[WebAssembly] Model weak aliases as wasm exports

[sbc100]

Summary

Previouly these were being included as both imports and
exports, with the import being satisfied by the export
(or some strong symbol) at runtime. However this proved
unnecesary and actually complicated linking as it meant
there was not a 1-to-1 mapping between a wasm function
/global index and a linker symbol.

[dschuff]

lib/Object/WasmObjectFile.cpp

340 ↗

(On Diff #116586)

I just remembered that try_emplace is a C++17 function, so it may not be in the stdlib on all supported LLVM platforms; we should probably switch to something else.

[dschuff]

That can be a separate change though.
Is this in tool-conventions yet? If so, should update.

[sbc100]

lib/Object/WasmObjectFile.cpp

340 ↗

(On Diff #116586)

Its been in here for months now.. so at least there are not bots that can't build this.

Oh I see, this is a StringMap which does define its own try_emplace() method.. so we should be fine.

llvm/trunk/lib/MC/WasmObjectWriter.cpp

for (const MCSymbol &S : Asm.symbols()) {
 
     if (WS.isTemporary())
       continue;
 
     if (WS.isFunction())
       registerFunctionType(WS);
 
     // If the symbol is not defined in this translation unit, import it.
-    if (!WS.isDefined(/*SetUsed=*/false) || WS.isVariable()) {
+    if (!WS.isDefined(/*SetUsed=*/false)) {
       WasmImport Import;
       Import.ModuleName = WS.getModuleName();
       Import.FieldName = WS.getName();
 
       if (WS.isFunction()) {
         Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
         Import.Type = getFunctionType(WS);
         SymbolIndices[&WS] = NumFuncImports;
for (const MCSymbol &S : Asm.symbols()) {
     // Find the target symbol of this weak alias and export that index
     const auto &WS = static_cast<const MCSymbolWasm &>(S);
     const MCSymbolWasm *ResolvedSym = ResolveSymbol(WS);
     DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym << "'\n");
     assert(SymbolIndices.count(ResolvedSym) > 0);
     uint32_t Index = SymbolIndices.find(ResolvedSym)->second;
     DEBUG(dbgs() << "  -> index:" << Index << "\n");
 
+    SymbolIndices[&WS] = Index;
     WasmExport Export;
     Export.FieldName = WS.getName();
     Export.Index = Index;
     if (WS.isFunction())
       Export.Kind = wasm::WASM_EXTERNAL_FUNCTION;
     else
       Export.Kind = wasm::WASM_EXTERNAL_GLOBAL;
     DEBUG(dbgs() << "  -> export " << Exports.size() << "\n");

llvm/trunk/lib/Object/WasmObjectFile.cpp

void WasmObjectFile::populateSymbolTable() {
   // Add exports to symbol table
   for (const wasm::WasmExport& Export : Exports) {
     if (Export.Kind == wasm::WASM_EXTERNAL_FUNCTION ||
         Export.Kind == wasm::WASM_EXTERNAL_GLOBAL) {
       WasmSymbol::SymbolType ExportType =
           Export.Kind == wasm::WASM_EXTERNAL_FUNCTION
               ? WasmSymbol::SymbolType::FUNCTION_EXPORT
               : WasmSymbol::SymbolType::GLOBAL_EXPORT;
-      auto Pair = SymbolMap.try_emplace(Export.Name, Symbols.size());
-      if (Pair.second) {
-        Symbols.emplace_back(Export.Name, ExportType,
-                             ExportSection, Export.Index);
-        DEBUG(dbgs() << "Adding export: " << Symbols.back()
-                     << " sym index:" << Symbols.size() << "\n");
-      } else {
-        uint32_t SymIndex = Pair.first->second;
-        Symbols[SymIndex] =
-            WasmSymbol(Export.Name, ExportType, ExportSection, Export.Index);
-        DEBUG(dbgs() << "Replacing existing symbol:  " << Symbols[SymIndex]
-                     << " sym index:" << SymIndex << "\n");
-      }
+      SymbolMap.try_emplace(Export.Name, Symbols.size());
     }
   }
 }
 
 Error WasmObjectFile::parseLinkingSection(const uint8_t *Ptr,
                                           const uint8_t *End) {
   HasLinkingSection = true;
 

