::pred_iterator SI = MBB.succ_begin();
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while (SI != MBB.succ_end()) {
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if (*SI == DestA) {
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DestA = 0;
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++SI;
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} else if (*SI == DestB) {
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DestB = 0;
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++SI;
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} else {
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// Otherwise, this is a superfluous edge, remove it.
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MBB.removeSuccessor(SI);
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MadeChange = true;
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}
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}
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if (!AddedFallThrough) {
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assert(DestA == 0 && DestB == 0 &&
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"MachineCFG is missing edges!");
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} else if (isCond) {
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assert(DestA == 0 && "MachineCFG is missing edges!");
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}
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return MadeChange;
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}
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/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
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/// 'Old', change the code and CFG so that it branches to 'New' instead.
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static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
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MachineBasicBlock *Old,
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MachineBasicBlock *New,
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const TargetInstrInfo *TII) {
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assert(Old != New && "Cannot replace self with self!");
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MachineBasicBlock::iterator I = BB->end();
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while (I != BB->begin()) {
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--I;
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if (!TII->isTerminatorInstr(I->getOpcode())) break;
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// Scan the operands of this machine instruction, replacing any uses of Old
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// with New.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (I->getOperand(i).isMachineBasicBlock() &&
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I->getOperand(i).getMachineBasicBlock() == Old)
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I->getOperand(i).setMachineBasicBlock(New);
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}
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// Update the successor information.
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std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
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for (int i = Succs.size()-1; i >= 0; --i)
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if (Succs[i] == Old) {
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BB->removeSuccessor(Old);
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BB->addSuccessor(New);
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}
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}
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/// OptimizeBlock - Analyze and optimize control flow related to the specified
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/// block. This is never called on the entry block.
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void BranchFolder::OptimizeBlock(MachineFunction::iterator MBB) {
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// If this block is empty, make everyone use its fall-through, not the block
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// explicitly.
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if (MBB->empty()) {
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// Dead block? Leave for cleanup later.
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if (MBB->pred_empty()) return;
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MachineFunction::iterator FallThrough = next(MBB);
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if (FallThrough == MBB->getParent()->end()) {
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// TODO: Simplify preds to not branch here if possible!
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} else {
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// Rewrite all predecessors of the old block to go to the fallthrough
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// instead.
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while (!MBB->pred_empty()) {
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MachineBasicBlock *Pred = *(MBB->pred_end()-1);
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ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
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}
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// If MBB was the target of a jump table, update jump tables to go to the
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// fallthrough instead.
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MBB->getParent()->getJumpTableInfo()->ReplaceMBBInJumpTables(MBB,
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FallThrough);
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MadeChange = true;
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}
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return;
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}
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// Check to see if we can simplify the terminator of the block before this
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// one.
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MachineBasicBlock &PrevBB = *prior(MBB);
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MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
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std::vector<MachineOperand> PriorCond;
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bool PriorUnAnalyzable = false;
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PriorUnAnalyzable = TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
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if (!PriorUnAnalyzable) {
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// If the CFG for the prior block has extra edges, remove them.
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MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB,
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!PriorCond.empty(), MBB);
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// If the previous branch is conditional and both conditions go to the same
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// destination, remove the branch, replacing it with an unconditional one or
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// a fall-through.
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if (PriorTBB && PriorTBB == PriorFBB) {
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TII->RemoveBranch(PrevBB);
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PriorCond.clear();
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if (PriorTBB != &*MBB)
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TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
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MadeChange = true;
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++NumBranchOpts;
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return OptimizeBlock(MBB);
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}
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// If the previous branch *only* branches to *this* block (conditional or
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// not) remove the branch.
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if (PriorTBB == &*MBB && PriorFBB == 0) {
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TII->RemoveBranch(PrevBB);
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MadeChange = true;
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++NumBranchOpts;
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return OptimizeBlock(MBB);
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}
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// If the prior block branches somewhere else on the condition and here if
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// the condition is false, remove the uncond second branch.
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if (PriorFBB == &*MBB) {
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TII->RemoveBranch(PrevBB);
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TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
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MadeChange = true;
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++NumBranchOpts;
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return OptimizeBlock(MBB);
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}
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// If the prior block branches here on true and somewhere else on false, and
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// if the branch condition is reversible, reverse the branch to create a
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// fall-through.
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if (PriorTBB == &*MBB) {
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std::vector<MachineOperand> NewPriorCond(PriorCond);
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if (!TII->ReverseBranchCondition(NewPriorCond)) {
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TII->RemoveBranch(PrevBB);
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TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
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MadeChange = true;
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++NumBranchOpts;
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return OptimizeBlock(MBB);
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}
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}
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}
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// Analyze the branch in the current block.
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MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
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std::vector<MachineOperand> CurCond;
|
|
if (!TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond)) {
|
|
// If the CFG for the prior block has extra edges, remove them.
|
|
MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB,
|
|
!CurCond.empty(), next(MBB));
|
|
|
|
// If this branch is the only thing in its block, see if we can forward
|
|
// other blocks across it.
|
|
if (CurTBB && CurCond.empty() && CurFBB == 0 &&
|
|
TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != &*MBB) {
|
|
// This block may contain just an unconditional branch. Because there can
|
|
// be 'non-branch terminators' in the block, try removing the branch and
|
|
// then seeing if the block is empty.
|
|
TII->RemoveBranch(*MBB);
|
|
|
|
// If this block is just an unconditional branch to CurTBB, we can
|
|
// usually completely eliminate the block. The only case we cannot
|
|
// completely eliminate the block is when the block before this one
|
|
// falls through into MBB and we can't understand the prior block's branch
|
|
// condition.
|
|
if (MBB->empty() && (!PriorUnAnalyzable || !PrevBB.isSuccessor(MBB))) {
|
|
// If the prior block falls through into us, turn it into an
|
|
// explicit branch to us to make updates simpler.
|
|
if (PrevBB.isSuccessor(MBB) && PriorTBB != &*MBB && PriorFBB != &*MBB) {
|
|
if (PriorTBB == 0) {
|
|
assert(PriorCond.empty() && PriorFBB == 0 && "Bad branch analysis");
|
|
PriorTBB = MBB;
|
|
} else {
|
|
assert(PriorFBB == 0 && "Machine CFG out of date!");
|
|
PriorFBB = MBB;
|
|
}
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
|
|
}
|
|
|
|
// Iterate through all the predecessors, revectoring each in-turn.
|
|
MachineBasicBlock::pred_iterator PI = MBB->pred_begin();
|
|
bool DidChange = false;
|
|
bool HasBranchToSelf = false;
|
|
while (PI != MBB->pred_end()) {
|
|
if (*PI == &*MBB) {
|
|
// If this block has an uncond branch to itself, leave it.
|
|
++PI;
|
|
HasBranchToSelf = true;
|
|
} else {
|
|
DidChange = true;
|
|
ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII);
|
|
}
|
|
}
|
|
|
|
// Change any jumptables to go to the new MBB.
|
|
MBB->getParent()->getJumpTableInfo()->ReplaceMBBInJumpTables(MBB,
|
|
CurTBB);
|
|
if (DidChange) {
|
|
++NumBranchOpts;
|
|
MadeChange = true;
|
|
if (!HasBranchToSelf) return;
|
|
}
|
|
}
|
|
|
|
// Add the branch back if the block is more than just an uncond branch.
|
|
TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
|
|
}
|
|
}
|
|
}
|