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90c579de5a
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110460 91177308-0d34-0410-b5e6-96231b3b80d8
1340 lines
51 KiB
C++
1340 lines
51 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 is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This 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 "BranchFolding.h"
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#include "llvm/Function.h"
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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/CodeGen/RegisterScavenging.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/Target/TargetRegisterInfo.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/ADT/SetVector.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<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
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cl::init(cl::BOU_UNSET), cl::Hidden);
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// Throttle for huge numbers of predecessors (compile speed problems)
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static cl::opt<unsigned>
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TailMergeThreshold("tail-merge-threshold",
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cl::desc("Max number of predecessors to consider tail merging"),
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cl::init(150), cl::Hidden);
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// Heuristic for tail merging (and, inversely, tail duplication).
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// TODO: This should be replaced with a target query.
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static cl::opt<unsigned>
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TailMergeSize("tail-merge-size",
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cl::desc("Min number of instructions to consider tail merging"),
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cl::init(3), cl::Hidden);
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namespace {
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/// BranchFolderPass - Wrap branch folder in a machine function pass.
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class BranchFolderPass : public MachineFunctionPass,
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public BranchFolder {
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public:
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static char ID;
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explicit BranchFolderPass(bool defaultEnableTailMerge)
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: MachineFunctionPass(ID), BranchFolder(defaultEnableTailMerge) {}
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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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};
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}
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char BranchFolderPass::ID = 0;
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FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
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return new BranchFolderPass(DefaultEnableTailMerge);
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}
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bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
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return OptimizeFunction(MF,
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MF.getTarget().getInstrInfo(),
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MF.getTarget().getRegisterInfo(),
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getAnalysisIfAvailable<MachineModuleInfo>());
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}
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BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
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switch (FlagEnableTailMerge) {
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case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
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case cl::BOU_TRUE: EnableTailMerge = true; break;
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case cl::BOU_FALSE: EnableTailMerge = false; break;
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}
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}
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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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DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
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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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// Remove the block.
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MF->erase(MBB);
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}
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/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
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/// followed by terminators, and if the implicitly defined registers are not
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/// used by the terminators, remove those implicit_def's. e.g.
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/// BB1:
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/// r0 = implicit_def
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/// r1 = implicit_def
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/// br
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/// This block can be optimized away later if the implicit instructions are
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/// removed.
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bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
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SmallSet<unsigned, 4> ImpDefRegs;
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MachineBasicBlock::iterator I = MBB->begin();
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while (I != MBB->end()) {
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if (!I->isImplicitDef())
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break;
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unsigned Reg = I->getOperand(0).getReg();
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ImpDefRegs.insert(Reg);
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for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
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unsigned SubReg = *SubRegs; ++SubRegs)
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ImpDefRegs.insert(SubReg);
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++I;
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}
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if (ImpDefRegs.empty())
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return false;
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MachineBasicBlock::iterator FirstTerm = I;
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while (I != MBB->end()) {
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if (!TII->isUnpredicatedTerminator(I))
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return false;
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// See if it uses any of the implicitly defined registers.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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MachineOperand &MO = I->getOperand(i);
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if (!MO.isReg() || !MO.isUse())
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continue;
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unsigned Reg = MO.getReg();
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if (ImpDefRegs.count(Reg))
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return false;
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}
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++I;
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}
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I = MBB->begin();
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while (I != FirstTerm) {
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MachineInstr *ImpDefMI = &*I;
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++I;
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MBB->erase(ImpDefMI);
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}
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return true;
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}
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/// OptimizeFunction - Perhaps branch folding, tail merging and other
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/// CFG optimizations on the given function.
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bool BranchFolder::OptimizeFunction(MachineFunction &MF,
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const TargetInstrInfo *tii,
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const TargetRegisterInfo *tri,
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MachineModuleInfo *mmi) {
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if (!tii) return false;
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TII = tii;
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TRI = tri;
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MMI = mmi;
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RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
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// Fix CFG. The later algorithms expect it to be right.
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bool MadeChange = false;
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
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MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
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SmallVector<MachineOperand, 4> Cond;
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if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
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MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
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MadeChange |= OptimizeImpDefsBlock(MBB);
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}
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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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MadeChange |= MadeChangeThisIteration;
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}
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// See if any jump tables have become dead as the code generator
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// did its thing.
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MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
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if (JTI == 0) {
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delete RS;
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return MadeChange;
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}
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// Walk the function to find jump tables that are live.
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BitVector JTIsLive(JTI->getJumpTables().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.isJTI()) continue;
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// Remember that this JT is live.
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JTIsLive.set(Op.getIndex());
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}
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}
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// Finally, remove dead jump tables. This happens when the
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// indirect jump was unreachable (and thus deleted).
