[ELF] Handle -u before input files

If both a.a and b.so define foo

```
ld.bfd -u foo a.a b.so  # foo is defined
ld.bfd a.a b.so -u foo  # foo is defined
ld.bfd -u foo b.so a.a  # foo is undefined (provided at runtime by b.so)
ld.bfd b.so a.a -u foo  # foo is undefined (provided at runtime by b.so)
```

In all cases we make foo undefined in the output.  I tend to think the
GNU ld behavior makes more sense.

* In their model, they have to treat -u as a fake object file with an
  undefined symbol before all input files, otherwise the first archive would not be fetched.
* Following their behavior allows us to drop a --warn-backrefs special case.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D81052

lld/ELF/Driver.cpp

@@ -1470,16 +1470,12 @@ static void excludeLibs(opt::InputArgList &args) {
     visit(file);
 }
 
-// Force Sym to be entered in the output. Used for -u or equivalent.
+// Force Sym to be entered in the output.
 static void handleUndefined(Symbol *sym) {
   // Since a symbol may not be used inside the program, LTO may
   // eliminate it. Mark the symbol as "used" to prevent it.
   sym->isUsedInRegularObj = true;
 
-  // GNU linkers allow -u foo -ldef -lref. We should not treat it as a backward
-  // reference.
-  backwardReferences.erase(sym);
-
   if (sym->isLazy())
     sym->fetch();
 }
@@ -1675,6 +1671,12 @@ static Symbol *addUndefined(StringRef name) {
       Undefined{nullptr, name, STB_GLOBAL, STV_DEFAULT, 0});
 }
 
+static Symbol *addUnusedUndefined(StringRef name) {
+  Undefined sym{nullptr, name, STB_GLOBAL, STV_DEFAULT, 0};
+  sym.isUsedInRegularObj = false;
+  return symtab->addSymbol(sym);
+}
+
 // This function is where all the optimizations of link-time
 // optimization takes place. When LTO is in use, some input files are
 // not in native object file format but in the LLVM bitcode format.
@@ -1851,6 +1853,11 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   for (auto *arg : args.filtered(OPT_trace_symbol))
     symtab->insert(arg->getValue())->traced = true;
 
+  // Handle -u/--undefined before input files. If both a.a and b.so define foo,
+  // -u foo a.a b.so will fetch a.a.
+  for (StringRef name : config->undefined)
+    addUnusedUndefined(name);
+
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table. This process might
   // add files to the link, via autolinking, these files are always
@@ -1875,10 +1882,10 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   for (StringRef name : script->referencedSymbols)
     addUndefined(name);
 
-  // Handle the `--undefined <sym>` options.
-  for (StringRef arg : config->undefined)
-    if (Symbol *sym = symtab->find(arg))
-      handleUndefined(sym);
+  // Prevent LTO from removing any definition referenced by -u.
+  for (StringRef name : config->undefined)
+    if (Defined *sym = dyn_cast_or_null<Defined>(symtab->find(name)))
+      sym->isUsedInRegularObj = true;
 
   // If an entry symbol is in a static archive, pull out that file now.
   if (Symbol *sym = symtab->find(config->entry))

lld/test/ELF/undefined-opt.s

@@ -54,13 +54,19 @@
 # UNK-UNDEFINED-SO: ]
 
 # Added undefined symbols should appear in the dynamic table if necessary.
-# RUN: ld.lld -shared -o %t5 %t.o -u export
-# RUN: llvm-readobj --dyn-symbols %t5 | \
+# RUN: ld.lld -shared -soname=t -o %t.so %t.o -u export
+# RUN: llvm-readobj --dyn-symbols %t.so | \
 # RUN:     FileCheck --check-prefix=EXPORT-SO %s
 # EXPORT-SO: DynamicSymbols [
 # EXPORT-SO:   Name: export
 # EXPORT-SO: ]
 
+## Test that we handle -u before input files: if we handle -u after
+## %t.so, foo would be undefined in the output.
+# RUN: rm -f %t.a && llvm-ar rc %t.a %t.o
+# RUN: ld.lld -u export %t.a %t.so -o %t5
+# RUN: llvm-readobj --dyn-symbols %t5 | FileCheck --check-prefix=EXPORT-SO %s
+
 .globl _start
 _start: