[WebAssembly] Don't generate unused table entries.

[sbc100]

Summary

When generating PIC output only relocations of type
R_WASM_TABLE_INDEX_REL_SLEB should generate table entries.

R_WASM_TABLE_INDEX_I32 get resolved at runtime via the auto-generated
__wasm_apply_relocs functions.

R_WASM_TABLE_INDEX_SLEB are not allowed in PIC code.

[sbc100]

• remove comment

[ruiu]

LGTM

lld/wasm/Writer.cpp

1159 ↗

(On Diff #198094)

nit: auto* -> auto *

[sbc100]

• feedback

[kripken]

I bisected the failure on emscripten's wasm0.test_dylink_rtti (and two others, also failing on the waterfall) which hits

exception thrown: RuntimeError: function signature mismatch,RuntimeError: function signature mismatch

To bisect, I ran

./emcc --clear-cache && ./tests/runner.py wasm0.test_dylink_rtti

on each iteration to make sure there isn't some local influence.

[sbc100]

OK, I'll revert this for now while I investigate.

test/wasm/pie.ll

 ; CHECK:        - Type:            IMPORT
 ; CHECK-NEXT:     Imports:
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __indirect_function_table
 ; CHECK-NEXT:         Kind:            TABLE
 ; CHECK-NEXT:         Table:
 ; CHECK-NEXT:           ElemType:        FUNCREF
 ; CHECK-NEXT:           Limits:
-; CHECK-NEXT:             Initial:         0x00000001
+; CHECK-NEXT:             Initial:         0x00000000
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __memory_base
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         GlobalType:      I32
 ; CHECK-NEXT:         GlobalMutable:   false
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __table_base
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         GlobalType:      I32
 ; CHECK-NEXT:         GlobalMutable:   false
 
 

test/wasm/shared.ll

 
 ; check for dylink section at start
 
 ; CHECK:      Sections:
 ; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            dylink
 ; CHECK-NEXT:     MemorySize:      24
 ; CHECK-NEXT:     MemoryAlignment: 2
-; CHECK-NEXT:     TableSize:       3
+; CHECK-NEXT:     TableSize:       1
 ; CHECK-NEXT:     TableAlignment:  0
 ; CHECK-NEXT:     Needed:          []
 ; CHECK-NEXT:   - Type:            TYPE
 
 ; check for import of __table_base and __memory_base globals
 
 ; CHECK:        - Type:            IMPORT
 ; CHECK-NEXT:     Imports:
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           memory
 ; CHECK-NEXT:         Kind:            MEMORY
 ; CHECK-NEXT:         Memory:
 ; CHECK-NEXT:           Initial:       0x0000000
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __indirect_function_table
 ; CHECK-NEXT:         Kind:            TABLE
 ; CHECK-NEXT:         Table:
 ; CHECK-NEXT:           ElemType:        FUNCREF
 ; CHECK-NEXT:           Limits:
-; CHECK-NEXT:             Initial:         0x00000003
+; CHECK-NEXT:             Initial:         0x00000001
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __stack_pointer
 ; CHECK-NEXT:         Kind:            GLOBAL
 ; CHECK-NEXT:         GlobalType:      I32
 ; CHECK-NEXT:         GlobalMutable:   true
 ; CHECK-NEXT:       - Module:          env
 ; CHECK-NEXT:         Field:           __memory_base
 ; CHECK-NEXT:         Kind:            GLOBAL

 
 ; check for elem segment initialized with __table_base global as offset
 
 ; CHECK:        - Type:            ELEM
 ; CHECK-NEXT:     Segments:
 ; CHECK-NEXT:       - Offset:
 ; CHECK-NEXT:           Opcode:          GLOBAL_GET
 ; CHECK-NEXT:           Index:           2
-; CHECK-NEXT:         Functions:       [ 5, 3, 0 ]
+; CHECK-NEXT:         Functions:       [ 5 ]
 
 ; check the generated code in __wasm_call_ctors and __wasm_apply_relocs functions
 ; TODO(sbc): Disassemble and verify instructions.
 
 ; CHECK:        - Type:            CODE
 ; CHECK-NEXT:     Functions:
 ; CHECK-NEXT:       - Index:           1
 ; CHECK-NEXT:         Locals:          []
 ; CHECK-NEXT:         Body:            10020B
 ; CHECK-NEXT:       - Index:           2
 ; CHECK-NEXT:         Locals:          []
-; CHECK-NEXT:         Body:            230141046A230241016A360200230141086A23043602002301410C6A230141006A360200230141106A2303360200230141146A230541046A3602000B
+; CHECK-NEXT:         Body:            230141046A230241006A360200230141086A23043602002301410C6A230141006A360200230141106A2303360200230141146A230541046A3602000B
 
