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//===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains a pass that scans a machine function to determine which
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// conditional branches need more than 10 bits of displacement to reach their
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// target basic block. It does this in two passes; a calculation of basic block
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// positions pass, and a branch pseudo op to machine branch opcode pass. This
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// pass should be run last, just before the assembly printer.
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//
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//===----------------------------------------------------------------------===//
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#include "MSP430.h"
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#include "MSP430InstrInfo.h"
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#include "MSP430Subtarget.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/Support/MathExtras.h"
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#include "llvm/Target/TargetMachine.h"
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using namespace llvm;
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#define DEBUG_TYPE "msp430-branch-select"
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static cl::opt<bool>
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BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
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cl::desc("Expand out of range branches"));
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STATISTIC(NumSplit, "Number of machine basic blocks split");
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STATISTIC(NumExpanded, "Number of branches expanded to long format");
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namespace {
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class MSP430BSel : public MachineFunctionPass {
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typedef SmallVector<int, 16> OffsetVector;
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MachineFunction *MF;
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const MSP430InstrInfo *TII;
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unsigned measureFunction(OffsetVector &BlockOffsets,
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MachineBasicBlock *FromBB = nullptr);
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bool expandBranches(OffsetVector &BlockOffsets);
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public:
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static char ID;
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MSP430BSel() : MachineFunctionPass(ID) {}
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bool runOnMachineFunction(MachineFunction &MF) override;
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MachineFunctionProperties getRequiredProperties() const override {
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return MachineFunctionProperties().set(
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MachineFunctionProperties::Property::NoVRegs);
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}
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StringRef getPassName() const override { return "MSP430 Branch Selector"; }
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};
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char MSP430BSel::ID = 0;
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}
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static bool isInRage(int DistanceInBytes) {
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// According to CC430 Family User's Guide, Section 4.5.1.3, branch
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// instructions have the signed 10-bit word offset field, so first we need to
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// convert the distance from bytes to words, then check if it fits in 10-bit
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// signed integer.
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const int WordSize = 2;
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assert((DistanceInBytes % WordSize == 0) &&
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"Branch offset should be word aligned!");
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int Words = DistanceInBytes / WordSize;
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return isInt<10>(Words);
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}
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/// Measure each basic block, fill the BlockOffsets, and return the size of
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/// the function, starting with BB
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unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
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MachineBasicBlock *FromBB) {
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// Give the blocks of the function a dense, in-order, numbering.
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MF->RenumberBlocks(FromBB);
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MachineFunction::iterator Begin;
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if (FromBB == nullptr) {
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Begin = MF->begin();
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} else {
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Begin = FromBB->getIterator();
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}
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BlockOffsets.resize(MF->getNumBlockIDs());
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unsigned TotalSize = BlockOffsets[Begin->getNumber()];
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for (auto &MBB : make_range(Begin, MF->end())) {
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BlockOffsets[MBB.getNumber()] = TotalSize;
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for (MachineInstr &MI : MBB) {
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TotalSize += TII->getInstSizeInBytes(MI);
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}
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}
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return TotalSize;
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}
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/// Do expand branches and split the basic blocks if necessary.
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/// Returns true if made any change.
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bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
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// For each conditional branch, if the offset to its destination is larger
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// than the offset field allows, transform it into a long branch sequence
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// like this:
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// short branch:
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// bCC MBB
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// long branch:
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// b!CC $PC+6
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// b MBB
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//
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bool MadeChange = false;
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for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
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unsigned MBBStartOffset = 0;
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for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
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MBBStartOffset += TII->getInstSizeInBytes(*MI);
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// If this instruction is not a short branch then skip it.
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if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
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continue;
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}
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MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
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// Determine the distance from the current branch to the destination
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// block. MBBStartOffset already includes the size of the current branch
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// instruction.
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int BlockDistance =
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BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
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int BranchDistance = BlockDistance - MBBStartOffset;
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// If this branch is in range, ignore it.
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if (isInRage(BranchDistance)) {
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continue;
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}
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DEBUG(dbgs() << " Found a branch that needs expanding, "
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<< printMBBReference(*DestBB) << ", Distance "
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<< BranchDistance << "\n");
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// If JCC is not the last instruction we need to split the MBB.
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if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
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DEBUG(dbgs() << " Found a basic block that needs to be split, "
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<< printMBBReference(*MBB) << "\n");
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// Create a new basic block.
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MachineBasicBlock *NewBB =
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MF->CreateMachineBasicBlock(MBB->getBasicBlock());
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MF->insert(std::next(MBB), NewBB);
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// Splice the instructions following MI over to the NewBB.
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NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
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// Update the successor lists.
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for (MachineBasicBlock *Succ : MBB->successors()) {
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if (Succ == DestBB) {
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continue;
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}
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MBB->replaceSuccessor(Succ, NewBB);
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NewBB->addSuccessor(Succ);
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}
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// We introduced a new MBB so all following blocks should be numbered
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// and measured again.
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measureFunction(BlockOffsets, &*MBB);
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++NumSplit;
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// It may be not necessary to start all over at this point, but it's
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// safer do this anyway.
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return true;
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}
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MachineInstr &OldBranch = *MI;
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DebugLoc dl = OldBranch.getDebugLoc();
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int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
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if (MI->getOpcode() == MSP430::JCC) {
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MachineBasicBlock *NextMBB = &*std::next(MBB);
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assert(MBB->isSuccessor(NextMBB) &&
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"This block must have a layout successor!");
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// The BCC operands are:
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// 0. Target MBB
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// 1. MSP430 branch predicate
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SmallVector<MachineOperand, 1> Cond;
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Cond.push_back(MI->getOperand(1));
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// Jump over the long branch on the opposite condition
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TII->reverseBranchCondition(Cond);
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MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
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.addMBB(NextMBB)
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.add(Cond[0]);
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InstrSizeDiff += TII->getInstSizeInBytes(*MI);
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++MI;
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}
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// Unconditional branch to the real destination.
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MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
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InstrSizeDiff += TII->getInstSizeInBytes(*MI);
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// Remove the old branch from the function.
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OldBranch.eraseFromParent();
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// The size of a new instruction is different from the old one, so we need
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// to correct all block offsets.
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for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
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BlockOffsets[i] += InstrSizeDiff;
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}
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MBBStartOffset += InstrSizeDiff;
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++NumExpanded;
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MadeChange = true;
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}
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}
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return MadeChange;
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}
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bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
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MF = &mf;
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TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
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// If the pass is disabled, just bail early.
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if (!BranchSelectEnabled)
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return false;
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DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
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// BlockOffsets - Contains the distance from the beginning of the function to
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// the beginning of each basic block.
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OffsetVector BlockOffsets;
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unsigned FunctionSize = measureFunction(BlockOffsets);
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// If the entire function is smaller than the displacement of a branch field,
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// we know we don't need to expand any branches in this
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// function. This is a common case.
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if (isInRage(FunctionSize)) {
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return false;
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}
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// Iteratively expand branches until we reach a fixed point.
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bool MadeChange = false;
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while (expandBranches(BlockOffsets))
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MadeChange = true;
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return MadeChange;
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}
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/// Returns an instance of the Branch Selection Pass
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FunctionPass *llvm::createMSP430BranchSelectionPass() {
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return new MSP430BSel();
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}
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