COFF: Support /force flag.

This option is to ignore remaining undefined symbols and force
the linker to create an output file anyways.

The existing code assumes that there's no undefined symbol after
reportRemainingUndefines(). That assumption is legitimate.
I also don't want to mess up the existing code for this minor feature.
In order to keep it as is, remaining undefined symbols are replaced
with dummy defined symbols.

llvm-svn: 240913

lld/COFF/Config.h

@@ -46,6 +46,7 @@ struct Configuration {
   std::string OutputFile;
   bool DoGC = true;
   bool Relocatable = true;
+  bool Force = false;
 
   // Symbols in this set are considered as live by the garbage collector.
   std::set<StringRef> GCRoots;

lld/COFF/Driver.cpp

@@ -265,6 +265,10 @@ bool LinkerDriver::link(llvm::ArrayRef<const char *> ArgsArr) {
   if (Args.hasArg(OPT_verbose))
     Config->Verbose = true;
 
+  // Handle /force or /force:unresolved
+  if (Args.hasArg(OPT_force) || Args.hasArg(OPT_force_unresolved))
+    Config->Force = true;
+
   // Handle /entry
   if (auto *Arg = Args.getLastArg(OPT_entry)) {
     Config->EntryName = Arg->getValue();

lld/COFF/SymbolTable.cpp

@@ -92,6 +92,12 @@ bool SymbolTable::reportRemainingUndefines() {
       }
     }
     llvm::errs() << "undefined symbol: " << Name << "\n";
+    // Remaining undefined symbols are not fatal if /force is specified.
+    // They are replaced with dummy defined symbols.
+    if (Config->Force) {
+      Sym->Body = new (Alloc) DefinedAbsolute(Name, 0);
+      continue;
+    }
     Ret = true;
   }
   return Ret;

lld/test/COFF/force.test

@@ -0,0 +1,43 @@
+# RUN: yaml2obj < %s > %t.obj
+# RUN: not lld -flavor link2 /out:%t.exe %t.obj
+# RUN: FileCheck %s < %t.log
+# RUN: lld -flavor link2 /out:%t.exe %t.obj /force >& %t.log
+# RUN: FileCheck %s < %t.log
+
+# CHECK: undefined symbol: foo
+
+---
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     000000000000
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          6
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            mainCRTStartup
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            foo
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...