1 //===-- MSP430BranchSelector.cpp - Emit long conditional branches ---------===//
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 pseudo op to machine branch opcode pass. This
14 // pass should be run last, just before the assembly printer.
16 //===----------------------------------------------------------------------===//
19 #include "MSP430InstrInfo.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/CodeGen/MachineFunctionPass.h"
22 #include "llvm/CodeGen/MachineInstrBuilder.h"
23 #include "llvm/Support/MathExtras.h"
24 #include "llvm/Target/TargetMachine.h"
27 #define DEBUG_TYPE "msp430-branch-select"
29 STATISTIC(NumExpanded, "Number of branches expanded to long format");
32 struct MSP430BSel : public MachineFunctionPass {
34 MSP430BSel() : MachineFunctionPass(ID) {}
36 /// BlockSizes - The sizes of the basic blocks in the function.
37 std::vector<unsigned> BlockSizes;
39 bool runOnMachineFunction(MachineFunction &Fn) override;
41 const char *getPassName() const override {
42 return "MSP430 Branch Selector";
45 char MSP430BSel::ID = 0;
48 /// createMSP430BranchSelectionPass - returns an instance of the Branch
51 FunctionPass *llvm::createMSP430BranchSelectionPass() {
52 return new MSP430BSel();
55 bool MSP430BSel::runOnMachineFunction(MachineFunction &Fn) {
56 const MSP430InstrInfo *TII =
57 static_cast<const MSP430InstrInfo*>(Fn.getTarget().getInstrInfo());
58 // Give the blocks of the function a dense, in-order, numbering.
60 BlockSizes.resize(Fn.getNumBlockIDs());
62 // Measure each MBB and compute a size for the entire function.
63 unsigned FuncSize = 0;
64 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
66 MachineBasicBlock *MBB = MFI;
68 unsigned BlockSize = 0;
69 for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
71 BlockSize += TII->GetInstSizeInBytes(MBBI);
73 BlockSizes[MBB->getNumber()] = BlockSize;
74 FuncSize += BlockSize;
77 // If the entire function is smaller than the displacement of a branch field,
78 // we know we don't need to shrink any branches in this function. This is a
80 if (FuncSize < (1 << 9)) {
85 // For each conditional branch, if the offset to its destination is larger
86 // than the offset field allows, transform it into a long branch sequence
94 bool MadeChange = true;
95 bool EverMadeChange = false;
97 // Iteratively expand branches until we reach a fixed point.
100 for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
102 MachineBasicBlock &MBB = *MFI;
103 unsigned MBBStartOffset = 0;
104 for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
106 if ((I->getOpcode() != MSP430::JCC || I->getOperand(0).isImm()) &&
107 I->getOpcode() != MSP430::JMP) {
108 MBBStartOffset += TII->GetInstSizeInBytes(I);
112 // Determine the offset from the current branch to the destination
114 MachineBasicBlock *Dest = I->getOperand(0).getMBB();
117 if (Dest->getNumber() <= MBB.getNumber()) {
118 // If this is a backwards branch, the delta is the offset from the
119 // start of this block to this branch, plus the sizes of all blocks
120 // from this block to the dest.
121 BranchSize = MBBStartOffset;
123 for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
124 BranchSize += BlockSizes[i];
126 // Otherwise, add the size of the blocks between this block and the
127 // dest to the number of bytes left in this block.
128 BranchSize = -MBBStartOffset;
130 for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
131 BranchSize += BlockSizes[i];
134 // If this branch is in range, ignore it.
135 if (isInt<10>(BranchSize)) {
140 // Otherwise, we have to expand it to a long branch.
142 MachineInstr *OldBranch = I;
143 DebugLoc dl = OldBranch->getDebugLoc();
145 if (I->getOpcode() == MSP430::JMP) {
148 // The BCC operands are:
149 // 0. MSP430 branch predicate
151 SmallVector<MachineOperand, 1> Cond;
152 Cond.push_back(I->getOperand(1));
154 // Jump over the uncond branch inst (i.e. $+6) on opposite condition.
155 TII->ReverseBranchCondition(Cond);
156 BuildMI(MBB, I, dl, TII->get(MSP430::JCC))
157 .addImm(4).addOperand(Cond[0]);
161 // Uncond branch to the real destination.
162 I = BuildMI(MBB, I, dl, TII->get(MSP430::Bi)).addMBB(Dest);
164 // Remove the old branch from the function.
165 OldBranch->eraseFromParent();
167 // Remember that this instruction is NewSize bytes, increase the size of the
168 // block by NewSize-2, remember to iterate.
169 BlockSizes[MBB.getNumber()] += NewSize-2;
170 MBBStartOffset += NewSize;
176 EverMadeChange |= MadeChange;