[lld][WebAssembly] Allow symbols with explict import names to be undefined at link time.

Differential Revision: https://reviews.llvm.org/D74110

Author: sbc100

lld/docs/WebAssembly.rst

@@ -112,8 +112,8 @@ The default behaviour is to generate these stub function and to produce
 a warning.  The ``--fatal-warnings`` flag can be used to disable this behaviour
 and error out if mismatched are found.
 
-Imports and Exports
-~~~~~~~~~~~~~~~~~~~
+Exports
+~~~~~~~
 
 When building a shared library any symbols marked as ``visibility=default`` will
 be exported.
@@ -130,6 +130,17 @@ Finally, just like with native ELF linker the ``--export-dynamic`` flag can be
 used to export symbols in the executable which are marked as
 ``visibility=default``.
 
+Imports
+~~~~~~~
+
+By default no undefined symbols are allowed in the final binary.  The flag
+``--allow-undefined`` results in a WebAssembly import being defined for each
+undefined symbol.  It is then up to the runtime to provide such symbols.
+
+Alternativly symbols can be marked in the source code as with the
+``import_name`` and/or ``import_module`` clang attributes which signals that
+they are expected to be undefined at static link time.
+
 Garbage Collection
 ~~~~~~~~~~~~~~~~~~
 

lld/test/wasm/import-name.ll

@@ -1,5 +1,5 @@
 ; RUN: llc -filetype=obj %s -o %t.o
-; RUN: wasm-ld --allow-undefined -o %t.wasm %t.o
+; RUN: wasm-ld -o %t.wasm %t.o
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 target triple = "wasm32-unknown-unknown"

lld/wasm/Relocations.cpp

@@ -28,6 +28,11 @@ static bool allowUndefined(const Symbol* sym) {
   // compiling with -fPIC)
   if (isa<DataSymbol>(sym))
     return false;
+  // Undefined functions with explicit import name are allowed to be undefined
+  // at link time.
+  if (auto *F = dyn_cast<UndefinedFunction>(sym))
+    if (F->importName)
+      return true;
   return (config->allowUndefined ||
           config->allowUndefinedSymbols.count(sym->getName()) != 0);
 }