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 /// \brief 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/MCSymbolWasm.h"
24 #include "llvm/MC/MCWasmObjectWriter.h"
25 #include "llvm/MC/MCValue.h"
26 #include "llvm/Support/Casting.h"
27 #include "llvm/Support/ErrorHandling.h"
28 
29 using namespace llvm;
30 
31 namespace {
32 class WebAssemblyWasmObjectWriter final : public MCWasmObjectTargetWriter {
33 public:
34   explicit WebAssemblyWasmObjectWriter(bool Is64Bit);
35 
36 private:
37   unsigned getRelocType(const MCValue &Target,
38                         const MCFixup &Fixup) const override;
39 };
40 } // end anonymous namespace
41 
42 WebAssemblyWasmObjectWriter::WebAssemblyWasmObjectWriter(bool Is64Bit)
43     : MCWasmObjectTargetWriter(Is64Bit) {}
44 
45 // Test whether the given expression computes a function address.
46 static bool IsFunctionExpr(const MCExpr *Expr) {
47   if (auto SyExp = dyn_cast<MCSymbolRefExpr>(Expr))
48     return cast<MCSymbolWasm>(SyExp->getSymbol()).isFunction();
49 
50   if (auto BinOp = dyn_cast<MCBinaryExpr>(Expr))
51     return IsFunctionExpr(BinOp->getLHS()) != IsFunctionExpr(BinOp->getRHS());
52 
53   if (auto UnOp = dyn_cast<MCUnaryExpr>(Expr))
54     return IsFunctionExpr(UnOp->getSubExpr());
55 
56   return false;
57 }
58 
59 static bool IsFunctionType(const MCValue &Target) {
60   const MCSymbolRefExpr *RefA = Target.getSymA();
61   return RefA && RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX;
62 }
63 
64 unsigned
65 WebAssemblyWasmObjectWriter::getRelocType(const MCValue &Target,
66                                           const MCFixup &Fixup) const {
67   // WebAssembly functions are not allocated in the data address space. To
68   // resolve a pointer to a function, we must use a special relocation type.
69   bool IsFunction = IsFunctionExpr(Fixup.getValue());
70 
71   switch (unsigned(Fixup.getKind())) {
72   case WebAssembly::fixup_code_global_index:
73     return wasm::R_WEBASSEMBLY_GLOBAL_INDEX_LEB;
74   case WebAssembly::fixup_code_sleb128_i32:
75     if (IsFunction)
76       return wasm::R_WEBASSEMBLY_TABLE_INDEX_SLEB;
77     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_SLEB;
78   case WebAssembly::fixup_code_sleb128_i64:
79     llvm_unreachable("fixup_sleb128_i64 not implemented yet");
80   case WebAssembly::fixup_code_uleb128_i32:
81     if (IsFunctionType(Target))
82       return wasm::R_WEBASSEMBLY_TYPE_INDEX_LEB;
83     if (IsFunction)
84       return wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB;
85     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_LEB;
86   case FK_Data_4:
87     if (IsFunction)
88       return wasm::R_WEBASSEMBLY_TABLE_INDEX_I32;
89     return wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32;
90   case FK_Data_8:
91     llvm_unreachable("FK_Data_8 not implemented yet");
92   default:
93     llvm_unreachable("unimplemented fixup kind");
94   }
95 }
96 
97 std::unique_ptr<MCObjectWriter>
98 llvm::createWebAssemblyWasmObjectWriter(raw_pwrite_stream &OS,
99                                         bool Is64Bit) {
100   auto MOTW = llvm::make_unique<WebAssemblyWasmObjectWriter>(Is64Bit);
101   return createWasmObjectWriter(std::move(MOTW), OS);
102 }
103