[lld][WebAssembly] Fix static linking of -fPIC code with external undefined data

[sbc100]

Thanks!

lld/test/wasm/pic-static.ll

 ; Test that PIC code can be linked into static binaries.
 ; In this case the GOT entries will end up as internalized wasm globals with
 ; fixed values.
 ; RUN: llc -relocation-model=pic -filetype=obj %p/Inputs/ret32.ll -o %t.ret32.o
 ; RUN: llc -relocation-model=pic -filetype=obj %s -o %t.o
 ; RUN: wasm-ld --allow-undefined --export-all -o %t.wasm %t.o %t.ret32.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target triple = "wasm32-unknown-emscripten"
 
 declare i32 @ret32(float)
 declare i32 @missing_function(float)
 @global_float = global float 1.0
 @hidden_float = hidden global float 2.0
+@missing_float = extern_weak global float
 
 @ret32_ptr = global i32 (float)* @ret32, align 4
 
 define i32 (float)* @getaddr_external() {
   ret i32 (float)* @ret32;
 }
 
 define i32 (float)* @getaddr_missing_function() {
   ret i32 (float)* @missing_function;
 }
 
 define i32 ()* @getaddr_hidden() {
   ret i32 ()* @hidden_func;
 }
 
+define float* @getaddr_missing_float() {
+  ret float* @missing_float
+}
+
 define hidden i32 @hidden_func() {
   ret i32 1
 }
 
 define void @_start() {
 entry:
   %f = load float, float* @hidden_float, align 4
   %addr = load i32 (float)*, i32 (float)** @ret32_ptr, align 4

 ; __table_base
 ; CHECK-NEXT:       - Index:           3
 ; CHECK-NEXT:         Type:            I32
 ; CHECK-NEXT:         Mutable:         false
 ; CHECK-NEXT:         InitExpr:
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           1
 
-; GOT.mem.global_float
+; GOT.mem.missing_float
 ; CHECK-NEXT:       - Index:           4
 ; CHECK-NEXT:         Type:            I32
 ; CHECK-NEXT:         Mutable:         false
 ; CHECK-NEXT:         InitExpr:
 ; CHECK-NEXT:           Opcode:          I32_CONST
+; CHECK-NEXT:           Value:           0
+
+; GOT.mem.global_float
+; CHECK-NEXT:       - Index:           5
+; CHECK-NEXT:         Type:            I32
+; CHECK-NEXT:         Mutable:         false
+; CHECK-NEXT:         InitExpr:
+; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           1024
 
 ; GOT.mem.ret32_ptr
-; CHECK-NEXT:       - Index:           5
+; CHECK-NEXT:       - Index:           6
 ; CHECK-NEXT:         Type:            I32
 ; CHECK-NEXT:         Mutable:         false
 ; CHECK-NEXT:         InitExpr:
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           1032
 
 ; __memory_base
-; CHECK-NEXT:       - Index:           6
+; CHECK-NEXT:       - Index:           7
 ; CHECK-NEXT:         Type:            I32
 ; CHECK-NEXT:         Mutable:         false
 ; CHECK-NEXT:         InitExpr:
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           1024

lld/wasm/SyntheticSections.cpp

void GlobalSection::writeBody() {
   for (InputGlobal *g : inputGlobals)
     writeGlobal(os, g->global);
   for (const Symbol *sym : staticGotSymbols) {
     WasmGlobal global;
     global.Type = {WASM_TYPE_I32, false};
     global.InitExpr.Opcode = WASM_OPCODE_I32_CONST;
     if (auto *d = dyn_cast<DefinedData>(sym))
       global.InitExpr.Value.Int32 = d->getVirtualAddress();
-    else if (auto *f = cast<FunctionSymbol>(sym))
+    else if (auto *f = dyn_cast<FunctionSymbol>(sym))
       global.InitExpr.Value.Int32 = f->getTableIndex();
+    else {
+      assert(isa<UndefinedData>(sym));
+      global.InitExpr.Value.Int32 = 0;
+    }
     writeGlobal(os, global);
   }
   for (const DefinedData *sym : dataAddressGlobals) {
     WasmGlobal global;
     global.Type = {WASM_TYPE_I32, false};
     global.InitExpr.Opcode = WASM_OPCODE_I32_CONST;
     global.InitExpr.Value.Int32 = sym->getVirtualAddress();
     writeGlobal(os, global);