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//===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains a pass that scans a machine function to determine which
// conditional branches need more than 10 bits of displacement to reach their
// target basic block. It does this in two passes; a calculation of basic block
// positions pass, and a branch pseudo op to machine branch opcode pass. This
// pass should be run last, just before the assembly printer.
//
//===----------------------------------------------------------------------===//
#include "MSP430.h"
#include "MSP430InstrInfo.h"
#include "MSP430Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
#define DEBUG_TYPE "msp430-branch-select"
static cl::opt<bool>
BranchSelectEnabled("msp430-branch-select", cl::Hidden, cl::init(true),
cl::desc("Expand out of range branches"));
STATISTIC(NumSplit, "Number of machine basic blocks split");
STATISTIC(NumExpanded, "Number of branches expanded to long format");
namespace {
class MSP430BSel : public MachineFunctionPass {
typedef SmallVector<int, 16> OffsetVector;
MachineFunction *MF;
const MSP430InstrInfo *TII;
unsigned measureFunction(OffsetVector &BlockOffsets,
MachineBasicBlock *FromBB = nullptr);
bool expandBranches(OffsetVector &BlockOffsets);
public:
static char ID;
MSP430BSel() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
StringRef getPassName() const override { return "MSP430 Branch Selector"; }
};
char MSP430BSel::ID = 0;
}
static bool isInRage(int DistanceInBytes) {
// According to CC430 Family User's Guide, Section 4.5.1.3, branch
// instructions have the signed 10-bit word offset field, so first we need to
// convert the distance from bytes to words, then check if it fits in 10-bit
// signed integer.
const int WordSize = 2;
assert((DistanceInBytes % WordSize == 0) &&
"Branch offset should be word aligned!");
int Words = DistanceInBytes / WordSize;
return isInt<10>(Words);
}
/// Measure each basic block, fill the BlockOffsets, and return the size of
/// the function, starting with BB
unsigned MSP430BSel::measureFunction(OffsetVector &BlockOffsets,
MachineBasicBlock *FromBB) {
// Give the blocks of the function a dense, in-order, numbering.
MF->RenumberBlocks(FromBB);
MachineFunction::iterator Begin;
if (FromBB == nullptr) {
Begin = MF->begin();
} else {
Begin = FromBB->getIterator();
}
BlockOffsets.resize(MF->getNumBlockIDs());
unsigned TotalSize = BlockOffsets[Begin->getNumber()];
for (auto &MBB : make_range(Begin, MF->end())) {
BlockOffsets[MBB.getNumber()] = TotalSize;
for (MachineInstr &MI : MBB) {
TotalSize += TII->getInstSizeInBytes(MI);
}
}
return TotalSize;
}
/// Do expand branches and split the basic blocks if necessary.
/// Returns true if made any change.
bool MSP430BSel::expandBranches(OffsetVector &BlockOffsets) {
// For each conditional branch, if the offset to its destination is larger
// than the offset field allows, transform it into a long branch sequence
// like this:
// short branch:
// bCC MBB
// long branch:
// b!CC $PC+6
// b MBB
//
bool MadeChange = false;
for (auto MBB = MF->begin(), E = MF->end(); MBB != E; ++MBB) {
unsigned MBBStartOffset = 0;
for (auto MI = MBB->begin(), EE = MBB->end(); MI != EE; ++MI) {
MBBStartOffset += TII->getInstSizeInBytes(*MI);
// If this instruction is not a short branch then skip it.
if (MI->getOpcode() != MSP430::JCC && MI->getOpcode() != MSP430::JMP) {
continue;
}
MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
// Determine the distance from the current branch to the destination
// block. MBBStartOffset already includes the size of the current branch
// instruction.
int BlockDistance =
BlockOffsets[DestBB->getNumber()] - BlockOffsets[MBB->getNumber()];
int BranchDistance = BlockDistance - MBBStartOffset;
// If this branch is in range, ignore it.
if (isInRage(BranchDistance)) {
continue;
}
LLVM_DEBUG(dbgs() << " Found a branch that needs expanding, "
<< printMBBReference(*DestBB) << ", Distance "
<< BranchDistance << "\n");
// If JCC is not the last instruction we need to split the MBB.
if (MI->getOpcode() == MSP430::JCC && std::next(MI) != EE) {
LLVM_DEBUG(dbgs() << " Found a basic block that needs to be split, "
<< printMBBReference(*MBB) << "\n");
// Create a new basic block.