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for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
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if (!JTIsLive.test(i)) {
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JTI->RemoveJumpTable(i);
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MadeChange = true;
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}
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delete RS;
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return MadeChange;
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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.getMBB()->getNumber();
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break;
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case MachineOperand::MO_FrameIndex:
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case MachineOperand::MO_ConstantPoolIndex:
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case MachineOperand::MO_JumpTableIndex:
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OperandHash = Op.getIndex();
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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 instruction in the MBB.
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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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// Skip debug info so it will not affect codegen.
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while (I->isDebugValue()) {
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if (I==MBB->begin())
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return 0; // MBB empty except for debug info.
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--I;
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}
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return HashMachineInstr(I);
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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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// Skip debugging pseudos; necessary to avoid changing the code.
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while (I1->isDebugValue()) {
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if (I1==MBB1->begin()) {
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while (I2->isDebugValue()) {
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if (I2==MBB2->begin())
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// I1==DBG at begin; I2==DBG at begin
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return TailLen;
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--I2;
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}
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++I2;
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// I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
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return TailLen;
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}
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--I1;
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}
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// I1==first (untested) non-DBG preceding known match
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while (I2->isDebugValue()) {
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if (I2==MBB2->begin()) {
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++I1;
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// I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
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return TailLen;
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}
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--I2;
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}
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// I1, I2==first (untested) non-DBGs preceding known match
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if (!I1->isIdenticalTo(I2) ||
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// FIXME: This check is dubious. It's used to get around a problem where
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// people incorrectly expect inline asm directives to remain in the same
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// relative order. This is untenable because normal compiler
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// optimizations (like this one) may reorder and/or merge these
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// directives.
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I1->isInlineAsm()) {
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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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// Back past possible debugging pseudos at beginning of block. This matters
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// when one block differs from the other only by whether debugging pseudos
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// are present at the beginning. (This way, the various checks later for
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// I1==MBB1->begin() work as expected.)
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if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
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--I2;
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while (I2->isDebugValue()) {
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if (I2 == MBB2->begin()) {
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return TailLen;
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}
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--I2;
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}
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++I2;
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}
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if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
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--I1;
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while (I1->isDebugValue()) {
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if (I1 == MBB1->begin())
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return TailLen;
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--I1;
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}
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++I1;
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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.
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void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
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MachineBasicBlock *NewDest) {
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TII->ReplaceTailWithBranchTo(OldInst, 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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if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
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return 0;
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MachineFunction &MF = *CurMBB.getParent();
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// Create the fall-through block.
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MachineFunction::iterator MBBI = &CurMBB;
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MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
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CurMBB.getParent()->insert(++MBBI, NewMBB);
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// Move all the successors of this block to the specified block.
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NewMBB->transferSuccessors(&CurMBB);
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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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// For targets that use the register scavenger, we must maintain LiveIns.
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if (RS) {
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RS->enterBasicBlock(&CurMBB);
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if (!CurMBB.empty())
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RS->forward(prior(CurMBB.end()));
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BitVector RegsLiveAtExit(TRI->getNumRegs());
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RS->getRegsUsed(RegsLiveAtExit, false);
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for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
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if (RegsLiveAtExit[i])
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NewMBB->addLiveIn(i);
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}
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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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unsigned Time = 0;
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for (; I != E; ++I) {
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if (I->isDebugValue())
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continue;
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const TargetInstrDesc &TID = I->getDesc();
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if (TID.isCall())
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Time += 10;
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else if (TID.mayLoad() || TID.mayStore())
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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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// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
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// branches temporarily for tail merging). In the case where CurMBB ends
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// with a conditional branch to the next block, optimize by reversing the
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// test and conditionally branching to SuccMBB instead.
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static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
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const TargetInstrInfo *TII) {
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MachineFunction *MF = CurMBB->getParent();
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MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
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MachineBasicBlock *TBB = 0, *FBB = 0;
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SmallVector<MachineOperand, 4> Cond;
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DebugLoc dl; // FIXME: this is nowhere
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if (I != MF->end() &&
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!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
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MachineBasicBlock *NextBB = I;
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if (TBB == NextBB && !Cond.empty() && !FBB) {
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if (!TII->ReverseBranchCondition(Cond)) {
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TII->RemoveBranch(*CurMBB);
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TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond, dl);
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return;
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}
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}
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}
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TII->InsertBranch(*CurMBB, SuccBB, NULL,
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SmallVector<MachineOperand, 0>(), dl);
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}
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bool
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BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
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if (getHash() < o.getHash())
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return true;
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else if (getHash() > o.getHash())
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return false;
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else if (getBlock()->getNumber() < o.getBlock()->getNumber())
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return true;
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else if (getBlock()->getNumber() > o.getBlock()->getNumber())
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return false;
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else {
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// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
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// an object with itself.
