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https://github.com/c64scene-ar/llvm-6502.git
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6b896cec8b
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34359 91177308-0d34-0410-b5e6-96231b3b80d8
910 lines
34 KiB
C++
910 lines
34 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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#define DEBUG_TYPE "branchfolding"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineModuleInfo.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/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include <algorithm>
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using namespace llvm;
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STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
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STATISTIC(NumBranchOpts, "Number of branches optimized");
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STATISTIC(NumTailMerge , "Number of block tails merged");
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static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden);
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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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MachineModuleInfo *MMI;
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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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MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
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MachineBasicBlock::iterator BBI1);
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// Branch optzn.
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bool OptimizeBranches(MachineFunction &MF);
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void OptimizeBlock(MachineBasicBlock *MBB);
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void RemoveDeadBlock(MachineBasicBlock *MBB);
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bool CanFallThrough(MachineBasicBlock *CurBB);
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bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
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MachineBasicBlock *TBB, MachineBasicBlock *FBB,
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const std::vector<MachineOperand> &Cond);
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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 LABEL
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// records in the basic block. If so, unregister them from MachineModuleInfo.
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if (MMI && !MBB->empty()) {
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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() == TargetInstrInfo::LABEL) {
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// The label ID # is always operand #0, an immediate.
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MMI->InvalidateLabel(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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MMI = getAnalysisToUpdate<MachineModuleInfo>();
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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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// See if any jump tables have become mergable or dead as the code generator
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// did its thing.
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MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
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const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
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if (!JTs.empty()) {
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// Figure out how these jump tables should be merged.
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std::vector<unsigned> JTMapping;
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JTMapping.reserve(JTs.size());
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// We always keep the 0th jump table.
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JTMapping.push_back(0);
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// Scan the jump tables, seeing if there are any duplicates. Note that this
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// is N^2, which should be fixed someday.
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for (unsigned i = 1, e = JTs.size(); i != e; ++i)
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JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
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// If a jump table was merge with another one, walk the function rewriting
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// references to jump tables to reference the new JT ID's. Keep track of
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// whether we see a jump table idx, if not, we can delete the JT.
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std::vector<bool> JTIsLive;
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JTIsLive.resize(JTs.size());
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for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
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BB != E; ++BB) {
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for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
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I != E; ++I)
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for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
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MachineOperand &Op = I->getOperand(op);
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if (!Op.isJumpTableIndex()) continue;
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unsigned NewIdx = JTMapping[Op.getJumpTableIndex()];
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Op.setJumpTableIndex(NewIdx);
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// Remember that this JT is live.
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JTIsLive[NewIdx] = true;
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}
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}
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// Finally, remove dead jump tables. This happens either because the
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// indirect jump was unreachable (and thus deleted) or because the jump
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// table was merged with some other one.
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for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
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if (!JTIsLive[i]) {
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JTI->RemoveJumpTable(i);
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EverMadeChange = true;
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}
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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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/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
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/// MBB so that the part before the iterator falls into the part starting at the
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/// iterator. This returns the new MBB.
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MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
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MachineBasicBlock::iterator BBI1) {
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// Create the fall-through block.
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MachineFunction::iterator MBBI = &CurMBB;
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MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock());
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CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB);
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// Move all the successors of this block to the specified block.
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while (!CurMBB.succ_empty()) {
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MachineBasicBlock *S = *(CurMBB.succ_end()-1);
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NewMBB->addSuccessor(S);
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CurMBB.removeSuccessor(S);
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}
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// Add an edge from CurMBB to NewMBB for the fall-through.
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CurMBB.addSuccessor(NewMBB);
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// Splice the code over.
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NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
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return NewMBB;
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}
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/// EstimateRuntime - Make a rough estimate for how long it will take to run
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/// the specified code.
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static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
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MachineBasicBlock::iterator E,
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const TargetInstrInfo *TII) {
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unsigned Time = 0;
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for (; I != E; ++I) {
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const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
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if (TID.Flags & M_CALL_FLAG)
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Time += 10;
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else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
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Time += 2;
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else
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++Time;
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}
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return Time;
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}
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/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
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/// MBB2I and then insert an unconditional branch in the other block. Determine
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/// which is the best to split
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static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
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MachineBasicBlock::iterator MBB1I,
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MachineBasicBlock *MBB2,
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MachineBasicBlock::iterator MBB2I,
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const TargetInstrInfo *TII) {
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// TODO: if we had some notion of which block was hotter, we could split
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// the hot block, so it is the fall-through. Since we don't have profile info
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// make a decision based on which will hurt most to split.
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unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII);
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unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII);
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// If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
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// MBB1 block so it falls through. This will penalize the MBB2 path, but will
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// have a lower overall impact on the program execution.
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return MBB1Time < MBB2Time;
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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) 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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MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
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// If neither block is the entire common tail, split the tail of one block
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// to make it redundant with the other tail.
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if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) {
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if (0) { // Enable this to disable partial tail merges.
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MergePotentials.pop_back();
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continue;
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}
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// Decide whether we want to split CurMBB or MBB2.
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if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) {
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CurMBB = SplitMBBAt(*CurMBB, BBI1);
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BBI1 = CurMBB->begin();
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MergePotentials.back().second = CurMBB;
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} else {
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MBB2 = SplitMBBAt(*MBB2, BBI2);
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BBI2 = MBB2->begin();
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(MergePotentials.end()-2)->second = MBB2;
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}
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}
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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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} else {
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assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?");
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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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}
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MadeChange = true;
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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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// Make sure blocks are numbered in order
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MF.RenumberBlocks();
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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()) {
|
|
DestA = FallThru;
|
|
AddedFallThrough = true;
|
|
}
|
|
}
|
|
|
|
MachineBasicBlock::pred_iterator SI = MBB.succ_begin();
|
|
while (SI != MBB.succ_end()) {
|
|
if (*SI == DestA) {
|
|
DestA = 0;
|
|
++SI;
|
|
} else if (*SI == DestB) {
|
|
DestB = 0;
|
|
++SI;
|
|
} else {
|
|
// Otherwise, this is a superfluous edge, remove it.
|
|
MBB.removeSuccessor(SI);
|
|
MadeChange = true;
|
|
}
|
|
}
|
|
if (!AddedFallThrough) {
|
|
assert(DestA == 0 && DestB == 0 &&
|
|
"MachineCFG is missing edges!");
|
|
} else if (isCond) {
|
|
assert(DestA == 0 && "MachineCFG is missing edges!");
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
|
|
/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
|
|
/// 'Old', change the code and CFG so that it branches to 'New' instead.
|
|
static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
|
|
MachineBasicBlock *Old,
|
|
MachineBasicBlock *New,
|
|
const TargetInstrInfo *TII) {
|
|
assert(Old != New && "Cannot replace self with self!");
|
|
|
|
MachineBasicBlock::iterator I = BB->end();
|
|
while (I != BB->begin()) {
|
|
--I;
|
|
if (!TII->isTerminatorInstr(I->getOpcode())) break;
|
|
|
|
// Scan the operands of this machine instruction, replacing any uses of Old
|
|
// with New.
|
|
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
|
|
if (I->getOperand(i).isMachineBasicBlock() &&
|
|
I->getOperand(i).getMachineBasicBlock() == Old)
|
|
I->getOperand(i).setMachineBasicBlock(New);
|
|
}
|
|
|
|
// Update the successor information.
|
|
std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
|
|
for (int i = Succs.size()-1; i >= 0; --i)
|
|
if (Succs[i] == Old) {
|
|
BB->removeSuccessor(Old);
|
|
BB->addSuccessor(New);
|
|
}
|
|
}
|
|
|
|
/// CanFallThrough - Return true if the specified block (with the specified
|
|
/// branch condition) can implicitly transfer control to the block after it by
|
|
/// falling off the end of it. This should return false if it can reach the
|
|
/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
|
|
///
|
|
/// True is a conservative answer.