MachineBasicBlock *NewBB =
MF->CreateMachineBasicBlock(MBB->getBasicBlock());
MF->insert(std::next(MBB), NewBB);
// Splice the instructions following MI over to the NewBB.
NewBB->splice(NewBB->end(), &*MBB, std::next(MI), MBB->end());
// Update the successor lists.
for (MachineBasicBlock *Succ : MBB->successors()) {
if (Succ == DestBB) {
continue;
}
MBB->replaceSuccessor(Succ, NewBB);
NewBB->addSuccessor(Succ);
}
// We introduced a new MBB so all following blocks should be numbered
// and measured again.
measureFunction(BlockOffsets, &*MBB);
++NumSplit;
// It may be not necessary to start all over at this point, but it's
// safer do this anyway.
return true;
}
MachineInstr &OldBranch = *MI;
DebugLoc dl = OldBranch.getDebugLoc();
int InstrSizeDiff = -TII->getInstSizeInBytes(OldBranch);
if (MI->getOpcode() == MSP430::JCC) {
MachineBasicBlock *NextMBB = &*std::next(MBB);
assert(MBB->isSuccessor(NextMBB) &&
"This block must have a layout successor!");
// The BCC operands are:
// 0. Target MBB
// 1. MSP430 branch predicate
SmallVector<MachineOperand, 1> Cond;
Cond.push_back(MI->getOperand(1));
// Jump over the long branch on the opposite condition
TII->reverseBranchCondition(Cond);
MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::JCC))
.addMBB(NextMBB)
.add(Cond[0]);
InstrSizeDiff += TII->getInstSizeInBytes(*MI);
++MI;
}
// Unconditional branch to the real destination.
MI = BuildMI(*MBB, MI, dl, TII->get(MSP430::Bi)).addMBB(DestBB);
InstrSizeDiff += TII->getInstSizeInBytes(*MI);
// Remove the old branch from the function.
OldBranch.eraseFromParent();
// The size of a new instruction is different from the old one, so we need
// to correct all block offsets.
for (int i = MBB->getNumber() + 1, e = BlockOffsets.size(); i < e; ++i) {
BlockOffsets[i] += InstrSizeDiff;
}
MBBStartOffset += InstrSizeDiff;
++NumExpanded;
MadeChange = true;
}
}
return MadeChange;
}
bool MSP430BSel::runOnMachineFunction(MachineFunction &mf) {
MF = &mf;
TII = static_cast<const MSP430InstrInfo *>(MF->getSubtarget().getInstrInfo());
// If the pass is disabled, just bail early.
if (!BranchSelectEnabled)
return false;
LLVM_DEBUG(dbgs() << "\n********** " << getPassName() << " **********\n");
// BlockOffsets - Contains the distance from the beginning of the function to
// the beginning of each basic block.
OffsetVector BlockOffsets;
unsigned FunctionSize = measureFunction(BlockOffsets);
// If the entire function is smaller than the displacement of a branch field,
// we know we don't need to expand any branches in this
// function. This is a common case.
if (isInRage(FunctionSize)) {
return false;
}
// Iteratively expand branches until we reach a fixed point.
bool MadeChange = false;
while (expandBranches(BlockOffsets))
MadeChange = true;
return MadeChange;
}
/// Returns an instance of the Branch Selection Pass
FunctionPass *llvm::createMSP430BranchSelectionPass() {
return new MSP430BSel();
}