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1 //===-- lib/CodeGen/ELFCodeEmitter.cpp ------------------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #define DEBUG_TYPE "elfce"
11 
12 #include "ELF.h"
13 #include "ELFWriter.h"
14 #include "ELFCodeEmitter.h"
15 #include "llvm/Constants.h"
16 #include "llvm/DerivedTypes.h"
17 #include "llvm/Function.h"
18 #include "llvm/CodeGen/BinaryObject.h"
19 #include "llvm/CodeGen/MachineConstantPool.h"
20 #include "llvm/CodeGen/MachineFunction.h"
21 #include "llvm/CodeGen/MachineJumpTableInfo.h"
22 #include "llvm/CodeGen/MachineRelocation.h"
23 #include "llvm/Target/TargetData.h"
24 #include "llvm/Target/TargetELFWriterInfo.h"
25 #include "llvm/Target/TargetMachine.h"
26 #include "llvm/MC/MCAsmInfo.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Support/raw_ostream.h"
30 
31 //===----------------------------------------------------------------------===//
32 //                       ELFCodeEmitter Implementation
33 //===----------------------------------------------------------------------===//
34 
35 namespace llvm {
36 
37 /// startFunction - This callback is invoked when a new machine function is
38 /// about to be emitted.
startFunction(MachineFunction & MF)39 void ELFCodeEmitter::startFunction(MachineFunction &MF) {
40   DEBUG(dbgs() << "processing function: "
41         << MF.getFunction()->getName() << "\n");
42 
43   // Get the ELF Section that this function belongs in.
44   ES = &EW.getTextSection(MF.getFunction());
45 
46   // Set the desired binary object to be used by the code emitters
47   setBinaryObject(ES);
48 
49   // Get the function alignment in bytes
50   unsigned Align = (1 << MF.getAlignment());
51 
52   // The function must start on its required alignment
53   ES->emitAlignment(Align);
54 
55   // Update the section alignment if needed.
56   ES->Align = std::max(ES->Align, Align);
57 
58   // Record the function start offset
59   FnStartOff = ES->getCurrentPCOffset();
60 
61   // Emit constant pool and jump tables to their appropriate sections.
62   // They need to be emitted before the function because in some targets
63   // the later may reference JT or CP entry address.
64   emitConstantPool(MF.getConstantPool());
65   if (MF.getJumpTableInfo())
66     emitJumpTables(MF.getJumpTableInfo());
67 }
68 
69 /// finishFunction - This callback is invoked after the function is completely
70 /// finished.
finishFunction(MachineFunction & MF)71 bool ELFCodeEmitter::finishFunction(MachineFunction &MF) {
72   // Add a symbol to represent the function.
73   const Function *F = MF.getFunction();
74   ELFSym *FnSym = ELFSym::getGV(F, EW.getGlobalELFBinding(F), ELF::STT_FUNC,
75                                 EW.getGlobalELFVisibility(F));
76   FnSym->SectionIdx = ES->SectionIdx;
77   FnSym->Size = ES->getCurrentPCOffset()-FnStartOff;
78   EW.AddPendingGlobalSymbol(F, true);
79 
80   // Offset from start of Section
81   FnSym->Value = FnStartOff;
82 
83   if (!F->hasPrivateLinkage())
84     EW.SymbolList.push_back(FnSym);
85 
86   // Patch up Jump Table Section relocations to use the real MBBs offsets
87   // now that the MBB label offsets inside the function are known.