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#ifndef _GLIBCXX_DEBUG
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llvm_unreachable("Predecessor appears twice");
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#endif
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return false;
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}
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}
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/// CountTerminators - Count the number of terminators in the given
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/// block and set I to the position of the first non-terminator, if there
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/// is one, or MBB->end() otherwise.
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static unsigned CountTerminators(MachineBasicBlock *MBB,
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MachineBasicBlock::iterator &I) {
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I = MBB->end();
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unsigned NumTerms = 0;
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for (;;) {
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if (I == MBB->begin()) {
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I = MBB->end();
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break;
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}
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--I;
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if (!I->getDesc().isTerminator()) break;
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++NumTerms;
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}
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return NumTerms;
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|
}
|
|
|
|
/// ProfitableToMerge - Check if two machine basic blocks have a common tail
|
|
/// and decide if it would be profitable to merge those tails. Return the
|
|
/// length of the common tail and iterators to the first common instruction
|
|
/// in each block.
|
|
static bool ProfitableToMerge(MachineBasicBlock *MBB1,
|
|
MachineBasicBlock *MBB2,
|
|
unsigned minCommonTailLength,
|
|
unsigned &CommonTailLen,
|
|
MachineBasicBlock::iterator &I1,
|
|
MachineBasicBlock::iterator &I2,
|
|
MachineBasicBlock *SuccBB,
|
|
MachineBasicBlock *PredBB) {
|
|
CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
|
|
MachineFunction *MF = MBB1->getParent();
|
|
|
|
if (CommonTailLen == 0)
|
|
return false;
|
|
|
|
// It's almost always profitable to merge any number of non-terminator
|
|
// instructions with the block that falls through into the common successor.
|
|
if (MBB1 == PredBB || MBB2 == PredBB) {
|
|
MachineBasicBlock::iterator I;
|
|
unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
|
|
if (CommonTailLen > NumTerms)
|
|
return true;
|
|
}
|
|
|
|
// If one of the blocks can be completely merged and happens to be in
|
|
// a position where the other could fall through into it, merge any number
|
|
// of instructions, because it can be done without a branch.
|
|
// TODO: If the blocks are not adjacent, move one of them so that they are?
|
|
if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
|
|
return true;
|
|
if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
|
|
return true;
|
|
|
|
// If both blocks have an unconditional branch temporarily stripped out,
|
|
// count that as an additional common instruction for the following
|
|
// heuristics.
|
|
unsigned EffectiveTailLen = CommonTailLen;
|
|
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
|
|
!MBB1->back().getDesc().isBarrier() &&
|
|
!MBB2->back().getDesc().isBarrier())
|
|
++EffectiveTailLen;
|
|
|
|
// Check if the common tail is long enough to be worthwhile.
|
|
if (EffectiveTailLen >= minCommonTailLength)
|
|
return true;
|
|
|
|
// If we are optimizing for code size, 2 instructions in common is enough if
|
|
// we don't have to split a block. At worst we will be introducing 1 new
|
|
// branch instruction, which is likely to be smaller than the 2
|
|
// instructions that would be deleted in the merge.
|
|
if (EffectiveTailLen >= 2 &&
|
|
MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
|
|
(I1 == MBB1->begin() || I2 == MBB2->begin()))
|
|
return true;
|
|
|
|
return false;
|
|
}
|
|
|
|
/// ComputeSameTails - Look through all the blocks in MergePotentials that have
|
|
/// hash CurHash (guaranteed to match the last element). Build the vector
|
|
/// SameTails of all those that have the (same) largest number of instructions
|
|
/// in common of any pair of these blocks. SameTails entries contain an
|
|
/// iterator into MergePotentials (from which the MachineBasicBlock can be
|
|
/// found) and a MachineBasicBlock::iterator into that MBB indicating the
|
|
/// instruction where the matching code sequence begins.
|
|
/// Order of elements in SameTails is the reverse of the order in which
|
|
/// those blocks appear in MergePotentials (where they are not necessarily
|
|
/// consecutive).