|
|
///
|
|
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
|
|
bool BranchUnAnalyzable,
|
|
MachineBasicBlock *TBB, MachineBasicBlock *FBB,
|
|
const std::vector<MachineOperand> &Cond) {
|
|
MachineFunction::iterator Fallthrough = CurBB;
|
|
++Fallthrough;
|
|
// If FallthroughBlock is off the end of the function, it can't fall through.
|
|
if (Fallthrough == CurBB->getParent()->end())
|
|
return false;
|
|
|
|
// If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
|
|
if (!CurBB->isSuccessor(Fallthrough))
|
|
return false;
|
|
|
|
// If we couldn't analyze the branch, assume it could fall through.
|
|
if (BranchUnAnalyzable) return true;
|
|
|
|
// If there is no branch, control always falls through.
|
|
if (TBB == 0) return true;
|
|
|
|
// If there is some explicit branch to the fallthrough block, it can obviously
|
|
// reach, even though the branch should get folded to fall through implicitly.
|
|
if (MachineFunction::iterator(TBB) == Fallthrough ||
|
|
MachineFunction::iterator(FBB) == Fallthrough)
|
|
return true;
|
|
|
|
// If it's an unconditional branch to some block not the fall through, it
|
|
// doesn't fall through.
|
|
if (Cond.empty()) return false;
|
|
|
|
// Otherwise, if it is conditional and has no explicit false block, it falls
|
|
// through.
|
|
return FBB == 0;
|
|
}
|
|
|
|
/// CanFallThrough - Return true if the specified can implicitly transfer
|
|
/// control to the block after it by falling off the end of it. This should
|
|
/// return false if it can reach the block after it, but it uses an explicit
|
|
/// branch to do so (e.g. a table jump).
|
|
///
|
|
/// True is a conservative answer.
|
|
///
|
|
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
|
|
MachineBasicBlock *TBB = 0, *FBB = 0;
|
|
std::vector<MachineOperand> Cond;
|
|
bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond);
|
|
return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
|
|
}
|
|
|
|
/// IsBetterFallthrough - Return true if it would be clearly better to
|
|
/// fall-through to MBB1 than to fall through into MBB2. This has to return
|
|
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
|
|
/// result in infinite loops.
|
|
static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
|
|
MachineBasicBlock *MBB2,
|
|
const TargetInstrInfo &TII) {
|
|
// Right now, we use a simple heuristic. If MBB2 ends with a call, and
|
|
// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
|
|
// optimize branches that branch to either a return block or an assert block
|
|
// into a fallthrough to the return.
|
|
if (MBB1->empty() || MBB2->empty()) return false;
|
|
|
|
MachineInstr *MBB1I = --MBB1->end();
|
|
MachineInstr *MBB2I = --MBB2->end();
|
|
return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode());
|
|
}
|
|
|
|
/// OptimizeBlock - Analyze and optimize control flow related to the specified
|
|
/// block. This is never called on the entry block.
|
|
void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
|
|
MachineFunction::iterator FallThrough = MBB;
|
|
++FallThrough;
|
|
|
|
// If this block is empty, make everyone use its fall-through, not the block
|
|
// explicitly.
|
|
if (MBB->empty()) {
|
|
// Dead block? Leave for cleanup later.
|
|
if (MBB->pred_empty()) return;
|
|
|
|
if (FallThrough == MBB->getParent()->end()) {
|
|
// TODO: Simplify preds to not branch here if possible!
|
|
} else {
|
|
// Rewrite all predecessors of the old block to go to the fallthrough
|
|
// instead.
|
|
while (!MBB->pred_empty()) {
|
|
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
|
|
ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
|
|
}
|
|
|
|
// If MBB was the target of a jump table, update jump tables to go to the
|
|
// fallthrough instead.