 ; check the data segment initialized with __memory_base global as offset
 
 ; CHECK:        - Type:            DATA
 ; CHECK-NEXT:     Segments:
 ; CHECK-NEXT:       - SectionOffset:   6
 ; CHECK-NEXT:         InitFlags:       0
 ; CHECK-NEXT:         Offset:
 ; CHECK-NEXT:           Opcode:          GLOBAL_GET
 ; CHECK-NEXT:           Index:           1
-; CHECK-NEXT:         Content:         '020000000100000002000000000000000000000000000000'
+; CHECK-NEXT:         Content:         '020000000000000000000000000000000000000000000000'

wasm/InputFiles.cpp

if (isa<DataSymbol>(Sym) && Sym->isUndefined()) {
       }
     }
   }
 
   switch (Reloc.Type) {
   case R_WASM_TABLE_INDEX_I32:
   case R_WASM_TABLE_INDEX_SLEB:
   case R_WASM_TABLE_INDEX_REL_SLEB:
+    if (Config->Pic && !getFunctionSymbol(Reloc.Index)->hasTableIndex())
+      return 0;
     return getFunctionSymbol(Reloc.Index)->getTableIndex();
   case R_WASM_MEMORY_ADDR_SLEB:
   case R_WASM_MEMORY_ADDR_I32:
   case R_WASM_MEMORY_ADDR_LEB:
   case R_WASM_MEMORY_ADDR_REL_SLEB:
     return cast<DefinedData>(Sym)->getVirtualAddress() + Reloc.Addend;
   case R_WASM_TYPE_INDEX_LEB:
     return TypeMap[Reloc.Index];

wasm/Writer.cpp

 }
 
 void Writer::processRelocations(InputChunk *Chunk) {
   if (!Chunk->Live)
     return;
   ObjFile *File = Chunk->File;
   ArrayRef<WasmSignature> Types = File->getWasmObj()->types();
   for (const WasmRelocation &Reloc : Chunk->getRelocations()) {
+    if (Reloc.Type == R_WASM_TYPE_INDEX_LEB) {
+      // Mark target type as live
+      File->TypeMap[Reloc.Index] = registerType(Types[Reloc.Index]);
+      File->TypeIsUsed[Reloc.Index] = true;
+      continue;
+    }
+
+    // Other relocation types all have a corresponding symbol
+    auto *Sym = File->getSymbols()[Reloc.Index];
     switch (Reloc.Type) {
     case R_WASM_TABLE_INDEX_I32:
     case R_WASM_TABLE_INDEX_SLEB:
     case R_WASM_TABLE_INDEX_REL_SLEB: {
-      FunctionSymbol *Sym = File->getFunctionSymbol(Reloc.Index);
-      if (Sym->hasTableIndex() || !Sym->hasFunctionIndex())
-        continue;
-      Sym->setTableIndex(TableBase + IndirectFunctions.size());
-      IndirectFunctions.emplace_back(Sym);
+      if (Config->Pic && Reloc.Type != R_WASM_TABLE_INDEX_REL_SLEB)
+        break;
+      auto *F = cast<FunctionSymbol>(Sym);
+      if (F->hasTableIndex() || !F->hasFunctionIndex())
+        break;
+      F->setTableIndex(TableBase + IndirectFunctions.size());
+      IndirectFunctions.emplace_back(F);
       break;
     }
     case R_WASM_TYPE_INDEX_LEB:
-      // Mark target type as live
-      File->TypeMap[Reloc.Index] = registerType(Types[Reloc.Index]);
-      File->TypeIsUsed[Reloc.Index] = true;
       break;
-    case R_WASM_GLOBAL_INDEX_LEB: {
-      auto* Sym = File->getSymbols()[Reloc.Index];
+    case R_WASM_GLOBAL_INDEX_LEB:
       if (!isa<GlobalSymbol>(Sym) && !Sym->isInGOT()) {
         Sym->setGOTIndex(NumImportedGlobals++);
         GOTSymbols.push_back(Sym);
       }
       break;
-    }
     case R_WASM_MEMORY_ADDR_SLEB:
     case R_WASM_MEMORY_ADDR_LEB:
-    case R_WASM_MEMORY_ADDR_REL_SLEB: {
+    case R_WASM_MEMORY_ADDR_REL_SLEB:
       if (!Config->Relocatable) {
-        auto* Sym = File->getSymbols()[Reloc.Index];
         if (Sym->isUndefined() && !Sym->isWeak()) {
           error(toString(File) + ": cannot resolve relocation of type " +
                 relocTypeToString(Reloc.Type) +
                 " against undefined (non-weak) data symbol: " + toString(*Sym));
         }
       }
       break;
     }
-    }
 
     if (Config->Pic) {
       switch (Reloc.Type) {
       case R_WASM_TABLE_INDEX_SLEB:
       case R_WASM_MEMORY_ADDR_SLEB:
-      case R_WASM_MEMORY_ADDR_LEB: {
+      case R_WASM_MEMORY_ADDR_LEB:
         // Certain relocation types can't be used when building PIC output, since
         // they would require absolute symbol addresses at link time.
-        Symbol *Sym = File->getSymbols()[Reloc.Index];
         error(toString(File) + ": relocation " +
               relocTypeToString(Reloc.Type) + " cannot be used againt symbol " +
               toString(*Sym) + "; recompile with -fPIC");
         break;
-      }
       case R_WASM_TABLE_INDEX_I32:
-      case R_WASM_MEMORY_ADDR_I32: {
+      case R_WASM_MEMORY_ADDR_I32:
         // These relocation types are only present in the data section and
         // will be converted into code by `generateRelocationCode`.  This code
         // requires the symbols to have GOT entires.
-        auto* Sym = File->getSymbols()[Reloc.Index];
         if (!Sym->isHidden() && !Sym->isLocal() && !Sym->isInGOT()) {
           Sym->setGOTIndex(NumImportedGlobals++);
           GOTSymbols.push_back(Sym);
         }
         break;
       }
-      }
-    }
-  }
-}
 
 void Writer::assignIndexes() {
   assert(InputFunctions.empty());
   uint32_t FunctionIndex = NumImportedFunctions;
   auto AddDefinedFunction = [&](InputFunction *Func) {
     if (!Func->Live)
       return;
     InputFunctions.emplace_back(Func);