lib/Target/WebAssembly/MCTargetDesc/WebAssemblyWasmObjectWriter.cpp

   1 //===-- WebAssemblyWasmObjectWriter.cpp - WebAssembly Wasm Writer ---------===//
   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 handles Wasm-specific object emission, converting LLVM's
  11 /// internal fixups into the appropriate relocations.
  12 ///
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "MCTargetDesc/WebAssemblyFixupKinds.h"
  16 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
  17 #include "llvm/BinaryFormat/Wasm.h"
  18 #include "llvm/MC/MCAsmBackend.h"
  19 #include "llvm/MC/MCFixup.h"
  20 #include "llvm/MC/MCFixupKindInfo.h"
  21 #include "llvm/MC/MCObjectWriter.h"
  22 #include "llvm/MC/MCSectionWasm.h"
  23 #include "llvm/MC/MCSymbolWasm.h"
  24 #include "llvm/MC/MCValue.h"
  25 #include "llvm/MC/MCWasmObjectWriter.h"
  26 #include "llvm/Support/Casting.h"
  27 #include "llvm/Support/ErrorHandling.h"
  28
  29 using namespace llvm;
  30
  31 namespace {
  32 class WebAssemblyWasmObjectWriter final : public MCWasmObjectTargetWriter {
  33 public:
  34   explicit WebAssemblyWasmObjectWriter(bool Is64Bit);
  35
  36 private:
  37   unsigned getRelocType(const MCValue &Target,
  38                         const MCFixup &Fixup) const override;
  39 };
  40 } // end anonymous namespace
  41
  42 WebAssemblyWasmObjectWriter::WebAssemblyWasmObjectWriter(bool Is64Bit)
  43     : MCWasmObjectTargetWriter(Is64Bit) {}
  44
  45 // Test whether the given expression computes a function address.
  46 static bool isFunctionExpr(const MCExpr *Expr) {
  47   if (auto SyExp = dyn_cast<MCSymbolRefExpr>(Expr))
  48     return cast<MCSymbolWasm>(SyExp->getSymbol()).isFunction();
  49
  50   if (auto BinOp = dyn_cast<MCBinaryExpr>(Expr))
  51     return isFunctionExpr(BinOp->getLHS()) != isFunctionExpr(BinOp->getRHS());
  52
  53   if (auto UnOp = dyn_cast<MCUnaryExpr>(Expr))
  54     return isFunctionExpr(UnOp->getSubExpr());
  55
  56   return false;
  57 }
  58
  59 static bool isFunctionType(const MCValue &Target) {
  60   const MCSymbolRefExpr *RefA = Target.getSymA();
  61   return RefA && RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX;
  62 }
  63
  64 static const MCSection *getFixupSection(const MCExpr *Expr) {
  65   if (auto SyExp = dyn_cast<MCSymbolRefExpr>(Expr)) {
  66     if (SyExp->getSymbol().isInSection())
  67       return &SyExp->getSymbol().getSection();
  68     return nullptr;
  69   }
  70
  71   if (auto BinOp = dyn_cast<MCBinaryExpr>(Expr)) {
  72     auto SectionLHS = getFixupSection(BinOp->getLHS());
  73     auto SectionRHS = getFixupSection(BinOp->getRHS());
  74     return SectionLHS == SectionRHS ? nullptr : SectionLHS;
  75   }
  76
  77   if (auto UnOp = dyn_cast<MCUnaryExpr>(Expr))
  78     return getFixupSection(UnOp->getSubExpr());
  79
  80   return nullptr;
  81 }
  82
  83 static bool isGlobalType(const MCValue &Target) {
  84   const MCSymbolRefExpr *RefA = Target.getSymA();
  85   return RefA && RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_GLOBAL;
  86 }
  87
  88 static bool isEventType(const MCValue &Target) {
  89   const MCSymbolRefExpr *RefA = Target.getSymA();
  90   return RefA && RefA->getKind() == MCSymbolRefExpr::VK_WebAssembly_EVENT;
  91 }
  92
  93 unsigned WebAssemblyWasmObjectWriter::getRelocType(const MCValue &Target,
  94                                                    const MCFixup &Fixup) const {
  95   // WebAssembly functions are not allocated in the data address space. To
  96   // resolve a pointer to a function, we must use a special relocation type.
  97   bool IsFunction = isFunctionExpr(Fixup.getValue());
  98
  99   switch (unsigned(Fixup.getKind())) {
 100   case WebAssembly::fixup_code_sleb128_i32:
 101     if (IsFunction)
 102       return wasm::R_WASM_TABLE_INDEX_SLEB;
 103     return wasm::R_WASM_MEMORY_ADDR_SLEB;
 104   case WebAssembly::fixup_code_sleb128_i64:
 105     llvm_unreachable("fixup_sleb128_i64 not implemented yet");
 106   case WebAssembly::fixup_code_uleb128_i32:
 107     if (isGlobalType(Target))
 108       return wasm::R_WASM_GLOBAL_INDEX_LEB;
 109     if (isFunctionType(Target))
 110       return wasm::R_WASM_TYPE_INDEX_LEB;
 111     if (IsFunction)
 112       return wasm::R_WASM_FUNCTION_INDEX_LEB;
 113     if (isEventType(Target))
 114       return wasm::R_WASM_EVENT_INDEX_LEB;
 115     return wasm::R_WASM_MEMORY_ADDR_LEB;
 116   case FK_Data_4:
 117     if (IsFunction)
 118       return wasm::R_WASM_TABLE_INDEX_I32;
 119     if (auto Section = static_cast<const MCSectionWasm *>(
 120             getFixupSection(Fixup.getValue()))) {
 121       if (Section->getKind().isText())
 122         return wasm::R_WASM_FUNCTION_OFFSET_I32;
 123       else if (!Section->isWasmData())
 124         return wasm::R_WASM_SECTION_OFFSET_I32;
 125     }
 126     return wasm::R_WASM_MEMORY_ADDR_I32;
 127   case FK_Data_8:
 128     llvm_unreachable("FK_Data_8 not implemented yet");
 129   default:
 130     llvm_unreachable("unimplemented fixup kind");
 131   }
 132 }
 133
 134 std::unique_ptr<MCObjectTargetWriter>
 135 llvm::createWebAssemblyWasmObjectWriter(bool Is64Bit) {
 136   return llvm::make_unique<WebAssemblyWasmObjectWriter>(Is64Bit);
 137 }