[ELF2] - Implemented -e flag

[grimar]

Summary

Implemented -e flag ("Use entry as the explicit symbol for beginning execution of your program, rather than the default entry point.");
Actually docs also says that:

"If there is no symbol named entry, the linker will try to parse entry as a number, and use that as the entry address (the number will be interpreted in base 10; you may use a leading 0x for base 16, or a leading 0 for base 8)."

But I doubt that it is a real usecase and not sure if it should be implemented or not ?

[ruiu]

ELF/Config.h

32

Member variable names in this class are consistent with their corresponding command line options. s/EntryPoint/Entry/.

ELF/Driver.cpp

142

We don't have comments for other options above.

test/elf2/entry.s

8

Remove blank trailing lines.

[grimar]

ELF/Config.h

32

Ok. That was not obvious though.

ELF/Driver.cpp

142

I`ll remove if you wish. Other are self-explaining, I thought at least reference to "-e" makes it more clear.

[ruiu]

ELF/Driver.cpp

142

This is also self-explanatory since the name of the variable is OPT_e (thus this is for -e). Can you add both --entry and -e to Options.td and use OPT_entry instead?

[grimar]

ELF/Driver.cpp

142

Sure. Will be added in next patch.

[grimar]

ELF/Config.h

32

Also can suggest to sort them alphabetically then.

[grimar]

Review comments addressed.

[denis-protivensky]

ELF/SymbolTable.cpp

76–79

Did you rebase your changes? This code looks outdated.

[grimar]

Updated patch to match latest revision.

ELF/SymbolTable.cpp

76–79

Updated the patch.

[ruiu]

LGTM

[grimar]

Could please anyone commit it ?
Going to request writing access today :)

[ruiu]

Submitted as r248806.

[grimar]

Thanks a lot Rui !

ELF/Config.h

struct Configuration {
   std::vector<llvm::StringRef> InputSearchPaths;
   llvm::StringRef Sysroot;
   bool Shared = false;
   bool DiscardAll = false;
   bool DiscardLocals = false;
   bool DiscardNone = false;
   bool ExportDynamic = false;
   bool NoInhibitExec = false;
   bool AllowMultipleDefinition = false;
+  llvm::StringRef Entry;
 };
 
 extern Configuration *Config;
 
 } // namespace elf2
 } // namespace lld
 
 #endif

ELF/Driver.cpp

if (Args.hasArg(OPT_discard_none))
     Config->DiscardNone = true;
 
   if (Args.hasArg(OPT_export_dynamic))
     Config->ExportDynamic = true;
 
   if (Args.hasArg(OPT_noinhibit_exec))
     Config->NoInhibitExec = true;
 
   if (Args.hasArg(OPT_allow_multiple_definition))
     Config->AllowMultipleDefinition = true;
 
+  if (auto *Arg = Args.getLastArg(OPT_entry))
+    Config->Entry = Arg->getValue();
+
   // Create a list of input files.
   std::vector<MemoryBufferRef> Inputs;
 
   for (auto *Arg : Args.filtered(OPT_l, OPT_INPUT)) {
     StringRef Path = Arg->getValue();
     if (Arg->getOption().getID() == OPT_l) {
       Inputs.push_back(openFile(searchLibrary(Path)));
       continue;

ELF/Options.td

 def l : Joined<["-"], "l">, MetaVarName<"<libName>">,
      HelpText<"Root name of library to use">;
 
 def alias_l : Joined<["--"], "library=">,
      Alias<l>;
 
 def sysroot : Joined<["--"], "sysroot=">,
      HelpText<"Set the system root">;
+
+def entry : Separate<["--", "-"], "entry">, MetaVarName<"<entry>">,
+     HelpText<"Name of entry point symbol">;
+
+def alias_entry : Separate<["-"], "e">,
+     Alias<entry>;

ELF/SymbolTable.cpp

void SymbolTable::addSyntheticSym(StringRef Name, OutputSection<ELFT> &Section,
   ESym->st_value = Value;
   auto Sym = new (Alloc) DefinedSynthetic<ELFT>(Name, *ESym, Section);
   resolve<ELFT>(Sym);
 }
 
 template <class ELFT> void SymbolTable::init(uint16_t EMachine) {
   Target.reset(createTarget(EMachine));
   if (Config->Shared)
     return;
-  EntrySym = new (Alloc)
-      Undefined<ELFT>(Target->getDefaultEntry(), Undefined<ELFT>::Synthetic);
+  EntrySym = new (Alloc) Undefined<ELFT>(
+      Config->Entry.empty() ? Target->getDefaultEntry() : Config->Entry,
+      Undefined<ELFT>::Synthetic);
   resolve<ELFT>(EntrySym);
 
   // In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol is magical
   // and is used to produce a R_386_GOTPC relocation.
   // The R_386_GOTPC relocation value doesn't actually depend on the
   // symbol value, so it could use an index of STN_UNDEF which, according to the
   // spec, means the symbol value is 0.
   // Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in

test/elf2/entry.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
+# RUN: not lld -flavor gnu2 %t1 -o %t2
+# RUN: lld -flavor gnu2 %t1 -o %t2 -e _end
+
+.globl _end;
+_end: