[WebAssembly] Don't create MSSymbolWasm object for non-symbols

[sbc100]

Summary

__linear_memory and __indirect_function_table are both generated
as imports in wasm object files but are actually symbols and don't
appear in any symbols table or relocation entry. Indeed we
don't have any symbol type to meaningfully represent either of them.

[tlively]

LGTM. Makes me wonder why it was ever done the other way.

[sbc100]

In D58487#1405260, @tlively wrote:

LGTM. Makes me wonder why it was ever done the other way.

Sorry I know this messes with you open PR.. should I hold of committing this until we reach consensus on your change?

[tlively]

In D58487#1405275, @sbc100 wrote:

In D58487#1405260, @tlively wrote:

LGTM. Makes me wonder why it was ever done the other way.

Sorry I know this messes with you open PR.. should I hold of committing this until we reach consensus on your change?

No need to wait. My PR can easily be updated to be rebased on top of this change.

llvm/trunk/lib/MC/WasmObjectWriter.cpp

static bool isInSymtab(const MCSymbolWasm &Sym) {
   return true;
 }
 
 uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
                                        const MCAsmLayout &Layout) {
   uint64_t StartOffset = W.OS.tell();
 
   LLVM_DEBUG(dbgs() << "WasmObjectWriter::writeObject\n");
-  MCContext &Ctx = Asm.getContext();
 
   // Collect information from the available symbols.
   SmallVector<WasmFunction, 4> Functions;
   SmallVector<uint32_t, 4> TableElems;
   SmallVector<wasm::WasmImport, 4> Imports;
   SmallVector<wasm::WasmExport, 4> Exports;
   SmallVector<wasm::WasmEventType, 1> Events;
   SmallVector<wasm::WasmSymbolInfo, 4> SymbolInfos;
   SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
   std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
   uint32_t DataSize = 0;
 
   // For now, always emit the memory import, since loads and stores are not
   // valid without it. In the future, we could perhaps be more clever and omit
   // it if there are no loads or stores.
-  auto *MemorySym =
-      cast<MCSymbolWasm>(Ctx.getOrCreateSymbol("__linear_memory"));
   wasm::WasmImport MemImport;
-  MemImport.Module = MemorySym->getImportModule();
-  MemImport.Field = MemorySym->getImportName();
+  MemImport.Module = "env";
+  MemImport.Field = "__linear_memory";
   MemImport.Kind = wasm::WASM_EXTERNAL_MEMORY;
   Imports.push_back(MemImport);
 
   // For now, always emit the table section, since indirect calls are not
   // valid without it. In the future, we could perhaps be more clever and omit
   // it if there are no indirect calls.
-  auto *TableSym =
-      cast<MCSymbolWasm>(Ctx.getOrCreateSymbol("__indirect_function_table"));
   wasm::WasmImport TableImport;
-  TableImport.Module = TableSym->getImportModule();
-  TableImport.Field = TableSym->getImportName();
+  TableImport.Module = "env";
+  TableImport.Field = "__indirect_function_table";
   TableImport.Kind = wasm::WASM_EXTERNAL_TABLE;
   TableImport.Table.ElemType = wasm::WASM_TYPE_FUNCREF;
   Imports.push_back(TableImport);
 
   // Populate SignatureIndices, and Imports and WasmIndices for undefined
   // symbols.  This must be done before populating WasmIndices for defined
   // symbols.
   for (const MCSymbol &S : Asm.symbols()) {