229 lines · cpp
1//===- lib/MC/MCWasmStreamer.cpp - Wasm Object Output ---------------------===//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-exception6//7//===----------------------------------------------------------------------===//8//9// This file assembles .s files and emits Wasm .o object files.10//11//===----------------------------------------------------------------------===//12 13#include "llvm/MC/MCWasmStreamer.h"14#include "llvm/MC/MCAsmBackend.h"15#include "llvm/MC/MCAssembler.h"16#include "llvm/MC/MCCodeEmitter.h"17#include "llvm/MC/MCContext.h"18#include "llvm/MC/MCExpr.h"19#include "llvm/MC/MCFixup.h"20#include "llvm/MC/MCObjectStreamer.h"21#include "llvm/MC/MCSection.h"22#include "llvm/MC/MCSectionWasm.h"23#include "llvm/MC/MCSymbol.h"24#include "llvm/MC/MCSymbolWasm.h"25#include "llvm/MC/SectionKind.h"26#include "llvm/MC/TargetRegistry.h"27#include "llvm/Support/Alignment.h"28#include "llvm/Support/ErrorHandling.h"29#include "llvm/Support/MathExtras.h"30 31namespace llvm {32class MCContext;33class MCStreamer;34class MCSubtargetInfo;35} // namespace llvm36 37using namespace llvm;38 39MCWasmStreamer::~MCWasmStreamer() = default; // anchor.40 41void MCWasmStreamer::emitLabel(MCSymbol *S, SMLoc Loc) {42 auto *Symbol = static_cast<MCSymbolWasm *>(S);43 MCObjectStreamer::emitLabel(Symbol, Loc);44 45 const MCSectionWasm &Section =46 static_cast<const MCSectionWasm &>(*getCurrentSectionOnly());47 if (Section.getSegmentFlags() & wasm::WASM_SEG_FLAG_TLS)48 Symbol->setTLS();49}50 51void MCWasmStreamer::emitLabelAtPos(MCSymbol *S, SMLoc Loc, MCFragment &F,52 uint64_t Offset) {53 auto *Symbol = static_cast<MCSymbolWasm *>(S);54 MCObjectStreamer::emitLabelAtPos(Symbol, Loc, F, Offset);55 56 const MCSectionWasm &Section =57 static_cast<const MCSectionWasm &>(*getCurrentSectionOnly());58 if (Section.getSegmentFlags() & wasm::WASM_SEG_FLAG_TLS)59 Symbol->setTLS();60}61 62void MCWasmStreamer::emitAssignment(MCSymbol *S, const MCExpr *Value) {63 auto *Symbol = static_cast<MCSymbolWasm *>(S);64 65 // wasm has no absolute addressing. A symbol assigned a bare integer constant66 // ("sym = N" / ".set sym, N", with no symbol reference) cannot be modeled as67 // an absolute symbol: WasmObjectWriter's alias pass calls getBaseSymbol(),68 // which is null for a pure constant, and previously aborted MC finalization69 // with "absolute addressing not supported". glibc emits exactly this for70 // address-taken linkage markers (e.g. _nl_current_LC_*_used from71 // _NL_CURRENT_DEFINE in locale/localeinfo.h), where the symbol's *address* is72 // taken to force a translation unit to be linked, and any read of the symbol73 // is expected to yield the constant.74 //75 // Lower a named (non-temporary) absolute-constant symbol to a real defined76 // data object that holds the constant in its own data section. It then has a77 // genuine wasm linear-memory address that can be taken and relocated at link78 // (resolving an undefined reference in another object), and a load of it79 // yields the constant. This mirrors what GNU-as effectively provides for the80 // construct on ELF (a defined symbol with an address). It is never-silent:81 // the address is real, the value is preserved, and nothing is dropped.82 //83 // Assignments that reference other symbols (true aliases, label differences)84 // are left to the normal assembler-alias machinery (the WasmObjectWriter85 // alias pass and the prior alias-to-undefined / .symver fixes). Temporary,86 // assembler-internal absolute constants are also left as plain assignments:87 // they are folded into immediates at encode time and never need an address.88 int64_t Const;89 if (!Symbol->isTemporary() && Value->evaluateAsAbsolute(Const)) {90 MCContext &Ctx = getContext();91 92 // Store the constant in the target pointer width, widened only if needed so93 // the stored bytes equal the constant exactly (no silent truncation).94 unsigned Width = Ctx.getTargetTriple().isArch64Bit() ? 