1 //===-- WebAssemblyAsmBackend.cpp - WebAssembly Assembler Backend ---------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// \file
10 /// This file implements the WebAssemblyAsmBackend class.
11 ///
12 //===----------------------------------------------------------------------===//
13 
14 #include "MCTargetDesc/WebAssemblyFixupKinds.h"
15 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
16 #include "llvm/MC/MCAsmBackend.h"
17 #include "llvm/MC/MCAssembler.h"
18 #include "llvm/MC/MCDirectives.h"
19 #include "llvm/MC/MCExpr.h"
20 #include "llvm/MC/MCFixupKindInfo.h"
21 #include "llvm/MC/MCObjectWriter.h"
22 #include "llvm/MC/MCSubtargetInfo.h"
23 #include "llvm/MC/MCSymbol.h"
24 #include "llvm/MC/MCWasmObjectWriter.h"
25 #include "llvm/Support/ErrorHandling.h"
26 #include "llvm/Support/raw_ostream.h"
27 
28 using namespace llvm;
29 
30 namespace {
31 
32 class WebAssemblyAsmBackend final : public MCAsmBackend {
33   bool Is64Bit;
34   bool IsEmscripten;
35 
36 public:
37   explicit WebAssemblyAsmBackend(bool Is64Bit, bool IsEmscripten)
38       : MCAsmBackend(support::little), Is64Bit(Is64Bit),
39         IsEmscripten(IsEmscripten) {}
40 
41   unsigned getNumFixupKinds() const override {
42     return WebAssembly::NumTargetFixupKinds;
43   }
44 
45   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
46 
47   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
48                   const MCValue &Target, MutableArrayRef<char> Data,
49                   uint64_t Value, bool IsPCRel,
50                   const MCSubtargetInfo *STI) const override;
51 
52   std::unique_ptr<MCObjectTargetWriter>
53   createObjectTargetWriter() const override;
54 
55   // No instruction requires relaxation
56   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
57                             const MCRelaxableFragment *DF,
58                             const MCAsmLayout &Layout) const override {
59     return false;
60   }
61 
62   bool writeNopData(raw_ostream &OS, uint64_t Count,
63                     const MCSubtargetInfo *STI) const override;
64 };
65 
66 const MCFixupKindInfo &
67 WebAssemblyAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
68   const static MCFixupKindInfo Infos[WebAssembly::NumTargetFixupKinds] = {
69       // This table *must* be in the order that the fixup_* kinds are defined in
70       // WebAssemblyFixupKinds.h.
71       //
72       // Name                     Offset (bits) Size (bits)     Flags
73       {"fixup_sleb128_i32", 0, 5 * 8, 0},
74       {"fixup_sleb128_i64", 0, 10 * 8, 0},
75       {"fixup_uleb128_i32", 0, 5 * 8, 0},
76       {"fixup_uleb128_i64", 0, 10 * 8, 0},
77   };
78 
79   if (Kind < FirstTargetFixupKind)
80     return MCAsmBackend::getFixupKindInfo(Kind);
81 
82   assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
83          "Invalid kind!");
84   return Infos[Kind - FirstTargetFixupKind];
85 }
86 
87 bool WebAssemblyAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count,
88                                          const MCSubtargetInfo *STI) const {
89   for (uint64_t I = 0; I < Count; ++I)
90     OS << char(WebAssembly::Nop);
91 
92   return true;
93 }
94 
95 void WebAssemblyAsmBackend::applyFixup(const MCAssembler &Asm,
96                                        const MCFixup &Fixup,
97                                        const MCValue &Target,
98                                        MutableArrayRef<char> Data,
99                                        uint64_t Value, bool IsPCRel,
100                                        const MCSubtargetInfo *STI) const {
101   const MCFixupKindInfo &Info = getFixupKindInfo(Fixup.getKind());
102   assert(Info.Flags == 0 && "WebAssembly does not use MCFixupKindInfo flags");
103 
104   unsigned NumBytes = alignTo(Info.TargetSize, 8) / 8;
105   if (Value == 0)
106     return; // Doesn't change encoding.
107 
108   // Shift the value into position.
109   Value <<= Info.TargetOffset;
110 
111   unsigned Offset = Fixup.getOffset();
112   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
113 
114   // For each byte of the fragment that the fixup touches, mask in the
115   // bits from the fixup value.
116   for (unsigned I = 0; I != NumBytes; ++I)
117     Data[Offset + I] |= uint8_t((Value >> (I * 8)) & 0xff);
118 }
119 
120 std::unique_ptr<MCObjectTargetWriter>
121 WebAssemblyAsmBackend::createObjectTargetWriter() const {
122   return createWebAssemblyWasmObjectWriter(Is64Bit, IsEmscripten);
123 }
124 
125 } // end anonymous namespace
126 
127 MCAsmBackend *llvm::createWebAssemblyAsmBackend(const Triple &TT) {
128   return new WebAssemblyAsmBackend(TT.isArch64Bit(), TT.isOSEmscripten());
129 }
130