[lld][WebAssembly] Fix crash with `-pie` without `--allow-undefined`

[sbc100]

Summary

shouldImport was not returning true in PIC mode even though our
assumption elsewhere (in Relocations.cpp:scanRelocations) is that we
don't report undefined symbols in PIC mode (at least not today.
This was resulting functions that were undefined and but also not
imported which hits an assert later on that all functions have valid
indexes.

[dschuff]

So this change means that unresolved symbols will turn into imports when linking?

[sbc100]

In D101716#2734815, @dschuff wrote:

So this change means that unresolved symbols will turn into imports when linking?

Yes, this was already the assumption of code that reported undefined symbols. In emscripten this had gone unnoticed because we have traditionally also always passed --allow-undefined.

Our experimental -pie and -shared modes implicitly include --allow-undefined (at least for now they do).

lld/test/wasm/pie.ll

 ; RUN: llc -relocation-model=pic -mattr=+mutable-globals -filetype=obj %s -o %t.o
-; RUN: wasm-ld --no-gc-sections --allow-undefined --experimental-pic -pie -o %t.wasm %t.o
+; RUN: wasm-ld --no-gc-sections --experimental-pic -pie -o %t.wasm %t.o
 
 target triple = "wasm32-unknown-emscripten"
 
 @data = global i32 2, align 4
 @data_external = external global i32
 @indirect_func = local_unnamed_addr global i32 ()* @foo, align 4
 
 @data_addr = local_unnamed_addr global i32* @data, align 4

 }
 
 define default i32** @get_data_address() {
 entry:
   ret i32** @data_addr_external
 }
 
 define void @_start() {
+  call void @external_func()
   ret void
 }
 
+declare void @external_func()
+
 ;      CHECK: Sections:
 ; CHECK-NEXT:   - Type:            CUSTOM
 ; CHECK-NEXT:     Name:            dylink
 ; CHECK-NEXT:     MemorySize:      16
 ; CHECK-NEXT:     MemoryAlignment: 2
 ; CHECK-NEXT:     TableSize:       1
 ; CHECK-NEXT:     TableAlignment:  0
 ; CHECK-NEXT:     Needed:          []

 ; to be generated along with __wasm_start as the start
 ; function.
 
 ; RUN: llc -relocation-model=pic -mattr=+mutable-globals,+atomics,+bulk-memory -filetype=obj %s -o %t.shmem.o
 ; RUN: wasm-ld --no-gc-sections --shared-memory --allow-undefined --experimental-pic -pie -o %t.shmem.wasm %t.shmem.o
 ; RUN: obj2yaml %t.shmem.wasm | FileCheck %s --check-prefix=SHMEM
 
 ; SHMEM:         - Type:            CODE
-; SHMEM:           - Index:           5
+; SHMEM:           - Index:           6
 ; SHMEM-NEXT:        Locals:          []
-; SHMEM-NEXT:        Body:            100210040B
+; SHMEM-NEXT:        Body:            100310050B
 
 ; SHMEM:         FunctionNames:
 ; SHMEM-NEXT:      - Index:           0
-; SHMEM-NEXT:        Name:            __wasm_call_ctors
+; SHMEM-NEXT:        Name:            external_func
 ; SHMEM-NEXT:      - Index:           1
-; SHMEM-NEXT:        Name:            __wasm_init_tls
+; SHMEM-NEXT:        Name:            __wasm_call_ctors
 ; SHMEM-NEXT:      - Index:           2
-; SHMEM-NEXT:        Name:            __wasm_init_memory
+; SHMEM-NEXT:        Name:            __wasm_init_tls
 ; SHMEM-NEXT:      - Index:           3
-; SHMEM-NEXT:        Name:            __wasm_apply_data_relocs
+; SHMEM-NEXT:        Name:            __wasm_init_memory
 ; SHMEM-NEXT:      - Index:           4
-; SHMEM-NEXT:        Name:            __wasm_apply_global_relocs
+; SHMEM-NEXT:        Name:            __wasm_apply_data_relocs
 ; SHMEM-NEXT:      - Index:           5
-; SHMEM-NEXT:        Name:            __wasm_start
+; SHMEM-NEXT:        Name:            __wasm_apply_global_relocs
 ; SHMEM-NEXT:      - Index:           6
-; SHMEM-NEXT:        Name:            foo
+; SHMEM-NEXT:        Name:            __wasm_start
 ; SHMEM-NEXT:      - Index:           7
-; SHMEM-NEXT:        Name:            get_data_address
+; SHMEM-NEXT:        Name:            foo
 ; SHMEM-NEXT:      - Index:           8
+; SHMEM-NEXT:        Name:            get_data_address
+; SHMEM-NEXT:      - Index:           9
 ; SHMEM-NEXT:        Name:            _start
 

lld/wasm/Relocations.cpp

if (config->isPic) {
       case R_WASM_MEMORY_ADDR_I64:
         // 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.
         if (requiresGOTAccess(sym))
           addGOTEntry(sym);
         break;
       }
-    } else {
+    } else if (sym->isUndefined() && !config->relocatable && !sym->isWeak()) {
       // Report undefined symbols
-      if (sym->isUndefined() && !config->relocatable && !sym->isWeak())
-        reportUndefined(sym);
     }
   }
 }
 
 } // namespace wasm
 } // namespace lld

lld/wasm/Writer.cpp

static bool shouldImport(Symbol *sym) {
   if (!sym->isUsedInRegularObj)
     return false;
 
   // We don't generate imports for data symbols. They however can be imported
   // as GOT entries.
   if (isa<DataSymbol>(sym))
     return false;
 
-  if (config->relocatable ||
+  if ((config->isPic || config->relocatable) ||
       config->unresolvedSymbols == UnresolvedPolicy::ImportFuncs)
     return true;
   if (config->allowUndefinedSymbols.count(sym->getName()) != 0)
     return true;
   if (auto *g = dyn_cast<UndefinedGlobal>(sym))
     return g->importName.hasValue();
   if (auto *f = dyn_cast<UndefinedFunction>(sym))
     return f->importName.hasValue();