|
|
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
|
|
unsigned minCommonTailLength,
|
|
MachineBasicBlock *SuccBB,
|
|
MachineBasicBlock *PredBB) {
|
|
unsigned maxCommonTailLength = 0U;
|
|
SameTails.clear();
|
|
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
|
|
MPIterator HighestMPIter = prior(MergePotentials.end());
|
|
for (MPIterator CurMPIter = prior(MergePotentials.end()),
|
|
B = MergePotentials.begin();
|
|
CurMPIter != B && CurMPIter->getHash() == CurHash;
|
|
--CurMPIter) {
|
|
for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
|
|
unsigned CommonTailLen;
|
|
if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
|
|
minCommonTailLength,
|
|
CommonTailLen, TrialBBI1, TrialBBI2,
|
|
SuccBB, PredBB)) {
|
|
if (CommonTailLen > maxCommonTailLength) {
|
|
SameTails.clear();
|
|
maxCommonTailLength = CommonTailLen;
|
|
HighestMPIter = CurMPIter;
|
|
SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
|
|
}
|
|
if (HighestMPIter == CurMPIter &&
|
|
CommonTailLen == maxCommonTailLength)
|
|
SameTails.push_back(SameTailElt(I, TrialBBI2));
|
|
}
|
|
if (I == B)
|
|
break;
|
|
}
|
|
}
|
|
return maxCommonTailLength;
|
|
}
|
|
|
|
/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
|
|
/// MergePotentials, restoring branches at ends of blocks as appropriate.
|
|
void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
|
|
MachineBasicBlock *SuccBB,
|
|
MachineBasicBlock *PredBB) {
|
|
MPIterator CurMPIter, B;
|
|
for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
|
|
CurMPIter->getHash() == CurHash;
|
|
--CurMPIter) {
|
|
// Put the unconditional branch back, if we need one.
|
|
MachineBasicBlock *CurMBB = CurMPIter->getBlock();
|
|
if (SuccBB && CurMBB != PredBB)
|
|
FixTail(CurMBB, SuccBB, TII);
|
|
if (CurMPIter == B)
|
|
break;
|
|
}
|
|
if (CurMPIter->getHash() != CurHash)
|
|
CurMPIter++;
|
|
MergePotentials.erase(CurMPIter, MergePotentials.end());
|
|
}
|
|
|
|
/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
|
|
/// only of the common tail. Create a block that does by splitting one.
|
|
bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
|
|
unsigned maxCommonTailLength,
|
|
unsigned &commonTailIndex) {
|
|
commonTailIndex = 0;
|
|
unsigned TimeEstimate = ~0U;
|
|
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
|
|
// Use PredBB if possible; that doesn't require a new branch.
|
|
if (SameTails[i].getBlock() == PredBB) {
|
|
commonTailIndex = i;
|
|
break;
|
|
}
|
|
// Otherwise, make a (fairly bogus) choice based on estimate of
|
|
// how long it will take the various blocks to execute.
|
|
unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
|
|
SameTails[i].getTailStartPos());
|
|
if (t <= TimeEstimate) {
|
|
TimeEstimate = t;
|
|
commonTailIndex = i;
|
|
}
|
|
}
|
|
|
|
MachineBasicBlock::iterator BBI =
|
|
SameTails[commonTailIndex].getTailStartPos();
|
|
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
|
|
|
|
// If the common tail includes any debug info we will take it pretty
|
|
// randomly from one of the inputs. Might be better to remove it?
|
|
DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
|
|
<< maxCommonTailLength);
|
|
|
|
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
|
|
if (!newMBB) {
|
|
DEBUG(dbgs() << "... failed!");
|
|
return false;
|
|
}
|
|
|
|
SameTails[commonTailIndex].setBlock(newMBB);
|
|
SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
|
|
|
|
// If we split PredBB, newMBB is the new predecessor.
|
|
if (PredBB == MBB)
|
|
PredBB = newMBB;
|
|
|
|
return true;
|
|
}
|
|
|
|
// See if any of the blocks in MergePotentials (which all have a common single
|
|
// successor, or all have no successor) can be tail-merged. If there is a
|
|
// successor, any blocks in MergePotentials that are not tail-merged and
|
|
// are not immediately before Succ must have an unconditional branch to
|
|
// Succ added (but the predecessor/successor lists need no adjustment).
|
|
// The lone predecessor of Succ that falls through into Succ,
|
|
// if any, is given in PredBB.
|
|
|
|
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
|
|
MachineBasicBlock *PredBB) {
|
|
bool MadeChange = false;
|
|
|
|
// Except for the special cases below, tail-merge if there are at least
|
|
// this many instructions in common.
|
|
unsigned minCommonTailLength = TailMergeSize;
|
|
|
|
DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
|
|
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
|
|
dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
|
|
<< (i == e-1 ? "" : ", ");
|
|
dbgs() << "\n";
|
|
if (SuccBB) {
|
|
dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
|
|
if (PredBB)
|
|
dbgs() << " which has fall-through from BB#"
|
|
<< PredBB->getNumber() << "\n";
|
|
}
|
|
dbgs() << "Looking for common tails of at least "
|
|
<< minCommonTailLength << " instruction"
|
|
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
|
|
);
|
|
|
|
// Sort by hash value so that blocks with identical end sequences sort
|
|
// together.
