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   ~WebAssemblyAsmBackend() override {}
39 
40   unsigned getNumFixupKinds() const override {
41     return WebAssembly::NumTargetFixupKinds;
42   }
43 
44   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
45 
46   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
47                   const MCValue &Target, MutableArrayRef<char> Data,
48                   uint64_t Value, bool IsPCRel,
49                   const MCSubtargetInfo *STI) const override;
50 
51   std::unique_ptr<MCObjectTargetWriter>
52   createObjectTargetWriter() const override;
53 
54   // No instruction requires relaxation
55   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
56                             const MCRelaxableFragment *DF,
57                             const MCAsmLayout &Layout) const override {
58     return false;
59   }
60 
61   bool mayNeedRelaxation(const MCInst &Inst,
62                          const MCSubtargetInfo &STI) const override {
63     return false;
64   }
65 
66   void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
67                         MCInst &Res) const override {}
68 
69   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
70 };
71 
72 const MCFixupKindInfo &
73 WebAssemblyAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
74   const static MCFixupKindInfo Infos[WebAssembly::NumTargetFixupKinds] = {
75       // This table *must* be in the order that the fixup_* kinds are defined in
76       // WebAssemblyFixupKinds.h.
77       //
78       // Name                     Offset (bits) Size (bits)     Flags
79       {"fixup_code_sleb128_i32", 0, 5 * 8, 0},
80       {"fixup_code_sleb128_i64", 0, 10 * 8, 0},
81       {"fixup_code_uleb128_i32", 0, 5 * 8, 0},
82   };
83 
84   if (Kind < FirstTargetFixupKind)
85     return MCAsmBackend::getFixupKindInfo(Kind);
86 
87   assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
88          "Invalid kind!");
89   return Infos[Kind - FirstTargetFixupKind];
90 }
91 
92 bool WebAssemblyAsmBackend::writeNopData(raw_ostream &OS,
93                                          uint64_t Count) const {
94   for (uint64_t i = 0; i < Count; ++i)
95     OS << char(WebAssembly::Nop);
96 
97   return true;
98 }
99 
100 void WebAssemblyAsmBackend::applyFixup(const MCAssembler &Asm,
101                                        const MCFixup &Fixup,
102                                        const MCValue &Target,
103                                        MutableArrayRef<char> Data,
104                                        uint64_t Value, bool IsPCRel,
105                                        const MCSubtargetInfo *STI) const {
106   const MCFixupKindInfo &Info = getFixupKindInfo(Fixup.getKind());
107   assert(Info.Flags == 0 && "WebAssembly does not use MCFixupKindInfo flags");
108 
109   unsigned NumBytes = alignTo(Info.TargetSize, 8) / 8;
110   if (Value == 0)
111     return; // Doesn't change encoding.
112 
113   // Shift the value into position.
114   Value <<= Info.TargetOffset;
115 
116   unsigned Offset = Fixup.getOffset();
117   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
118 
119   // For each byte of the fragment that the fixup touches, mask in the
120   // bits from the fixup value.
121   for (unsigned i = 0; i != NumBytes; ++i)
122     Data[Offset + i] |= uint8_t((Value >> (i * 8)) & 0xff);
123 }
124 
125 std::unique_ptr<MCObjectTargetWriter>
126 WebAssemblyAsmBackend::createObjectTargetWriter() const {
127   return createWebAssemblyWasmObjectWriter(Is64Bit);
128 }
129 
130 } // end anonymous namespace
131 
132 MCAsmBackend *llvm::createWebAssemblyAsmBackend(const Triple &TT) {
133   return new WebAssemblyAsmBackend(TT.isArch64Bit());
134 }
135