[lld][WebAssembly] Allow export for optional symbols

[sbc100]

Summary

This moves the error checking until after all optional
symbols (including the section start/end symbols) have
been created.

[sbc100]

• revert

lld/test/wasm/export-optional.s

+# Test the --export of optional linker-synthetic symbols works.
+# Specifically the __start_xxx and __end_xx symbols.
+
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld --export=__start_foo %t.o -o %t.wasm
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+  .globl  _start
+_start:
+  .functype _start () -> ()
+  i32.load foo
+  drop
+  end_function
+
+  .globl  foo
+  .section    foo,"",@
+foo:
+  .int32 42
+  .size foo, 4
+
+#      CHECK:  - Type:            EXPORT
+# CHECK-NEXT:    Exports:
+# CHECK-NEXT:      - Name:            memory
+# CHECK-NEXT:        Kind:            MEMORY
+# CHECK-NEXT:        Index:           0
+# CHECK-NEXT:      - Name:            _start
+# CHECK-NEXT:        Kind:            FUNCTION
+# CHECK-NEXT:        Index:           0
+# CHECK-NEXT:      - Name:            __start_foo
+# CHECK-NEXT:        Kind:            GLOBAL
+# CHECK-NEXT:        Index:           1

lld/wasm/Driver.cpp

void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   // Apply symbol renames for -wrap.
   if (!wrapped.empty())
     wrapSymbols(wrapped);
 
   for (auto *arg : args.filtered(OPT_export)) {
     Symbol *sym = symtab->find(arg->getValue());
     if (sym && sym->isDefined())
       sym->forceExport = true;
-    else if (config->unresolvedSymbols == UnresolvedPolicy::ReportError)
-      error(Twine("symbol exported via --export not found: ") +
-            arg->getValue());
-    else if (config->unresolvedSymbols == UnresolvedPolicy::Warn)
-      warn(Twine("symbol exported via --export not found: ") + arg->getValue());
   }
 
   if (!config->relocatable && !config->isPic) {
     // Add synthetic dummies for weak undefined functions.  Must happen
     // after LTO otherwise functions may not yet have signatures.
     symtab->handleWeakUndefines();
   }
 

lld/wasm/Writer.cpp

void Writer::run() {
 
   if (!config->relocatable) {
     // Create linker synthesized __start_SECNAME/__stop_SECNAME symbols
     // This has to be done after memory layout is performed.
     for (const OutputSegment *seg : segments)
       addStartStopSymbols(seg);
   }
 
+  // Delay reporting error about explict exports until after addStartStopSymbols
+  // which can create optional symbols.
+  for (auto &entry : config->exportedSymbols) {
+    StringRef name = entry.first();
+    Symbol *sym = symtab->find(name);
+    if (sym && sym->isDefined())
+      sym->forceExport = true;
+    else if (config->unresolvedSymbols == UnresolvedPolicy::ReportError)
+      error(Twine("symbol exported via --export not found: ") + name);
+    else if (config->unresolvedSymbols == UnresolvedPolicy::Warn)
+      warn(Twine("symbol exported via --export not found: ") + name);
+  }
+
   log("-- scanRelocations");
   scanRelocations();
   log("-- finalizeIndirectFunctionTable");
   finalizeIndirectFunctionTable();
   log("-- createSyntheticInitFunctions");
   createSyntheticInitFunctions();
   log("-- assignIndexes");
   assignIndexes();