|
|
std::stable_sort(MergePotentials.begin(), MergePotentials.end());
|
|
|
|
// Walk through equivalence sets looking for actual exact matches.
|
|
while (MergePotentials.size() > 1) {
|
|
unsigned CurHash = MergePotentials.back().getHash();
|
|
|
|
// Build SameTails, identifying the set of blocks with this hash code
|
|
// and with the maximum number of instructions in common.
|
|
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
|
|
minCommonTailLength,
|
|
SuccBB, PredBB);
|
|
|
|
// If we didn't find any pair that has at least minCommonTailLength
|
|
// instructions in common, remove all blocks with this hash code and retry.
|
|
if (SameTails.empty()) {
|
|
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
|
|
continue;
|
|
}
|
|
|
|
// If one of the blocks is the entire common tail (and not the entry
|
|
// block, which we can't jump to), we can treat all blocks with this same
|
|
// tail at once. Use PredBB if that is one of the possibilities, as that
|
|
// will not introduce any extra branches.
|
|
MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
|
|
getParent()->begin();
|
|
unsigned commonTailIndex = SameTails.size();
|
|
// If there are two blocks, check to see if one can be made to fall through
|
|
// into the other.
|
|
if (SameTails.size() == 2 &&
|
|
SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
|
|
SameTails[1].tailIsWholeBlock())
|
|
commonTailIndex = 1;
|
|
else if (SameTails.size() == 2 &&
|
|
SameTails[1].getBlock()->isLayoutSuccessor(
|
|
SameTails[0].getBlock()) &&
|
|
SameTails[0].tailIsWholeBlock())
|
|
commonTailIndex = 0;
|
|
else {
|
|
// Otherwise just pick one, favoring the fall-through predecessor if
|
|
// there is one.
|
|
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
|
|
MachineBasicBlock *MBB = SameTails[i].getBlock();
|
|
if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
|
|
continue;
|
|
if (MBB == PredBB) {
|
|
commonTailIndex = i;
|
|
break;
|
|
}
|
|
if (SameTails[i].tailIsWholeBlock())
|
|
commonTailIndex = i;
|
|
}
|
|
}
|
|
|
|
if (commonTailIndex == SameTails.size() ||
|
|
(SameTails[commonTailIndex].getBlock() == PredBB &&
|
|
!SameTails[commonTailIndex].tailIsWholeBlock())) {
|
|
// None of the blocks consist entirely of the common tail.
|
|
// Split a block so that one does.
|
|
if (!CreateCommonTailOnlyBlock(PredBB,
|
|
maxCommonTailLength, commonTailIndex)) {
|
|
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
|
|
continue;
|
|
}
|
|
}
|
|
|
|
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
|
|
// MBB is common tail. Adjust all other BB's to jump to this one.
|
|
// Traversal must be forwards so erases work.
|
|
DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
|
|
<< " for ");
|
|
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
|
|
if (commonTailIndex == i)
|
|
continue;
|
|
DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
|
|
<< (i == e-1 ? "" : ", "));
|
|
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
|
|
ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
|
|
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
|
|
MergePotentials.erase(SameTails[i].getMPIter());
|
|
}
|
|
DEBUG(dbgs() << "\n");
|
|
// We leave commonTailIndex in the worklist in case there are other blocks
|
|
// that match it with a smaller number of instructions.
|
|
MadeChange = true;
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
|
|
|
|
if (!EnableTailMerge) return false;
|
|
|
|
bool MadeChange = false;
|
|
|
|
// First find blocks with no successors.
|
|
MergePotentials.clear();
|
|
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
|
|
if (I->succ_empty())
|
|
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
|
|
}
|
|
|
|
// See if we can do any tail merging on those.
|
|
if (MergePotentials.size() < TailMergeThreshold &&
|
|
MergePotentials.size() >= 2)
|
|
MadeChange |= TryTailMergeBlocks(NULL, NULL);
|
|
|
|
// Look at blocks (IBB) with multiple predecessors (PBB).
|
|
// We change each predecessor to a canonical form, by
|
|
// (1) temporarily removing any unconditional branch from the predecessor
|
|
// to IBB, and
|
|
// (2) alter conditional branches so they branch to the other block
|
|
// not IBB; this may require adding back an unconditional branch to IBB
|
|
// later, where there wasn't one coming in. E.g.
|
|
// Bcc IBB
|
|
// fallthrough to QBB
|
|
// here becomes
|
|
// Bncc QBB
|
|
// with a conceptual B to IBB after that, which never actually exists.
|
|
// With those changes, we see whether the predecessors' tails match,
|
|
// and merge them if so. We change things out of canonical form and
|
|
// back to the way they were later in the process. (OptimizeBranches
|
|
// would undo some of this, but we can't use it, because we'd get into
|
|
// a compile-time infinite loop repeatedly doing and undoing the same
|
|
// transformations.)