|
|
MBB->getParent()->getJumpTableInfo()->
|
|
ReplaceMBBInJumpTables(MBB, FallThrough);
|
|
MadeChange = true;
|
|
}
|
|
return;
|
|
}
|
|
|
|
// Check to see if we can simplify the terminator of the block before this
|
|
// one.
|
|
MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
|
|
|
|
MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
|
|
std::vector<MachineOperand> PriorCond;
|
|
bool PriorUnAnalyzable =
|
|
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
|
|
if (!PriorUnAnalyzable) {
|
|
// If the CFG for the prior block has extra edges, remove them.
|
|
MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB,
|
|
!PriorCond.empty(), MBB);
|
|
|
|
// If the previous branch is conditional and both conditions go to the same
|
|
// destination, remove the branch, replacing it with an unconditional one or
|
|
// a fall-through.
|
|
if (PriorTBB && PriorTBB == PriorFBB) {
|
|
TII->RemoveBranch(PrevBB);
|
|
PriorCond.clear();
|
|
if (PriorTBB != MBB)
|
|
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
|
|
// If the previous branch *only* branches to *this* block (conditional or
|
|
// not) remove the branch.
|
|
if (PriorTBB == MBB && PriorFBB == 0) {
|
|
TII->RemoveBranch(PrevBB);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
|
|
// If the prior block branches somewhere else on the condition and here if
|
|
// the condition is false, remove the uncond second branch.
|
|
if (PriorFBB == MBB) {
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
|
|
// If the prior block branches here on true and somewhere else on false, and
|
|
// if the branch condition is reversible, reverse the branch to create a
|
|
// fall-through.
|
|
if (PriorTBB == MBB) {
|
|
std::vector<MachineOperand> NewPriorCond(PriorCond);
|
|
if (!TII->ReverseBranchCondition(NewPriorCond)) {
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
}
|
|
|
|
// If this block doesn't fall through (e.g. it ends with an uncond branch or
|
|
// has no successors) and if the pred falls through into this block, and if
|
|
// it would otherwise fall through into the block after this, move this
|
|
// block to the end of the function.
|
|
//
|
|
// We consider it more likely that execution will stay in the function (e.g.
|
|
// due to loops) than it is to exit it. This asserts in loops etc, moving
|
|
// the assert condition out of the loop body.
|
|
if (!PriorCond.empty() && PriorFBB == 0 &&
|
|
MachineFunction::iterator(PriorTBB) == FallThrough &&
|
|
!CanFallThrough(MBB)) {
|
|
bool DoTransform = true;
|
|
|
|
// We have to be careful that the succs of PredBB aren't both no-successor
|
|
// blocks. If neither have successors and if PredBB is the second from
|
|
// last block in the function, we'd just keep swapping the two blocks for
|
|
// last. Only do the swap if one is clearly better to fall through than
|
|
// the other.
|
|
if (FallThrough == --MBB->getParent()->end() &&
|
|
!IsBetterFallthrough(PriorTBB, MBB, *TII))
|
|
DoTransform = false;
|
|
|
|
// We don't want to do this transformation if we have control flow like:
|
|
// br cond BB2
|
|
// BB1:
|
|
// ..
|
|
// jmp BBX
|
|
// BB2:
|
|
// ..
|
|
// ret
|
|
//
|
|
// In this case, we could actually be moving the return block *into* a
|
|
// loop!
|
|
if (DoTransform && !MBB->succ_empty() &&
|
|
(!CanFallThrough(PriorTBB) || PriorTBB->empty()))
|
|
DoTransform = false;
|
|
|
|
|
|
if (DoTransform) {
|
|
// Reverse the branch so we will fall through on the previous true cond.