8 : 4;95 while (Width < 8 && !isIntN(Width * 8, Const) &&96 !isUIntN(Width * 8, static_cast<uint64_t>(Const)))97 Width *= 2;98 99 // A dedicated data section so the marker is independently addressable and100 // garbage-collectable, exactly like any other defined data symbol.101 MCSection *DataSec = Ctx.getWasmSection(Twine(".data.") + Symbol->getName(),102 SectionKind::getData());103 104 MCSection *Prev = getCurrentSectionOnly();105 switchSection(DataSec);106 emitValueToAlignment(Align(Width));107 emitLabel(Symbol);108 emitIntValue(static_cast<uint64_t>(Const), Width);109 Symbol->setSize(MCConstantExpr::create(Width, Ctx));110 // Record the constant so a repeated, identical "sym = N" (which glibc emits111 // per-TU and llvm-link concatenates without dedup) is recognized in the112 // parser as an idempotent no-op rather than colliding as a redefinition of113 // this now-defined data label, while a repeat to a *different* constant is114 // still rejected loudly. See MCParserUtils::parseAssignmentExpression.115 Symbol->setAbsoluteConstant(Const);116 if (Prev)117 switchSection(Prev);118 return;119 }120 121 MCObjectStreamer::emitAssignment(S, Value);122}123 124void MCWasmStreamer::changeSection(MCSection *Section, uint32_t Subsection) {125 MCAssembler &Asm = getAssembler();126 auto *SectionWasm = static_cast<const MCSectionWasm *>(Section);127 const MCSymbol *Grp = SectionWasm->getGroup();128 if (Grp)129 Asm.registerSymbol(*Grp);130 131 this->MCObjectStreamer::changeSection(Section, Subsection);132 Asm.registerSymbol(*Section->getBeginSymbol());133}134 135bool MCWasmStreamer::emitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {136 assert(Attribute != MCSA_IndirectSymbol && "indirect symbols not supported");137 auto *Symbol = static_cast<MCSymbolWasm *>(S);138 139 // Adding a symbol attribute always introduces the symbol; note that an140 // important side effect of calling registerSymbol here is to register the141 // symbol with the assembler.142 getAssembler().registerSymbol(*Symbol);143 144 switch (Attribute) {145 case MCSA_LazyReference:146 case MCSA_Reference:147 case MCSA_SymbolResolver:148 case MCSA_PrivateExtern:149 case MCSA_WeakDefinition:150 case MCSA_WeakDefAutoPrivate:151 case MCSA_Invalid:152 case MCSA_IndirectSymbol:153 case MCSA_Protected:154 case MCSA_Exported:155 return false;156 157 case MCSA_Hidden:158 Symbol->setHidden(true);159 break;160 161 case MCSA_Weak:162 case MCSA_WeakReference:163 Symbol->setWeak(true);164 Symbol->setExternal(true);165 break;166 167 case MCSA_Global:168 Symbol->setExternal(true);169 break;170 171 case MCSA_ELF_TypeFunction:172 Symbol->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);173 break;174 175 case MCSA_ELF_TypeTLS:176 Symbol->setTLS();177 break;178 179 case MCSA_ELF_TypeObject:180 case MCSA_Cold:181 break;182 183 case MCSA_NoDeadStrip:184 Symbol->setNoStrip();185 break;186 187 default:188 // unrecognized directive189 llvm_unreachable("unexpected MCSymbolAttr");190 return false;191 }192 193 return true;194}195 196void MCWasmStreamer::emitCommonSymbol(MCSymbol *S, uint64_t Size,197 Align ByteAlignment) {198 llvm_unreachable("Common symbols are not yet implemented for Wasm");199}200 201void MCWasmStreamer::emitELFSize(MCSymbol *Symbol, const MCExpr *Value) {202 static_cast<MCSymbolWasm *>(Symbol)->setSize(Value);203}204 205void MCWasmStreamer::emitLocalCommonSymbol(MCSymbol *S, uint64_t Size,206 Align ByteAlignment) {207 llvm_unreachable("Local common symbols are not yet implemented for Wasm");208}209 210void MCWasmStreamer::emitIdent(StringRef IdentString) {211 // TODO(sbc): Add the ident section once we support mergable strings212 // sections in the object format213}214 215void MCWasmStreamer::finishImpl() {216 emitFrames();217 218 this->MCObjectStreamer::finishImpl();219}220 221MCStreamer *llvm::createWasmStreamer(MCContext &Context,222 std::unique_ptr<MCAsmBackend> &&MAB,223 std::unique_ptr<MCObjectWriter> &&OW,224 std::unique_ptr<MCCodeEmitter> &&CE) {225 MCWasmStreamer *S =226 new MCWasmStreamer(Context, std::move(MAB), std::move(OW), std::move(CE));227 return S;228}229