1 //===-- MSP430BranchSelector.cpp - Emit long conditional branches--*- C++ -*-=//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file contains a pass that scans a machine function to determine which
11 // conditional branches need more than 10 bits of displacement to reach their
12 // target basic block. It does this in two passes; a calculation of basic block
13 // positions pass, and a branch psuedo op to machine branch opcode pass. This
14 // pass should be run last, just before the assembly printer.
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "msp430-branch-select"
20 #include "MSP430InstrInfo.h"
21 #include "llvm/CodeGen/MachineInstrBuilder.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/Target/TargetMachine.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Support/MathExtras.h"
28 STATISTIC(NumExpanded, "Number of branches expanded to long format");
31 struct MSP430BSel : public MachineFunctionPass {
33 MSP430BSel() : MachineFunctionPass(&ID) {}
35 /// BlockSizes - The sizes of the basic blocks in the function.
36 std::vector<unsigned> BlockSizes;
38 virtual bool runOnMachineFunction(MachineFunction &Fn);
40 virtual const char *getPassName() const {
41 return "MSP430 Branch Selector";
44 char MSP430BSel::ID = 0;
47 /// createMSP430BranchSelectionPass - returns an instance of the Branch
50 FunctionPass *llvm::createMSP430BranchSelectionPass() {
51 return new MSP430BSel();
54 bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
55 const TargetInstrInfo *TII = Fn.getTarget().getInstrInfo();
56 // Give the blocks of the function a dense, in-order, numbering.
58 BlockSizes.resize(Fn.getNumBlockIDs());
60 // Measure each MBB and compute a size for the entire function.
61 unsigned FuncSize = 0;
62 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
64 MachineBasicBlock *MBB = MFI;
66 unsigned BlockSize = 0;
67 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
69 BlockSize += TII->GetInstSizeInBytes(MBBI);
71 BlockSizes[MBB->getNumber()] = BlockSize;
72 FuncSize += BlockSize;
75 // If the entire function is smaller than the displacement of a branch field,
76 // we know we don't need to shrink any branches in this function. This is a
78 if (FuncSize < (1 << 9)) {
83 // For each conditional branch, if the offset to its destination is larger
84 // than the offset field allows, transform it into a long branch sequence
92 bool MadeChange = true;
93 bool EverMadeChange = false;
95 // Iteratively expand branches until we reach a fixed point.
98 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
100 MachineBasicBlock &MBB = *MFI;
101 unsigned MBBStartOffset = 0;
102 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
104 if ((I->getOpcode() != MSP430::JCC || I->getOperand(0).isImm()) &&
105 I->getOpcode() != MSP430::JMP) {
106 MBBStartOffset += TII->GetInstSizeInBytes(I);
110 // Determine the offset from the current branch to the destination
112 MachineBasicBlock *Dest = I->getOperand(0).getMBB();
115 if (Dest->getNumber() <= MBB.getNumber()) {
116 // If this is a backwards branch, the delta is the offset from the
117 // start of this block to this branch, plus the sizes of all blocks
118 // from this block to the dest.
119 BranchSize = MBBStartOffset;
121 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
122 BranchSize += BlockSizes[i];
124 // Otherwise, add the size of the blocks between this block and the
125 // dest to the number of bytes left in this block.
126 BranchSize = -MBBStartOffset;
128 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
129 BranchSize += BlockSizes[i];
132 // If this branch is in range, ignore it.
133 if (isInt<10>(BranchSize)) {
138 // Otherwise, we have to expand it to a long branch.
140 MachineInstr *OldBranch = I;
141 DebugLoc dl = OldBranch->getDebugLoc();
143 if (I->getOpcode() == MSP430::JMP) {
146 // The BCC operands are:
147 // 0. MSP430 branch predicate
149 SmallVector<MachineOperand, 1> Cond;
150 Cond.push_back(I->getOperand(1));
152 // Jump over the uncond branch inst (i.e. $+6) on opposite condition.
153 TII->ReverseBranchCondition(Cond);
154 BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
155 .addImm(4).addOperand(Cond[0]);
159 // Uncond branch to the real destination.
160 I = BuildMI(MBB, I, dl, TII->get(MSP430::B)).addMBB(Dest);
162 // Remove the old branch from the function.
163 OldBranch->eraseFromParent();
165 // Remember that this instruction is NewSize bytes, increase the size of the
166 // block by NewSize-2, remember to iterate.
167 BlockSizes[MBB.getNumber()] += NewSize-2;
168 MBBStartOffset += NewSize;
174 EverMadeChange |= MadeChange;