1 //===-- WebAssemblyAsmBackend.cpp - WebAssembly Assembler Backend ---------===//
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 implements the WebAssemblyAsmBackend class.
12 ///
13 //===----------------------------------------------------------------------===//
14 
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/MCELFObjectWriter.h"
20 #include "llvm/MC/MCExpr.h"
21 #include "llvm/MC/MCFixupKindInfo.h"
22 #include "llvm/MC/MCObjectWriter.h"
23 #include "llvm/MC/MCSubtargetInfo.h"
24 #include "llvm/MC/MCSymbol.h"
25 #include "llvm/Support/ErrorHandling.h"
26 #include "llvm/Support/raw_ostream.h"
27 using namespace llvm;
28 
29 namespace {
30 class WebAssemblyAsmBackend final : public MCAsmBackend {
31   bool Is64Bit;
32 
33 public:
34   explicit WebAssemblyAsmBackend(bool Is64Bit)
35       : MCAsmBackend(), Is64Bit(Is64Bit) {}
36   ~WebAssemblyAsmBackend() override {}
37 
38   void applyFixup(const MCFixup &Fixup, char *Data, unsigned DataSize,
39                   uint64_t Value, bool IsPCRel) const override;
40 
41   MCObjectWriter *createObjectWriter(raw_pwrite_stream &OS) const override;
42 
43   // No instruction requires relaxation
44   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
45                             const MCRelaxableFragment *DF,
46                             const MCAsmLayout &Layout) const override {
47     return false;
48   }
49 
50   unsigned getNumFixupKinds() const override {
51     // We currently just use the generic fixups in MCFixup.h and don't have any
52     // target-specific fixups.
53     return 0;
54   }
55 
56   bool mayNeedRelaxation(const MCInst &Inst) const override { return false; }
57 
58   void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
59                         MCInst &Res) const override {}
60 
61   bool writeNopData(uint64_t Count, MCObjectWriter *OW) const override;
62 };
63 
64 bool WebAssemblyAsmBackend::writeNopData(uint64_t Count,
65                                          MCObjectWriter *OW) const {
66   if (Count == 0)
67     return true;
68 
69   for (uint64_t i = 0; i < Count; ++i)
70     OW->write8(WebAssembly::Nop);
71 
72   return true;
73 }
74 
75 void WebAssemblyAsmBackend::applyFixup(const MCFixup &Fixup, char *Data,
76                                        unsigned DataSize, uint64_t Value,
77                                        bool IsPCRel) const {
78   const MCFixupKindInfo &Info = getFixupKindInfo(Fixup.getKind());
79   assert(Info.Flags == 0 && "WebAssembly does not use MCFixupKindInfo flags");
80 
81   unsigned NumBytes = (Info.TargetSize + 7) / 8;
82   if (Value == 0)
83     return; // Doesn't change encoding.
84 
85   // Shift the value into position.
86   Value <<= Info.TargetOffset;
87 
88   unsigned Offset = Fixup.getOffset();
89   assert(Offset + NumBytes <= DataSize && "Invalid fixup offset!");
90 
91   // For each byte of the fragment that the fixup touches, mask in the
92   // bits from the fixup value.
93   for (unsigned i = 0; i != NumBytes; ++i)
94     Data[Offset + i] |= uint8_t((Value >> (i * 8)) & 0xff);
95 }
96 
97 MCObjectWriter *
98 WebAssemblyAsmBackend::createObjectWriter(raw_pwrite_stream &OS) const {
99   return createWebAssemblyELFObjectWriter(OS, Is64Bit, 0);
100 }
101 } // end anonymous namespace
102 
103 MCAsmBackend *llvm::createWebAssemblyAsmBackend(const Triple &TT) {
104   return new WebAssemblyAsmBackend(TT.isArch64Bit());
105 }
106