[ELF] - Do not create reserved symbols in case of relocatable output.

[grimar]

Summary

This combines D26599, D26545 patches and also
stops creating _end/_etext_edata symbols when using -r.

[ruiu]

LGTM. This is much better. Thanks!

ELF/Writer.cpp

return !Symtab<ELFT>::X->getSharedFiles().empty() &&
          !Script<ELFT>::X->ignoreInterpSection();
 }
 
 template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
 
 // The main function of the writer.
 template <class ELFT> void Writer<ELFT>::run() {
   createSyntheticSections();
+
+  if (!Config->Relocatable)
     addReservedSymbols();
 
   if (Target->NeedsThunks)
     forEachRelSec(createThunks<ELFT>);
 
   Script<ELFT>::X->OutputSections = &OutputSections;
   if (ScriptConfig->HasSections) {
     Script<ELFT>::X->createSections(Factory);
   } else {
template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
   S = Config->Rela ? "__rela_iplt_end" : "__rel_iplt_end";
   addOptionalSynthetic<ELFT>(S, Out<ELFT>::RelaPlt,
                              DefinedSynthetic<ELFT>::SectionEnd);
 }
 
 // The linker is expected to define some symbols depending on
 // the linking result. This function defines such symbols.
 template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
-  if (Config->EMachine == EM_MIPS && !Config->Relocatable) {
+  if (Config->EMachine == EM_MIPS) {
     // 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
     addRegular("_gp", In<ELFT>::Got, MipsGPOffset);
 
     // On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
     // start of function and 'gp' pointer into GOT.
template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
   // 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.
-  if (!Config->Relocatable)
-    Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
 
   // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
   // static linking the linker is required to optimize away any references to
   // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
   // to avoid the undefined symbol error. As usual special cases are ARM and
   // MIPS - the libc for these targets defines __tls_get_addr itself because
   // there are no TLS optimizations for these targets.
   if (!Out<ELFT>::DynSymTab &&

test/ELF/Inputs/relocatable-tls.s

+callq __tls_get_addr@PLT

test/ELF/edata-etext.s

 # CHECK-NEXT:    3 .bss          00000006 0000000000012004 BSS
 # CHECK:      SYMBOL TABLE:
 # CHECK-NEXT:  0000000000000000         *UND* 00000000
 # CHECK-NEXT:  0000000000012002         *ABS* 00000000 _edata
 # CHECK-NEXT:  000000000001200a         *ABS* 00000000 _end
 # CHECK-NEXT:  0000000000011001         *ABS* 00000000 _etext
 # CHECK-NEXT:  0000000000011000         .text 00000000 _start
 
+# RUN: ld.lld -r %t.o -o %t2
+# RUN: llvm-objdump -t %t2 | FileCheck %s --check-prefix=RELOCATABLE
+# RELOCATABLE:       0000000000000000 *UND* 00000000 _edata
+# RELOCATABLE-NEXT:  0000000000000000 *UND* 00000000 _end
+# RELOCATABLE-NEXT:  0000000000000000 *UND* 00000000 _etext
+
 .global _start,_end,_etext,_edata
 .text
 _start:
   nop
 .data
   .word 1
 .bss
   .align 4
   .space 6

test/ELF/ehdr_start.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t
 # RUN: llvm-objdump -t %t | FileCheck %s
 # CHECK: 0000000000010040 *ABS* 00000000 .hidden __ehdr_start
 
 .text
 .global _start, __ehdr_start
 _start:
   .quad __ehdr_start
+
+# RUN: ld.lld -r %t.o -o %t.r
+# RUN: llvm-objdump -t %t.r | FileCheck %s --check-prefix=RELOCATABLE
+
+# RELOCATABLE: 0000000000000000 *UND* 00000000 __ehdr_start

test/ELF/end.s

 // DEFAULT-NEXT:     Offset:
 // DEFAULT-NEXT:     Size: 6
 // DEFAULT: ]
 // DEFAULT: Symbols [
 // DEFAULT:     Name: _end
 // DEFAULT-NEXT:     Value: 0x12008
 // DEFAULT: ]
 
+// RUN: ld.lld -r %t.o -o %t2
+// RUN: llvm-objdump -t %t2 | FileCheck %s --check-prefix=RELOCATABLE
+// RELOCATABLE: 0000000000000000 *UND* 00000000 _end
+
 .global _start,_end
 .text
 _start:
     nop
 .data
     .word 1
 .bss
     .space 6

test/ELF/relocatable-tls.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
+# RUN:   %S/Inputs/relocatable-tls.s -o %t2.o
+
+# RUN: ld.lld -r %t2.o -o %t3.r
+# RUN: llvm-objdump -t %t3.r | FileCheck --check-prefix=RELOCATABLE %s
+# RELOCATABLE: SYMBOL TABLE:
+# RELOCATABLE: 0000000000000000 *UND* 00000000 __tls_get_addr
+
+# RUN: ld.lld -shared %t2.o %t3.r -o %t4.out
+# RUN: llvm-objdump -t %t4.out | FileCheck --check-prefix=DSO %s
+# DSO: SYMBOL TABLE:
+# DSO: 0000000000000000 *UND* 00000000 __tls_get_addr
+
+callq __tls_get_addr@PLT