Fix Weak External symbol handling.

The fallback atom was used only when it's searching for a symbol in a library;
if an undefined symbol was not found in a library, the LLD looked for its
fallback symbol in the library.

Although it worked in most cases, because symbols with fallbacks usually occur
only in OLDNAMES.LIB (a standard library), that behavior was incompatible with
link.exe. This patch fixes the issue so that the semantics is the same as
MSVC's link.exe

The new (and correct, I believe) behavior is this:

 - If there's no definition for an undefined atom, replace the undefined atom
   with its fallback and then proceed (e.g. look in the next file or stop
   linking as usual.)

Weak External symbols are underspecified in the Microsoft PE/COFF spec. However,
as long as I observed the behavior of link.exe, this seems to be what we want
for compatibility.

Differential Revision: http://llvm-reviews.chandlerc.com/D2162

llvm-svn: 195269

lld/lib/Core/Resolver.cpp

@@ -83,6 +83,16 @@ void Resolver::handleFile(const File &file) {
   for (const UndefinedAtom *undefAtom : file.undefined()) {
     doUndefinedAtom(*undefAtom);
     resolverState |= StateNewUndefinedAtoms;
+
+    // If the undefined symbol has an alternative name, try to resolve the
+    // symbol with the name to give it a second chance. This feature is used for
+    // COFF "weak external" symbol.
+    if (!_symbolTable.isDefined(undefAtom->name())) {
+      if (const UndefinedAtom *fallbackAtom = undefAtom->fallback()) {
+        doUndefinedAtom(*fallbackAtom);
+        _symbolTable.addReplacement(undefAtom, fallbackAtom);
+      }
+    }
   }
   for (const SharedLibraryAtom *shlibAtom : file.sharedLibrary()) {
     doSharedLibraryAtom(*shlibAtom);
@@ -108,15 +118,6 @@ Resolver::forEachUndefines(UndefCallback callback, bool searchForOverrides) {
       // load for previous undefine may also have loaded this undefine
       if (!_symbolTable.isDefined(undefName))
         callback(undefName, false);
-      // If the undefined symbol has an alternative name, try to resolve the
-      // symbol with the name to give it a second chance. This feature is used
-      // for COFF "weak external" symbol.
-      if (!_symbolTable.isDefined(undefName)) {
-        if (const UndefinedAtom *fallbackUndefAtom = undefAtom->fallback()) {
-          _symbolTable.addReplacement(undefAtom, fallbackUndefAtom);
-          _symbolTable.add(*fallbackUndefAtom);
-        }
-      }
     }
     // search libraries for overrides of common symbols
     if (searchForOverrides) {

lld/test/core/undef-fallback.objtxt

@@ -5,14 +5,33 @@
 
 ---
 defined-atoms:
-  - name: bar
-    type: code
+  - name: def1
+    scope: global
 
 undefined-atoms:
-  - name: foo
+  - name: undef1
     fallback:
-      name: bar
+      name: fallback1
+  - name: undef2
+    fallback:
+      name: fallback2
+---
+defined-atoms:
+  - name: fallback1
+
+undefined-atoms:
+  - name: def1
+    fallback:
+      name: fallback3
 ...
 
 # CHECK:      defined-atoms:
-# CHECK-NEXT:   - name: bar
+# CHECK-NEXT:   - name:            def1
+# CHECK-NEXT:     scope:           global
+# CHECK-NEXT:   - name:            fallback1
+# CHECK-NEXT:     ref-name:        fallback1
+# CHECK-NEXT: undefined-atoms:
+# CHECK-NEXT:   - name:            fallback1
+# CHECK-NEXT:   - name:            fallback2
+
+# CHECK-NOT:    - name:            fallback3

lld/test/core/undef-weak-coalesce.objtxt

@@ -1,5 +1,4 @@
-# RUN: lld -core %s 2> %t.err | FileCheck %s
-# RUN: FileCheck -check-prefix=ERROR %s < %t.err
+# RUN: lld -core %s | FileCheck %s
 
 #
 # Test that undefined symbols preserve their attributes and merge properly
@@ -29,12 +28,8 @@ undefined-atoms:
       can-be-null:       never
     - name:              bar8
       can-be-null:       at-runtime
-      fallback:
-        name: baz1
     - name:              bar9
       can-be-null:       at-buildtime
-      fallback:
-        name: baz2
 ---
 undefined-atoms:
     - name:              bar1
@@ -55,8 +50,6 @@ undefined-atoms:
       can-be-null:       never
     - name:              bar9
       can-be-null:       at-runtime
-      fallback:
-        name: baz3
 ...
 
 # CHECK:       - name:            regular_func
@@ -77,7 +70,3 @@ undefined-atoms:
 # CHECK-NEXT:  - name:            bar8
 # CHECK-NEXT:  - name:            bar9
 # CHECK-NEXT:    can-be-null:     at-runtime
-# CHECK-NEXT:    fallback:
-# CHECK-NEXT:      name:            baz3
-
-# ERROR: undefined symbol bar9 has different fallback: baz2 in and baz3 in

lld/test/pecoff/weak-external.test

@@ -4,6 +4,6 @@
 # RUN:    /entry:fn -- %t.obj %p/Inputs/static.lib 2> %t2.out
 # RUN: FileCheck %s < %t2.out
 
-CHECK-NOT: _no_such_symbol1
+CHECK: _no_such_symbol1
 CHECK-NOT: _no_such_symbol2
 CHECK: _no_such_symbol3