llvm-nm: Don't print value or size for undefined or weak symbols

[meadori]

Summary

Undefined and weak symbols don't have a meaningful size or value.
As such, nothing should be printed for those attributes (this is
already done for the address with 'U') with the BSD format. This
matches what GNU nm does.

Note that for the POSIX.2 format [1] zero values are still
printed for the size and value. This seems in spirit with
the format strings in that specification, but is debatable.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/

test/Object/nm-trivial-object.test

 COFF-COMMON-NEXT:          C _a
 
 
 ELF-NOT:      U
 ELF:          U SomeOtherFunction
 ELF: 00000000 T main
 ELF:          U puts
 
-FIXME: we should not print the size of undefined symbols.
-ELF-SIZE:               00000000 U SomeOtherFunction
+ELF-SIZE:                        U SomeOtherFunction
 ELF-SIZE-NEXT: 00000000 00000024 T main
-ELF-SIZE-NEXT:          00000000 U puts
+ELF-SIZE-NEXT:                   U puts
 
 ELF-o: {{.*}}/trivial-object-test.elf-i386:          U SomeOtherFunction
 ELF-o: {{.*}}/trivial-object-test.elf-i386: 00000000 T main
 ELF-o: {{.*}}/trivial-object-test.elf-i386:          U puts
 
 ELF-u:          U SomeOtherFunction
 ELF-u:          U puts
 

test/tools/llvm-nm/X86/weak.test

+# RUN: llvm-nm -B -S %p/Inputs/weak.obj.elf-x86_64 | FileCheck --match-full-lines %s
+
+CHECK:                  w weak_extern_func
+CHECK:                  w weak_extern_var
+CHECK: 0000000000000000 0000000000000011 W weak_func
+CHECK: 0000000000000000 0000000000000004 V weak_var

tools/llvm-nm/llvm-nm.cpp

static void darwinPrintStab(MachOObjectFile *MachO, SymbolListT::iterator I) {
   outs() << Str << ' ';
   if (const char *stabString = getDarwinStabString(NType))
     format("%5.5s", stabString).print(Str, sizeof(Str));
   else
     format("   %02x", NType).print(Str, sizeof(Str));
   outs() << Str;
 }
 
+static bool symbolIsDefined(const NMSymbol &Sym) {
+  return Sym.TypeChar != 'U' && Sym.TypeChar != 'w' && Sym.TypeChar != 'v';
+}
+
 static void sortAndPrintSymbolList(SymbolicFile &Obj, bool printName,
                                    const std::string &ArchiveName,
                                    const std::string &ArchitectureName) {
   if (!NoSort) {
     std::function<bool(const NMSymbol &, const NMSymbol &)> Cmp;
     if (NumericSort)
       Cmp = compareSymbolAddress;
     else if (SizeSort)
if ((JustSymbolName || (UndefinedOnly && isa<MachOObjectFile>(Obj) &&
                             OutputFormat != darwin)) && OutputFormat != posix) {
       outs() << I->Name << "\n";
       continue;
     }
 
     char SymbolAddrStr[18] = "";
     char SymbolSizeStr[18] = "";
 
-    if (OutputFormat == sysv || I->TypeChar == 'U') {
-      if (OutputFormat == posix)
+    // If the format is SysV or the symbol isn't defined, then print spaces.
+    if (OutputFormat == sysv || !symbolIsDefined(*I)) {
+      if (OutputFormat == posix) {
         format(printFormat, I->Address)
           .print(SymbolAddrStr, sizeof(SymbolAddrStr));
-      else
+        format(printFormat, I->Size)
+            .print(SymbolSizeStr, sizeof(SymbolSizeStr));
+      } else {
         strcpy(SymbolAddrStr, printBlanks);
-    }
-    if (OutputFormat == sysv)
         strcpy(SymbolSizeStr, printBlanks);
+      }
+    }
 
-    if (I->TypeChar != 'U') {
+    // Otherwise, print the symbol address and size.
+    if (symbolIsDefined(*I)) {
       if (Obj.isIR())
         strcpy(SymbolAddrStr, printDashes);
       else
         format(printFormat, I->Address)
           .print(SymbolAddrStr, sizeof(SymbolAddrStr));
-    }
       format(printFormat, I->Size).print(SymbolSizeStr, sizeof(SymbolSizeStr));
+    }
 
     // If OutputFormat is darwin or we are printing Mach-O symbols in hex and
     // we have a MachOObjectFile, call darwinPrintSymbol to print as darwin's
     // nm(1) -m output or hex, else if OutputFormat is darwin or we are
     // printing Mach-O symbols in hex and not a Mach-O object fall back to
     // OutputFormat bsd (see below).
     MachOObjectFile *MachO = dyn_cast<MachOObjectFile>(&Obj);
     if ((OutputFormat == darwin || FormatMachOasHex) && (MachO || Obj.isIR())) {