COFF: Fix bug involving archives defining a symbol multiple times.

Previously we were unnecessarily loading lazy symbols if they appeared in an
archive multiple times, as can happen with comdat symbols. This change fixes
the bug by only loading symbols from archives at load time if the original
symbol was undefined.

Differential Revision: http://reviews.llvm.org/D10980

llvm-svn: 241538

lld/COFF/SymbolTable.cpp

@@ -177,7 +177,8 @@ void SymbolTable::addLazy(Lazy *New, std::vector<Symbol *> *Accum) {
     if (!Sym->Body.compare_exchange_strong(Existing, New))
       continue;
     New->setBackref(Sym);
-    Accum->push_back(Sym);
+    if (isa<Undefined>(Existing))
+      Accum->push_back(Sym);
     return;
   }
 }

lld/test/COFF/ar-comdat.test

@@ -0,0 +1,38 @@
+# RUN: yaml2obj %s > %t1.obj
+# RUN: yaml2obj %s > %t2.obj
+# RUN: llvm-lib /out:%t.lib %t1.obj %t2.obj
+# RUN: lld -flavor link2 /out:%t.exe /lldmap:%t.map /entry:main /subsystem:console %p/Inputs/ret42.obj %t.lib
+# RUN: FileCheck %s < %t.map
+
+# CHECK-NOT: .lib
+
+---
+header:          
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:        
+  - Name:            .bss
+    Characteristics: [ IMAGE_SCN_CNT_UNINITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+symbols:         
+  - Name:            .bss
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          4
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          1
+      Selection:       IMAGE_COMDAT_SELECT_ANY
+  - Name:            x
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...