|
|
|
|
for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
|
|
I != E; ++I) {
|
|
if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
|
|
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
|
|
MachineBasicBlock *IBB = I;
|
|
MachineBasicBlock *PredBB = prior(I);
|
|
MergePotentials.clear();
|
|
for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
|
|
E2 = I->pred_end();
|
|
P != E2; ++P) {
|
|
MachineBasicBlock *PBB = *P;
|
|
// Skip blocks that loop to themselves, can't tail merge these.
|
|
if (PBB == IBB)
|
|
continue;
|
|
// Visit each predecessor only once.
|
|
if (!UniquePreds.insert(PBB))
|
|
continue;
|
|
MachineBasicBlock *TBB = 0, *FBB = 0;
|
|
SmallVector<MachineOperand, 4> Cond;
|
|
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
|
|
// Failing case: IBB is the target of a cbr, and
|
|
// we cannot reverse the branch.
|
|
SmallVector<MachineOperand, 4> NewCond(Cond);
|
|
if (!Cond.empty() && TBB == IBB) {
|
|
if (TII->ReverseBranchCondition(NewCond))
|
|
continue;
|
|
// This is the QBB case described above
|
|
if (!FBB)
|
|
FBB = llvm::next(MachineFunction::iterator(PBB));
|
|
}
|
|
// Failing case: the only way IBB can be reached from PBB is via
|
|
// exception handling. Happens for landing pads. Would be nice
|
|
// to have a bit in the edge so we didn't have to do all this.
|
|
if (IBB->isLandingPad()) {
|
|
MachineFunction::iterator IP = PBB; IP++;
|
|
MachineBasicBlock *PredNextBB = NULL;
|
|
if (IP != MF.end())
|
|
PredNextBB = IP;
|
|
if (TBB == NULL) {
|
|
if (IBB != PredNextBB) // fallthrough
|
|
continue;
|
|
} else if (FBB) {
|
|
if (TBB != IBB && FBB != IBB) // cbr then ubr
|
|
continue;
|
|
} else if (Cond.empty()) {
|
|
if (TBB != IBB) // ubr
|
|
continue;
|
|
} else {
|
|
if (TBB != IBB && IBB != PredNextBB) // cbr
|
|
continue;
|
|
}
|
|
}
|
|
// Remove the unconditional branch at the end, if any.
|
|
if (TBB && (Cond.empty() || FBB)) {
|
|
DebugLoc dl; // FIXME: this is nowhere
|
|
TII->RemoveBranch(*PBB);
|
|
if (!Cond.empty())
|
|
// reinsert conditional branch only, for now
|
|
TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl);
|
|
}
|
|
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
|
|
}
|
|
}
|
|
if (MergePotentials.size() >= 2)
|
|
MadeChange |= TryTailMergeBlocks(IBB, PredBB);
|
|
// Reinsert an unconditional branch if needed.
|
|
// The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
|
|
PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
|
|
if (MergePotentials.size() == 1 &&
|
|
MergePotentials.begin()->getBlock() != PredBB)
|
|
FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
|
|
}
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
// Branch Optimization
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
|
|
bool MadeChange = false;
|
|
|
|
// Make sure blocks are numbered in order
|
|
MF.RenumberBlocks();
|
|
|
|
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
|
|
MachineBasicBlock *MBB = I++;
|
|
MadeChange |= OptimizeBlock(MBB);
|
|
|
|
// If it is dead, remove it.
|
|
if (MBB->pred_empty()) {
|
|
RemoveDeadBlock(MBB);
|
|
MadeChange = true;
|
|
++NumDeadBlocks;
|
|
}
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
// Blocks should be considered empty if they contain only debug info;
|
|
// else the debug info would affect codegen.
|
|
static bool IsEmptyBlock(MachineBasicBlock *MBB) {
|
|
if (MBB->empty())
|
|
return true;
|
|
for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
|
|
MBBI!=MBBE; ++MBBI) {
|
|
if (!MBBI->isDebugValue())
|
|
return false;
|
|
}
|
|
return true;
|
|
}
|
|
|
|
// Blocks with only debug info and branches should be considered the same
|
|
// as blocks with only branches.
|
|
static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
|
|
MachineBasicBlock::iterator MBBI, MBBE;
|
|
for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
|
|
if (!MBBI->isDebugValue())
|
|
break;
|
|
}
|
|
return (MBBI->getDesc().isBranch());
|
|
}
|
|
|
|
/// 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) {
|
|
// 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 (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
|
|
|
|
// If there is a clear successor ordering we make sure that one block
|
|
// will fall through to the next
|
|
if (MBB1->isSuccessor(MBB2)) return true;
|
|
if (MBB2->isSuccessor(MBB1)) return false;
|
|
|
|
// Neither block consists entirely of debug info (per IsEmptyBlock check),
|
|
// so we needn't test for falling off the beginning here.