llvm/trunk/test/MC/WebAssembly/weak-alias.ll

 }
 
 
 ; CHECK:        - Type:            TYPE
 ; CHECK-NEXT:     Signatures:      
 ; CHECK-NEXT:       - Index:           0
 ; CHECK-NEXT:         ReturnType:      I32
 ; CHECK-NEXT:         ParamTypes:      
-
-; CHECK:        - Type:            IMPORT
-; CHECK-NEXT:     Imports:         
-; CHECK-NEXT:       - Module:          env
-; CHECK-NEXT:         Field:           foo_alias
-; CHECK-NEXT:         Kind:            FUNCTION
-; CHECK-NEXT:         SigIndex:        0
-; CHECK-NEXT:       - Module:          env
-; CHECK-NEXT:         Field:           bar_alias
-; CHECK-NEXT:         Kind:            GLOBAL
-; CHECK-NEXT:         GlobalType:      I32
-; CHECK-NEXT:         GlobalMutable:   false
-
-; CHECK:        - Type:            FUNCTION
+; CHECK-NEXT:   - Type:            FUNCTION
 ; CHECK-NEXT:     FunctionTypes:   [ 0, 0 ]
-
-; CHECK:        - Type:            EXPORT
+; CHECK-NEXT:   - Type:            TABLE
+; CHECK-NEXT:     Tables:          
+; CHECK-NEXT:       - ElemType:        ANYFUNC
+; CHECK-NEXT:         Limits:          
+; CHECK-NEXT:           Initial:         0x00000000
+; CHECK-NEXT:   - Type:            MEMORY
+; CHECK-NEXT:     Memories:        
+; CHECK-NEXT:       - Initial:         0x00000001
+; CHECK-NEXT:   - Type:            GLOBAL
+; CHECK-NEXT:     Globals:         
+; CHECK-NEXT:       - Type:            I32
+; CHECK-NEXT:         Mutable:         false
+; CHECK-NEXT:         InitExpr:        
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           0
+; CHECK-NEXT:       - Type:            I32
+; CHECK-NEXT:         Mutable:         false
+; CHECK-NEXT:         InitExpr:        
+; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           8
+; CHECK-NEXT:   - Type:            EXPORT
 ; CHECK-NEXT:     Exports:         
 ; CHECK-NEXT:       - Name:            call_alias
 ; CHECK-NEXT:         Kind:            FUNCTION
-; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:       - Name:            foo
 ; CHECK-NEXT:         Kind:            FUNCTION
-; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:       - Name:            bar
 ; CHECK-NEXT:         Kind:            GLOBAL
-; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:       - Name:            bar_alias_address
 ; CHECK-NEXT:         Kind:            GLOBAL
-; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:       - Name:            foo_alias
 ; CHECK-NEXT:         Kind:            FUNCTION
-; CHECK-NEXT:         Index:           2
+; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:       - Name:            bar_alias
 ; CHECK-NEXT:         Kind:            GLOBAL
-; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Index:           0
 
 ; CHECK:        - Type:            DATA
 ; CHECK-NEXT:     Relocations:     
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_MEMORY_ADDR_I32
 ; CHECK-NEXT:         Index:           0
 ; CHECK-NEXT:         Offset:          0x0000000F
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - SectionOffset:   6
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           0
 ; CHECK-NEXT:         Content:         '07000000'
 ; CHECK-NEXT:       - SectionOffset:   15
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           8
 ; CHECK-NEXT:         Content:         '00000000'
 
 ; CHECK:        - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            name
 ; CHECK-NEXT:     FunctionNames:   
 ; CHECK-NEXT:       - Index:           0
-; CHECK-NEXT:         Name:            foo_alias
-; CHECK-NEXT:       - Index:           1
 ; CHECK-NEXT:         Name:            call_alias
-; CHECK-NEXT:       - Index:           2
+; CHECK-NEXT:       - Index:           1
 ; CHECK-NEXT:         Name:            foo
 ; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            linking
 ; CHECK-NEXT:     DataSize:        12
 ; CHECK-NEXT:     DataAlignment:   8
 ; CHECK-NEXT:     SymbolInfo:      
 ; CHECK-NEXT:       - Name:            foo_alias
 ; CHECK-NEXT:         Flags:           1
 ; CHECK-NEXT:       - Name:            bar_alias
 ; CHECK-NEXT:         Flags:           1
 ; CHECK-NEXT:     SegmentNames:    
 ; CHECK-NEXT:       - Index:           0
 ; CHECK-NEXT:         Name:            .data.bar
 ; CHECK-NEXT:       - Index:           1
 ; CHECK-NEXT:         Name:            .data.bar_alias_address
 ; CHECK-NEXT: ...
 
 ; CHECK-SYMS: SYMBOL TABLE:
-; CHECK-SYMS-NEXT: 00000000 g     F name  foo_alias
-; CHECK-SYMS-NEXT: 00000001 g     F name  call_alias
-; CHECK-SYMS-NEXT: 00000002 g     F name  foo
-; CHECK-SYMS-NEXT: 00000002 gw    F EXPORT  foo_alias
-; CHECK-SYMS-NEXT: 00000000 gw      EXPORT  bar_alias
-; CHECK-SYMS-NEXT: 00000001 g     F EXPORT  call_alias
-; CHECK-SYMS-NEXT: 00000002 g     F EXPORT  foo
+; CHECK-SYMS-NEXT: 00000000 g     F name  call_alias
+; CHECK-SYMS-NEXT: 00000001 g     F name  foo
+; CHECK-SYMS-NEXT: 00000000 g     F EXPORT  call_alias
+; CHECK-SYMS-NEXT: 00000001 g     F EXPORT  foo
 ; CHECK-SYMS-NEXT: 00000000 g       EXPORT  bar
 ; CHECK-SYMS-NEXT: 00000008 g       EXPORT  bar_alias_address
+; CHECK-SYMS-NEXT: 00000001 gw    F EXPORT  foo_alias
+; CHECK-SYMS-NEXT: 00000000 gw      EXPORT  bar_alias