lld: allow -e with -shared

[emaste]

Summary

It is reasonable to specify an entry point for shared objects - e.g. FreeBSD rtld ld-elf.so.1.

Unlike GNU ld we leave it as 0 if -shared is specified without an explicit -e address or symbol.

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   SymbolTable<ELFT> Symtab;
   Target.reset(createTarget());
 
   if (!Config->Shared) {
     // Add entry symbol.
     if (Config->Entry.empty())
       Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
 
-    // Set either EntryAddr (if S is a number) or EntrySym (otherwise).
-    StringRef S = Config->Entry;
-    if (S.getAsInteger(0, Config->EntryAddr))
-      Config->EntrySym = Symtab.addUndefined(S);
-
     // 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
     // the object file.
     // The situation is even stranger on x86_64 where the assembly doesn't
     // need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
     // an undefined symbol in the .o files.
     // Given that the symbol is effectively unused, we just create a dummy
     // hidden one to avoid the undefined symbol error.
     Symtab.addIgnoredSym("_GLOBAL_OFFSET_TABLE_");
   }
 
+  if (!Config->Entry.empty()) {
+    // Set either EntryAddr (if S is a number) or EntrySym (otherwise).
+    StringRef S = Config->Entry;
+    if (S.getAsInteger(0, Config->EntryAddr))
+      Config->EntrySym = Symtab.addUndefined(S);
+  }
+
   // Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
   // so that it points to an absolute address which is relative to GOT.
   // See "Global Data Symbols" in Chapter 6 in the following document:
   // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
   if (Config->EMachine == EM_MIPS)
     Symtab.addAbsoluteSym("_gp", DefinedAbsolute<ELFT>::MipsGp);
 
   for (std::unique_ptr<InputFile> &F : Files)

lld/trunk/test/ELF/entry.s

 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
 # RUN: not ld.lld %t1 -o %t2
 # RUN: ld.lld %t1 -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=SYM %s
+# RUN: ld.lld %t1 -shared -o %t2 -e entry
+# RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DSO %s
 # RUN: ld.lld %t1 -o %t2 -e 4096
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DEC %s
 # RUN: ld.lld %t1 -o %t2 -e 0xcafe
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=HEX %s
 # RUN: ld.lld %t1 -o %t2 -e 0777
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=OCT %s
 
 # SYM: Entry: 0x11000
+# DSO: Entry: 0x1000
 # DEC: Entry: 0x1000
 # HEX: Entry: 0xCAFE
 # OCT: Entry: 0x1FF
 
 .globl entry
 entry: