1 //===-- WebAssemblyWasmObjectWriter.cpp - WebAssembly Wasm Writer ---------===//
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 /// \file
11 /// This file handles Wasm-specific object emission, converting LLVM's
12 /// internal fixups into the appropriate relocations.
13 ///
14 //===----------------------------------------------------------------------===//
15 
16 #include "MCTargetDesc/WebAssemblyFixupKinds.h"
17 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
18 #include "llvm/BinaryFormat/Wasm.h"
19 #include "llvm/MC/MCAsmBackend.h"
20 #include "llvm/MC/MCFixup.h"
21 #include "llvm/MC/MCFixupKindInfo.h"
22 #include "llvm/MC/MCObjectWriter.h"
23 #include "llvm/MC/MCSectionWasm.h"
24 #include "llvm/MC/MCSymbolWasm.h"
25 #include "llvm/MC/MCValue.h"
26 #include "llvm/MC/MCWasmObjectWriter.h"
27 #include "llvm/Support/Casting.h"
28 #include "llvm/Support/ErrorHandling.h"
29 
30 using namespace llvm;
31 
32 namespace {
33 class WebAssemblyWasmObjectWriter final : public MCWasmObjectTargetWriter {
34 public:
35   explicit WebAssemblyWasmObjectWriter(bool Is64Bit);
36 
37 private:
38   unsigned getRelocType(const MCValue &Target,
39                         const MCFixup &Fixup) const override;
40 };
41 } // end anonymous namespace
42 
WebAssemblyWasmObjectWriter(bool Is64Bit)43 WebAssemblyWasmObjectWriter::WebAssemblyWasmObjectWriter(bool Is64Bit)
44     : MCWasmObjectTargetWriter(Is64Bit) {}
45 
46 // Test whether the given expression computes a function address.
IsFunctionExpr(const MCExpr * Expr)47 static bool IsFunctionExpr(const MCExpr *Expr) {
48   if (auto SyExp = dyn_cast<MCSymbolRefExpr>(Expr))
49     return cast<MCSymbolWasm>(SyExp->getSymbol()).isFunction();
50 
51   if (auto BinOp = dyn_cast<MCBinaryExpr>(Expr))
52     return IsFunctionExpr(BinOp->getLHS()) != IsFunctionExpr(BinOp->getRHS());
53 
54   if (auto UnOp = dyn_cast<MCUnaryExpr>(Expr))
55     return IsFunctionExpr(UnOp->getSubExpr());
56 
57   return false;
58 }
59 
IsFunctionType(const MCValue & Target)60 static bool IsFunctionType(const MCValue &Target) {
61   const MCSymbolRefExpr *RefA = Target.getSymA();
62   return RefA && RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX;
63 }
64 
GetFixupSection(const MCExpr * Expr)65 static const MCSection *GetFixupSection(const MCExpr *Expr) {
66   if (auto SyExp = dyn_cast<MCSymbolRefExpr>(Expr)) {
67     if (SyExp->getSymbol().isInSection())
68       return &SyExp->getSymbol().getSection();
69     return nullptr;
70   }
71 
72   if (auto BinOp = dyn_cast<MCBinaryExpr>(Expr)) {
73     auto SectionLHS = GetFixupSection(BinOp->getLHS());
74     auto SectionRHS = GetFixupSection(BinOp->getRHS());
75     return SectionLHS == SectionRHS ? nullptr : SectionLHS;
76   }
77 
78   if (auto UnOp = dyn_cast<MCUnaryExpr>(Expr))
79     return GetFixupSection(UnOp->getSubExpr());
80 
81   return nullptr;
82 }
83 
84 unsigned
getRelocType(const MCValue & Target,const MCFixup & Fixup) const85 WebAssemblyWasmObjectWriter::getRelocType(const MCValue &Target,
86                                           const MCFixup &Fixup) const {
87   // WebAssembly functions are not allocated in the data address space. To
88   // resolve a pointer to a function, we must use a special relocation type.
89   bool IsFunction = IsFunctionExpr(Fixup.getValue());
90 
91   switch (unsigned(Fixup.getKind())) {
92   case WebAssembly::fixup_code_global_index:
93     return wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB;
94   case WebAssembly::fixup_code_sleb128_i32:
95     if (IsFunction)
96       return wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB;
97     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB;
98   case WebAssembly::fixup_code_sleb128_i64:
99     llvm_unreachable("fixup_sleb128_i64 not implemented yet");
100   case WebAssembly::fixup_code_uleb128_i32:
101     if (IsFunctionType(Target))
102       return wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB;
103     if (IsFunction)
104       return wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB;
105     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB;
106   case FK_Data_4:
107     if (IsFunction)
108       return wasm::R_WEBASSEMBLY_TABLE_INDEX_I32;
109     if (auto Section = static_cast<const MCSectionWasm *>(
110             GetFixupSection(Fixup.getValue()))) {
111       if (Section->getKind().isText())
112         return wasm::R_WEBASSEMBLY_FUNCTION_OFFSET_I32;
113       else if (!Section->isWasmData())
114         return wasm::R_WEBASSEMBLY_SECTION_OFFSET_I32;
115     }
116     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32;
117   case FK_Data_8:
118     llvm_unreachable("FK_Data_8 not implemented yet");
119   default:
120     llvm_unreachable("unimplemented fixup kind");
121   }
122 }
123 
124 std::unique_ptr<MCObjectTargetWriter>
createWebAssemblyWasmObjectWriter(bool Is64Bit)125 llvm::createWebAssemblyWasmObjectWriter(bool Is64Bit) {
126   return llvm::make_unique<WebAssemblyWasmObjectWriter>(Is64Bit);
127 }
128