|
|
MachineBasicBlock::iterator MBB1I = --MBB1->end();
|
|
while (MBB1I->isDebugValue())
|
|
--MBB1I;
|
|
MachineBasicBlock::iterator MBB2I = --MBB2->end();
|
|
while (MBB2I->isDebugValue())
|
|
--MBB2I;
|
|
return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
|
|
}
|
|
|
|
/// OptimizeBlock - Analyze and optimize control flow related to the specified
|
|
/// block. This is never called on the entry block.
|
|
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
|
|
bool MadeChange = false;
|
|
MachineFunction &MF = *MBB->getParent();
|
|
DebugLoc dl; // FIXME: this is nowhere
|
|
ReoptimizeBlock:
|
|
|
|
MachineFunction::iterator FallThrough = MBB;
|
|
++FallThrough;
|
|
|
|
// If this block is empty, make everyone use its fall-through, not the block
|
|
// explicitly. Landing pads should not do this since the landing-pad table
|
|
// points to this block. Blocks with their addresses taken shouldn't be
|
|
// optimized away.
|
|
if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
|
|
// Dead block? Leave for cleanup later.
|
|
if (MBB->pred_empty()) return MadeChange;
|
|
|
|
if (FallThrough == MF.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);
|
|
Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
|
|
}
|
|
// If MBB was the target of a jump table, update jump tables to go to the
|
|
// fallthrough instead.
|
|
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
|
|
MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
|
|
MadeChange = true;
|
|
}
|
|
return MadeChange;
|
|
}
|
|
|
|
// 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;
|
|
SmallVector<MachineOperand, 4> PriorCond;
|
|
bool PriorUnAnalyzable =
|
|
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
|
|
if (!PriorUnAnalyzable) {
|
|
// If the CFG for the prior block has extra edges, remove them.
|
|
MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
|
|
!PriorCond.empty());
|
|
|
|
// 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, dl);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
|
|
// If the previous block unconditionally falls through to this block and
|
|
// this block has no other predecessors, move the contents of this block
|
|
// into the prior block. This doesn't usually happen when SimplifyCFG
|
|
// has been used, but it can happen if tail merging splits a fall-through
|
|
// predecessor of a block.
|
|
// This has to check PrevBB->succ_size() because EH edges are ignored by
|
|
// AnalyzeBranch.
|
|
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
|
|
PrevBB.succ_size() == 1 &&
|
|
!MBB->hasAddressTaken()) {
|
|
DEBUG(dbgs() << "\nMerging into block: " << PrevBB
|
|
<< "From MBB: " << *MBB);
|
|
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
|
|
PrevBB.removeSuccessor(PrevBB.succ_begin());;
|
|
assert(PrevBB.succ_empty());
|
|
PrevBB.transferSuccessors(MBB);
|
|
MadeChange = true;
|
|
return MadeChange;
|
|
}
|
|
|
|
// 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;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
|
|
// 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, dl);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
|
|
// 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) {
|
|
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
|
|
if (!TII->ReverseBranchCondition(NewPriorCond)) {
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
}
|
|
|
|
// If this block has no successors (e.g. it is a return block or ends with
|
|
// a call to a no-return function like abort or __cxa_throw) 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 (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
|
|
MachineFunction::iterator(PriorTBB) == FallThrough &&
|
|
!MBB->canFallThrough()) {
|
|
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 == --MF.end() &&
|
|
!IsBetterFallthrough(PriorTBB, MBB))
|
|
DoTransform = false;
|
|
|
|
if (DoTransform) {
|
|
// Reverse the branch so we will fall through on the previous true cond.
|
|
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
|
|
if (!TII->ReverseBranchCondition(NewPriorCond)) {
|
|
DEBUG(dbgs() << "\nMoving MBB: " << *MBB
|
|
<< "To make fallthrough to: " << *PriorTBB << "\n");
|
|
|
|
TII->RemoveBranch(PrevBB);
|
|
TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl);
|
|
|
|
// Move this block to the end of the function.
|
|
MBB->moveAfter(--MF.end());
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
return MadeChange;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
// Analyze the branch in the current block.
|
|
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
|
|
SmallVector<MachineOperand, 4> CurCond;
|
|
bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
|
|
if (!CurUnAnalyzable) {
|
|
// If the CFG for the prior block has extra edges, remove them.
