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 
35 public:
36   explicit WebAssemblyAsmBackend(bool Is64Bit)
37       : MCAsmBackend(support::little), Is64Bit(Is64Bit) {}
38 
39   unsigned getNumFixupKinds() const override {
40     return WebAssembly::NumTargetFixupKinds;
41   }
42 
43   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
44 
45   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
46                   const MCValue &Target, MutableArrayRef<char> Data,
47                   uint64_t Value, bool IsPCRel,
48                   const MCSubtargetInfo *STI) const override;
49 
50   std::unique_ptr<MCObjectTargetWriter>
51   createObjectTargetWriter() const override;
52 
53   // No instruction requires relaxation
54   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
55                             const MCRelaxableFragment *DF,
56                             const MCAsmLayout &Layout) const override {
57     return false;
58   }
59 
60   bool mayNeedRelaxation(const MCInst &Inst,
61                          const MCSubtargetInfo &STI) const override {
62     return false;
63   }
64 
65   void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
66                         MCInst &Res) const override {}
67 
68   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
69 };
70 
71 const MCFixupKindInfo &
72 WebAssemblyAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
73   const static MCFixupKindInfo Infos[WebAssembly::NumTargetFixupKinds] = {
74       // This table *must* be in the order that the fixup_* kinds are defined in
75       // WebAssemblyFixupKinds.h.
76       //
77       // Name                     Offset (bits) Size (bits)     Flags
78       {"fixup_sleb128_i32", 0, 5 * 8, 0},
79       {"fixup_sleb128_i64", 0, 10 * 8, 0},
80       {"fixup_uleb128_i32", 0, 5 * 8, 0},
81   };
82 
83   if (Kind < FirstTargetFixupKind)
84     return MCAsmBackend::getFixupKindInfo(Kind);
85 
86   assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
87          "Invalid kind!");
88   return Infos[Kind - FirstTargetFixupKind];
89 }
90 
91 bool WebAssemblyAsmBackend::writeNopData(raw_ostream &OS,
92                                          uint64_t Count) const {
93   for (uint64_t I = 0; I < Count; ++I)
94     OS << char(WebAssembly::Nop);
95 
96   return true;
97 }
98 
99 void WebAssemblyAsmBackend::applyFixup(const MCAssembler &Asm,
100                                        const MCFixup &Fixup,
101                                        const MCValue &Target,
102                                        MutableArrayRef<char> Data,
103                                        uint64_t Value, bool IsPCRel,
104                                        const MCSubtargetInfo *STI) const {
105   const MCFixupKindInfo &Info = getFixupKindInfo(Fixup.getKind());
106   assert(Info.Flags == 0 && "WebAssembly does not use MCFixupKindInfo flags");
107 
108   unsigned NumBytes = alignTo(Info.TargetSize, 8) / 8;
109   if (Value == 0)
110     return; // Doesn't change encoding.
111 
112   // Shift the value into position.
113   Value <<= Info.TargetOffset;
114 
115   unsigned Offset = Fixup.getOffset();
116   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
117 
118   // For each byte of the fragment that the fixup touches, mask in the
119   // bits from the fixup value.
120   for (unsigned I = 0; I != NumBytes; ++I)
121     Data[Offset + I] |= uint8_t((Value >> (I * 8)) & 0xff);
122 }
123 
124 std::unique_ptr<MCObjectTargetWriter>
125 WebAssemblyAsmBackend::createObjectTargetWriter() const {
126   return createWebAssemblyWasmObjectWriter(Is64Bit);
127 }
128 
129 } // end anonymous namespace
130 
131 MCAsmBackend *llvm::createWebAssemblyAsmBackend(const Triple &TT) {
132   return new WebAssemblyAsmBackend(TT.isArch64Bit());
133 }
134