88   if (MF.getJumpTableInfo()) {
89     ELFSection &JTSection = EW.getJumpTableSection();
90     for (std::vector<MachineRelocation>::iterator MRI = JTRelocations.begin(),
91          MRE = JTRelocations.end(); MRI != MRE; ++MRI) {
92       MachineRelocation &MR = *MRI;
93       uintptr_t MBBOffset = getMachineBasicBlockAddress(MR.getBasicBlock());
94       MR.setResultPointer((void*)MBBOffset);
95       MR.setConstantVal(ES->SectionIdx);
96       JTSection.addRelocation(MR);
97     }
98   }
99 
100   // If we have emitted any relocations to function-specific objects such as
101   // basic blocks, constant pools entries, or jump tables, record their
102   // addresses now so that we can rewrite them with the correct addresses later
103   for (unsigned i = 0, e = Relocations.size(); i != e; ++i) {
104     MachineRelocation &MR = Relocations[i];
105     intptr_t Addr;
106     if (MR.isGlobalValue()) {
107       EW.AddPendingGlobalSymbol(MR.getGlobalValue());
108     } else if (MR.isExternalSymbol()) {
109       EW.AddPendingExternalSymbol(MR.getExternalSymbol());
110     } else if (MR.isBasicBlock()) {
111       Addr = getMachineBasicBlockAddress(MR.getBasicBlock());
112       MR.setConstantVal(ES->SectionIdx);
113       MR.setResultPointer((void*)Addr);
114     } else if (MR.isConstantPoolIndex()) {
115       Addr = getConstantPoolEntryAddress(MR.getConstantPoolIndex());
116       MR.setConstantVal(CPSections[MR.getConstantPoolIndex()]);
117       MR.setResultPointer((void*)Addr);
118     } else if (MR.isJumpTableIndex()) {
119       ELFSection &JTSection = EW.getJumpTableSection();
120       Addr = getJumpTableEntryAddress(MR.getJumpTableIndex());
121       MR.setConstantVal(JTSection.SectionIdx);
122       MR.setResultPointer((void*)Addr);
123     } else {
124       llvm_unreachable("Unhandled relocation type");
125     }
126     ES->addRelocation(MR);
127   }
128 
129   // Clear per-function data structures.
130   JTRelocations.clear();
131   Relocations.clear();
132   CPLocations.clear();
133   CPSections.clear();
134   JTLocations.clear();
135   MBBLocations.clear();
136   return false;
137 }
138 
139 /// emitConstantPool - For each constant pool entry, figure out which section
140 /// the constant should live in and emit the constant
emitConstantPool(MachineConstantPool * MCP)141 void ELFCodeEmitter::emitConstantPool(MachineConstantPool *MCP) {
142   const std::vector<MachineConstantPoolEntry> &CP = MCP->getConstants();
143   if (CP.empty()) return;
144 
145   // TODO: handle PIC codegen
146   assert(TM.getRelocationModel() != Reloc::PIC_ &&
147          "PIC codegen not yet handled for elf constant pools!");
148 
149   for (unsigned i = 0, e = CP.size(); i != e; ++i) {
150     MachineConstantPoolEntry CPE = CP[i];
151 
152     // Record the constant pool location and the section index
153     ELFSection &CstPool = EW.getConstantPoolSection(CPE);
154     CPLocations.push_back(CstPool.size());
155     CPSections.push_back(CstPool.SectionIdx);
156 
157     if (CPE.isMachineConstantPoolEntry())
158       assert(0 && "CPE.isMachineConstantPoolEntry not supported yet");
159 
160     // Emit the constant to constant pool section
161     EW.EmitGlobalConstant(CPE.Val.ConstVal, CstPool);
162   }
163 }
164 
165 /// emitJumpTables - Emit all the jump tables for a given jump table info
166 /// record to the appropriate section.
emitJumpTables(MachineJumpTableInfo * MJTI)167 void ELFCodeEmitter::emitJumpTables(MachineJumpTableInfo *MJTI) {
168   const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
169   if (JT.empty()) return;
170 
171   // FIXME: handle PIC codegen
172   assert(TM.getRelocationModel() != Reloc::PIC_ &&
173          "PIC codegen not yet handled for elf jump tables!");
174 
175   const TargetELFWriterInfo *TEW = TM.getELFWriterInfo();
176   unsigned EntrySize = 4; //MJTI->getEntrySize();
177 
178   // Get the ELF Section to emit the jump table
179   ELFSection &JTSection = EW.getJumpTableSection();
180 
181   // For each JT, record its offset from the start of the section
182   for (unsigned i = 0, e = JT.size(); i != e; ++i) {
183     const std::vector<MachineBasicBlock*> &MBBs = JT[i].MBBs;
184 
185     // Record JT 'i' offset in the JT section
186     JTLocations.push_back(JTSection.size());
187 
188     // Each MBB entry in the Jump table section has a relocation entry
189     // against the current text section.
190     for (unsigned mi = 0, me = MBBs.size(); mi != me; ++mi) {
191       unsigned MachineRelTy = TEW->getAbsoluteLabelMachineRelTy();
192       MachineRelocation MR =
193         MachineRelocation::getBB(JTSection.size(), MachineRelTy, MBBs[mi]);
194 
195       // Add the relocation to the Jump Table section
196       JTRelocations.push_back(MR);
197 
198       // Output placeholder for MBB in the JT section
199       for (unsigned s=0; s < EntrySize; ++s)
200         JTSection.emitByte(0);
201     }
202   }
203 }
204 
205 } // end namespace llvm
206