lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp

   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 mayNeedRelaxation(const MCInst &Inst,
  63                          const MCSubtargetInfo &STI) const override {
  64     return false;
  65   }
  66
  67   void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
  68                         MCInst &Res) const override {}
  69
  70   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
  71 };
  72
  73 const MCFixupKindInfo &
  74 WebAssemblyAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
  75   const static MCFixupKindInfo Infos[WebAssembly::NumTargetFixupKinds] = {
  76       // This table *must* be in the order that the fixup_* kinds are defined in
  77       // WebAssemblyFixupKinds.h.
  78       //
  79       // Name                     Offset (bits) Size (bits)     Flags
  80       {"fixup_sleb128_i32", 0, 5 * 8, 0},
  81       {"fixup_sleb128_i64", 0, 10 * 8, 0},
  82       {"fixup_uleb128_i32", 0, 5 * 8, 0},
  83   };
  84
  85   if (Kind < FirstTargetFixupKind)
  86     return MCAsmBackend::getFixupKindInfo(Kind);
  87
  88   assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
  89          "Invalid kind!");
  90   return Infos[Kind - FirstTargetFixupKind];
  91 }
  92
  93 bool WebAssemblyAsmBackend::writeNopData(raw_ostream &OS,
  94                                          uint64_t Count) const {
  95   for (uint64_t I = 0; I < Count; ++I)
  96     OS << char(WebAssembly::Nop);
  97
  98   return true;
  99 }
 100
 101 void WebAssemblyAsmBackend::applyFixup(const MCAssembler &Asm,
 102                                        const MCFixup &Fixup,
 103                                        const MCValue &Target,
 104                                        MutableArrayRef<char> Data,
 105                                        uint64_t Value, bool IsPCRel,
 106                                        const MCSubtargetInfo *STI) const {
 107   const MCFixupKindInfo &Info = getFixupKindInfo(Fixup.getKind());
 108   assert(Info.Flags == 0 && "WebAssembly does not use MCFixupKindInfo flags");
 109
 110   unsigned NumBytes = alignTo(Info.TargetSize, 8) / 8;
 111   if (Value == 0)
 112     return; // Doesn't change encoding.
 113
 114   // Shift the value into position.
 115   Value <<= Info.TargetOffset;
 116
 117   unsigned Offset = Fixup.getOffset();
 118   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
 119
 120   // For each byte of the fragment that the fixup touches, mask in the
 121   // bits from the fixup value.
 122   for (unsigned I = 0; I != NumBytes; ++I)
 123     Data[Offset + I] |= uint8_t((Value >> (I * 8)) & 0xff);
 124 }
 125
 126 std::unique_ptr<MCObjectTargetWriter>
 127 WebAssemblyAsmBackend::createObjectTargetWriter() const {
 128   return createWebAssemblyWasmObjectWriter(Is64Bit, IsEmscripten);
 129 }
 130
 131 } // end anonymous namespace
 132
 133 MCAsmBackend *llvm::createWebAssemblyAsmBackend(const Triple &TT) {
 134   return new WebAssemblyAsmBackend(TT.isArch64Bit(), TT.isOSEmscripten());
 135 }