|
|
std::vector<MachineOperand> NewPriorCond(PriorCond);
|
|
if (!TII->ReverseBranchCondition(NewPriorCond)) {
|
|
DOUT << "\nMoving MBB: " << *MBB;
|
|
DOUT << "To make fallthrough to: " << *PriorTBB << "\n";
|
|
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
|
|
|
|
// Move this block to the end of the function.
|
|
MBB->moveAfter(--MBB->getParent()->end());
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
// Analyze the branch in the current block.
|
|
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
|
|
std::vector<MachineOperand> CurCond;
|
|
bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond);
|
|
if (!CurUnAnalyzable) {
|
|
// If the CFG for the prior block has extra edges, remove them.
|
|
MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB,
|
|
!CurCond.empty(),
|
|
++MachineFunction::iterator(MBB));
|
|
|
|
// If this is a two-way branch, and the FBB branches to this block, reverse
|
|
// the condition so the single-basic-block loop is faster. Instead of:
|
|
// Loop: xxx; jcc Out; jmp Loop
|
|
// we want:
|
|
// Loop: xxx; jncc Loop; jmp Out
|
|
if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
|
|
std::vector<MachineOperand> NewCond(CurCond);
|
|
if (!TII->ReverseBranchCondition(NewCond)) {
|
|
TII->RemoveBranch(*MBB);
|
|
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return OptimizeBlock(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()) {
|
|
bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
|
|
if (PredHasNoFallThrough || !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 (!PredHasNoFallThrough && 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);
|
|
}
|
|
}
|
|
|
|
// If the prior block doesn't fall through into this block, and if this
|
|
// block doesn't fall through into some other block, see if we can find a
|
|
// place to move this block where a fall-through will happen.
|
|
if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
|
|
PriorTBB, PriorFBB, PriorCond)) {
|
|
// Now we know that there was no fall-through into this block, check to
|
|
// see if it has a fall-through into its successor.
|
|
bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
|
|
CurCond);
|
|
|
|
// Check all the predecessors of this block. If one of them has no fall
|
|
// throughs, move this block right after it.
|
|
for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
|
|
E = MBB->pred_end(); PI != E; ++PI) {
|
|
// Analyze the branch at the end of the pred.
|
|
MachineBasicBlock *PredBB = *PI;
|
|
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
|
|
if (PredBB != MBB && !CanFallThrough(PredBB)
|
|
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
|
|
// If the current block doesn't fall through, just move it.
|
|
// If the current block can fall through and does not end with a
|
|
// conditional branch, we need to append an unconditional jump to
|
|
// the (current) next block. To avoid a possible compile-time
|
|
// infinite loop, move blocks only backward in this case.
|
|
if (CurFallsThru) {
|
|
MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
|
|
CurCond.clear();
|
|
TII->InsertBranch(*MBB, NextBB, 0, CurCond);
|
|
}
|
|
MBB->moveAfter(PredBB);
|
|
MadeChange = true;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
}
|
|
|
|
if (!CurFallsThru) {
|
|
// Check all successors to see if we can move this block before it.
|
|
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
|
|
E = MBB->succ_end(); SI != E; ++SI) {
|
|
// Analyze the branch at the end of the block before the succ.
|
|
MachineBasicBlock *SuccBB = *SI;
|
|
MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
|
|
std::vector<MachineOperand> SuccPrevCond;
|
|
if (SuccBB != MBB && !CanFallThrough(SuccPrev)) {
|
|
MBB->moveBefore(SuccBB);
|
|
MadeChange = true;
|
|
return OptimizeBlock(MBB);
|
|
}
|
|
}
|
|
|
|
// Okay, there is no really great place to put this block. If, however,
|
|
// the block before this one would be a fall-through if this block were
|
|
// removed, move this block to the end of the function.
|
|
if (FallThrough != MBB->getParent()->end() &&
|
|
PrevBB.isSuccessor(FallThrough)) {
|
|
MBB->moveAfter(--MBB->getParent()->end());
|
|
MadeChange = true;
|
|
return;
|
|
}
|
|
}
|
|
}
|
|
}
|