|
|
MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
|
|
|
|
// 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) {
|
|
SmallVector<MachineOperand, 4> NewCond(CurCond);
|
|
if (!TII->ReverseBranchCondition(NewCond)) {
|
|
TII->RemoveBranch(*MBB);
|
|
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
}
|
|
|
|
// 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 &&
|
|
IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
|
|
!MBB->hasAddressTaken()) {
|
|
// 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 the only things remaining in the block are debug info, remove these
|
|
// as well, so this will behave the same as an empty block in non-debug
|
|
// mode.
|
|
if (!MBB->empty()) {
|
|
bool NonDebugInfoFound = false;
|
|
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
|
|
I != E; ++I) {
|
|
if (!I->isDebugValue()) {
|
|
NonDebugInfoFound = true;
|
|
break;
|
|
}
|
|
}
|
|
if (!NonDebugInfoFound)
|
|
// Make the block empty, losing the debug info (we could probably
|
|
// improve this in some cases.)
|
|
MBB->erase(MBB->begin(), MBB->end());
|
|
}
|
|
// 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 = !PrevBB.canFallThrough();
|
|
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, dl);
|
|
}
|
|
|
|
// Iterate through all the predecessors, revectoring each in-turn.
|
|
size_t PI = 0;
|
|
bool DidChange = false;
|
|
bool HasBranchToSelf = false;
|
|
while(PI != MBB->pred_size()) {
|
|
MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
|
|
if (PMBB == MBB) {
|
|
// If this block has an uncond branch to itself, leave it.
|
|
++PI;
|
|
HasBranchToSelf = true;
|
|
} else {
|
|
DidChange = true;
|
|
PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
|
|
// If this change resulted in PMBB ending in a conditional
|
|
// branch where both conditions go to the same destination,
|
|
// change this to an unconditional branch (and fix the CFG).
|
|
MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
|
|
SmallVector<MachineOperand, 4> NewCurCond;
|
|
bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
|
|
NewCurFBB, NewCurCond, true);
|
|
if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
|
|
TII->RemoveBranch(*PMBB);
|
|
NewCurCond.clear();
|
|
TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, dl);
|
|
MadeChange = true;
|
|
++NumBranchOpts;
|
|
PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
|
|
}
|
|
}
|
|
}
|
|
|
|
// Change any jumptables to go to the new MBB.
|
|
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
|
|
MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
|
|
if (DidChange) {
|
|
++NumBranchOpts;
|
|
MadeChange = true;
|
|
if (!HasBranchToSelf) return MadeChange;
|
|
}
|
|
}
|
|
}
|
|
|
|
// Add the branch back if the block is more than just an uncond branch.
|
|
TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl);
|
|
}
|
|
}
|
|
|
|
// 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 (!PrevBB.canFallThrough()) {
|
|
|
|
// 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 = MBB->canFallThrough();
|
|
|
|
if (!MBB->isLandingPad()) {
|
|
// 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;
|
|
MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
|
|
SmallVector<MachineOperand, 4> PredCond;
|
|
if (PredBB != MBB && !PredBB->canFallThrough() &&
|
|
!TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
|
|
&& (!CurFallsThru || !CurTBB || !CurFBB)
|
|
&& (!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.
|
|
// Also, if there are already 2 branches here, we cannot add a third;
|
|
// this means we have the case
|
|
// Bcc next
|
|
// B elsewhere
|
|
// next:
|
|
if (CurFallsThru) {
|
|
MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
|
|
CurCond.clear();
|
|
TII->InsertBranch(*MBB, NextBB, 0, CurCond, dl);
|
|
}
|
|
MBB->moveAfter(PredBB);
|
|
MadeChange = true;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
}
|
|
}
|
|
|
|
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;
|
|
|
|
// If this block doesn't already fall-through to that successor, and if
|
|
// the succ doesn't already have a block that can fall through into it,
|
|
// and if the successor isn't an EH destination, we can arrange for the
|
|
// fallthrough to happen.
|
|
if (SuccBB != MBB && &*SuccPrev != MBB &&
|
|
!SuccPrev->canFallThrough() && !CurUnAnalyzable &&
|
|
!SuccBB->isLandingPad()) {
|
|
MBB->moveBefore(SuccBB);
|
|
MadeChange = true;
|
|
goto ReoptimizeBlock;
|
|
}
|
|
}
|
|
|
|
// 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.
|
|
MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
|
|
SmallVector<MachineOperand, 4> PrevCond;
|
|
if (FallThrough != MF.end() &&
|
|
!TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
|
|
PrevBB.isSuccessor(FallThrough)) {
|
|
MBB->moveAfter(--MF.end());
|
|
MadeChange = true;
|
|
return MadeChange;
|
|
}
|
|
}
|
|
}
|
|
|
|
return MadeChange;
|
|
}
|