2012-02-18 20:03:15 +08:00
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//===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
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2010-01-16 05:19:05 +08:00
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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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2010-08-24 04:30:51 +08:00
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// positions pass, and a branch pseudo op to machine branch opcode pass. This
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2010-01-16 05:19:05 +08:00
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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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#define DEBUG_TYPE "msp430-branch-select"
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#include "MSP430.h"
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#include "MSP430InstrInfo.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/MathExtras.h"
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using namespace llvm;
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STATISTIC(NumExpanded, "Number of branches expanded to long format");
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namespace {
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struct MSP430BSel : public MachineFunctionPass {
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static char ID;
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2010-08-07 02:33:48 +08:00
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MSP430BSel() : MachineFunctionPass(ID) {}
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2010-01-16 05:19:05 +08:00
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/// BlockSizes - The sizes of the basic blocks in the function.
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std::vector<unsigned> BlockSizes;
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virtual bool runOnMachineFunction(MachineFunction &Fn);
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virtual const char *getPassName() const {
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return "MSP430 Branch Selector";
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}
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};
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char MSP430BSel::ID = 0;
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}
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/// createMSP430BranchSelectionPass - returns an instance of the Branch
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/// Selection Pass
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///
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FunctionPass *llvm::createMSP430BranchSelectionPass() {
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return new MSP430BSel();
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}
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bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
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2010-07-23 21:28:47 +08:00
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const MSP430InstrInfo *TII =
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static_cast<const MSP430InstrInfo*>(Fn.getTarget().getInstrInfo());
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2010-01-16 05:19:05 +08:00
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// Give the blocks of the function a dense, in-order, numbering.
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Fn.RenumberBlocks();
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BlockSizes.resize(Fn.getNumBlockIDs());
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// Measure each MBB and compute a size for the entire function.
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unsigned FuncSize = 0;
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for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
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++MFI) {
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MachineBasicBlock *MBB = MFI;
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unsigned BlockSize = 0;
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for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
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MBBI != EE; ++MBBI)
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BlockSize += TII->GetInstSizeInBytes(MBBI);
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BlockSizes[MBB->getNumber()] = BlockSize;
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FuncSize += BlockSize;
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}
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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 shrink any branches in this function. This is a
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// common case.
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if (FuncSize < (1 << 9)) {
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BlockSizes.clear();
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return false;
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}
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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 = true;
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bool EverMadeChange = false;
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while (MadeChange) {
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// Iteratively expand branches until we reach a fixed point.
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MadeChange = false;
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for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
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++MFI) {
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MachineBasicBlock &MBB = *MFI;
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unsigned MBBStartOffset = 0;
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for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
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I != E; ++I) {
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if ((I->getOpcode() != MSP430::JCC || I->getOperand(0).isImm()) &&
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I->getOpcode() != MSP430::JMP) {
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MBBStartOffset += TII->GetInstSizeInBytes(I);
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continue;
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}
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// Determine the offset from the current branch to the destination
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// block.
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MachineBasicBlock *Dest = I->getOperand(0).getMBB();
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int BranchSize;
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if (Dest->getNumber() <= MBB.getNumber()) {
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// If this is a backwards branch, the delta is the offset from the
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// start of this block to this branch, plus the sizes of all blocks
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// from this block to the dest.
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BranchSize = MBBStartOffset;
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for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
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BranchSize += BlockSizes[i];
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} else {
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// Otherwise, add the size of the blocks between this block and the
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// dest to the number of bytes left in this block.
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BranchSize = -MBBStartOffset;
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for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
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BranchSize += BlockSizes[i];
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}
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// If this branch is in range, ignore it.
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if (isInt<10>(BranchSize)) {
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MBBStartOffset += 2;
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continue;
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}
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// Otherwise, we have to expand it to a long branch.
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unsigned NewSize;
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MachineInstr *OldBranch = I;
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DebugLoc dl = OldBranch->getDebugLoc();
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if (I->getOpcode() == MSP430::JMP) {
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NewSize = 4;
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} else {
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// The BCC operands are:
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// 0. MSP430 branch predicate
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// 1. Target MBB
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SmallVector<MachineOperand, 1> Cond;
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Cond.push_back(I->getOperand(1));
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// Jump over the uncond branch inst (i.e. $+6) on opposite condition.
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TII->ReverseBranchCondition(Cond);
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BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
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.addImm(4).addOperand(Cond[0]);
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NewSize = 6;
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}
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// Uncond branch to the real destination.
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2010-05-01 20:28:21 +08:00
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I = BuildMI(MBB, I, dl, TII->get(MSP430::Bi)).addMBB(Dest);
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2010-01-16 05:19:05 +08:00
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// Remove the old branch from the function.
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OldBranch->eraseFromParent();
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// Remember that this instruction is NewSize bytes, increase the size of the
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// block by NewSize-2, remember to iterate.
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BlockSizes[MBB.getNumber()] += NewSize-2;
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MBBStartOffset += NewSize;
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++NumExpanded;
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MadeChange = true;
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}
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}
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EverMadeChange |= MadeChange;
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}
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BlockSizes.clear();
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return true;
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}
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