[WebAssembly] Honor --allow-undefined even for explicit exports

When a symbol is exported via --export=foo but --allow-undefined
is also specified, the symbol is now allowed to be undefined.
Previously we were special casing such symbols.

This combinations of behavior is exactly what emescripten
requires. Although we are trying hard not to allow emscripten
specific features in lld, this one makes sense.

Enforce this behavior by added this case to test/wasm/undefined.ll.

Differential Revision: https://reviews.llvm.org/D44237

llvm-svn: 326976

lld/test/wasm/undefined.ll

@@ -6,10 +6,13 @@
 ; CHECK: error: {{.*}}.o: undefined symbol: foo
 ; CHECK: error: undefined symbol: baz
 
-; But succeeds if we pass a file containing 'foo' as --allow-undefined-file.
+; Succeeds if we pass a file containing 'foo' as --allow-undefined-file.
 ; RUN: echo 'foo' > %t.txt
 ; RUN: wasm-ld --check-signatures --allow-undefined-file=%t.txt -o %t.wasm %t.o
 
+; Succeeds even if a missing symbol is added via --export
+; RUN: wasm-ld --check-signatures --allow-undefined --export=xxx -o %t.wasm %t.o
+
 target triple = "wasm32-unknown-unknown-wasm"
 
 ; Takes the address of the external foo() resulting in undefined external

lld/wasm/Driver.cpp

@@ -346,7 +346,7 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
     Symbol *Sym = Symtab->find(Name);
     if (Sym && Sym->isDefined())
       Sym->setHidden(false);
-    else
+    else if (!Config->AllowUndefined)
       error("symbol exported via --export not found: " + Name);
   }