1 //===- lib/MC/AArch64ELFStreamer.cpp - ELF Object Output for AArch64 ------===//
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 // This file assembles .s files and emits AArch64 ELF .o object files. Different
11 // from generic ELF streamer in emitting mapping symbols ($x and $d) to delimit
12 // regions of data and code.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include "llvm/MC/MCELFStreamer.h"
17 #include "llvm/ADT/SmallPtrSet.h"
18 #include "llvm/ADT/Twine.h"
19 #include "llvm/MC/MCAsmBackend.h"
20 #include "llvm/MC/MCAssembler.h"
21 #include "llvm/MC/MCCodeEmitter.h"
22 #include "llvm/MC/MCContext.h"
23 #include "llvm/MC/MCELF.h"
24 #include "llvm/MC/MCELFStreamer.h"
25 #include "llvm/MC/MCELFSymbolFlags.h"
26 #include "llvm/MC/MCExpr.h"
27 #include "llvm/MC/MCInst.h"
28 #include "llvm/MC/MCObjectStreamer.h"
29 #include "llvm/MC/MCSection.h"
30 #include "llvm/MC/MCSectionELF.h"
31 #include "llvm/MC/MCStreamer.h"
32 #include "llvm/MC/MCSymbol.h"
33 #include "llvm/MC/MCValue.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/Support/ELF.h"
36 #include "llvm/Support/ErrorHandling.h"
37 #include "llvm/Support/raw_ostream.h"
38 
39 using namespace llvm;
40 
41 namespace {
42 
43 /// Extend the generic ELFStreamer class so that it can emit mapping symbols at
44 /// the appropriate points in the object files. These symbols are defined in the
45 /// AArch64 ELF ABI:
46 ///    infocenter.arm.com/help/topic/com.arm.doc.ihi0056a/IHI0056A_aaelf64.pdf
47 ///
48 /// In brief: $x or $d should be emitted at the start of each contiguous region
49 /// of A64 code or data in a section. In practice, this emission does not rely
50 /// on explicit assembler directives but on inherent properties of the
51 /// directives doing the emission (e.g. ".byte" is data, "add x0, x0, x0" an
52 /// instruction).
53 ///
54 /// As a result this system is orthogonal to the DataRegion infrastructure used
55 /// by MachO. Beware!
56 class AArch64ELFStreamer : public MCELFStreamer {
57 public:
58   AArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB,
59                  raw_ostream &OS, MCCodeEmitter *Emitter)
60     : MCELFStreamer(Context, TAB, OS, Emitter),
61       MappingSymbolCounter(0), LastEMS(EMS_None) {
62   }
63 
64   ~AArch64ELFStreamer() {}
65 
66   virtual void ChangeSection(const MCSection *Section,
67                              const MCExpr *Subsection) {
68     // We have to keep track of the mapping symbol state of any sections we
69     // use. Each one should start off as EMS_None, which is provided as the
70     // default constructor by DenseMap::lookup.
71     LastMappingSymbols[getPreviousSection().first] = LastEMS;
72     LastEMS = LastMappingSymbols.lookup(Section);
73 
74     MCELFStreamer::ChangeSection(Section, Subsection);
75   }
76 
77   /// This function is the one used to emit instruction data into the ELF
78   /// streamer. We override it to add the appropriate mapping symbol if
79   /// necessary.
80   virtual void EmitInstruction(const MCInst& Inst) {
81     EmitA64MappingSymbol();
82     MCELFStreamer::EmitInstruction(Inst);
83   }
84 
85   /// This is one of the functions used to emit data into an ELF section, so the
86   /// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
87   /// if necessary.
88   virtual void EmitBytes(StringRef Data, unsigned AddrSpace) {
89     EmitDataMappingSymbol();
90     MCELFStreamer::EmitBytes(Data, AddrSpace);
91   }
92 
93   /// This is one of the functions used to emit data into an ELF section, so the
94   /// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
95   /// if necessary.
96   virtual void EmitValueImpl(const MCExpr *Value, unsigned Size,
97                              unsigned AddrSpace) {
98     EmitDataMappingSymbol();
99     MCELFStreamer::EmitValueImpl(Value, Size, AddrSpace);
100   }
101 
102 private:
103   enum ElfMappingSymbol {
104     EMS_None,
105     EMS_A64,
106     EMS_Data
107   };
108 
109   void EmitDataMappingSymbol() {
110     if (LastEMS == EMS_Data) return;
111     EmitMappingSymbol("$d");
112     LastEMS = EMS_Data;
113   }
114 
115   void EmitA64MappingSymbol() {
116     if (LastEMS == EMS_A64) return;
117     EmitMappingSymbol("$x");
118     LastEMS = EMS_A64;
119   }
120 
121   void EmitMappingSymbol(StringRef Name) {
122     MCSymbol *Start = getContext().CreateTempSymbol();
123     EmitLabel(Start);
124 
125     MCSymbol *Symbol =
126       getContext().GetOrCreateSymbol(Name + "." +
127                                      Twine(MappingSymbolCounter++));
128 
129     MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
130     MCELF::SetType(SD, ELF::STT_NOTYPE);
131     MCELF::SetBinding(SD, ELF::STB_LOCAL);
132     SD.setExternal(false);
133     Symbol->setSection(*getCurrentSection().first);
134 
135     const MCExpr *Value = MCSymbolRefExpr::Create(Start, getContext());
136     Symbol->setVariableValue(Value);
137   }
138 
139   int64_t MappingSymbolCounter;
140 
141   DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
142   ElfMappingSymbol LastEMS;
143 
144   /// @}
145 };
146 }
147 
148 namespace llvm {
149   MCELFStreamer* createAArch64ELFStreamer(MCContext &Context, MCAsmBackend &TAB,
150                                       raw_ostream &OS, MCCodeEmitter *Emitter,
151                                       bool RelaxAll, bool NoExecStack) {
152     AArch64ELFStreamer *S = new AArch64ELFStreamer(Context, TAB, OS, Emitter);
153     if (RelaxAll)
154       S->getAssembler().setRelaxAll(true);
155     if (NoExecStack)
156       S->getAssembler().setNoExecStack(true);
157     return S;
158   }
159 }
160 
161 
162