[ELF] Make __ehdr_start point to ELF file headers

[phosek]

Summary

__ehdr_start should be pointing to ELF file headers, not program headers.

This is a reland of D30319.

[pcc]

LGTM

lld/trunk/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
   if (!In<ELFT>::DynSymTab &&
       (Config->EMachine != EM_MIPS && Config->EMachine != EM_ARM))
     Symtab<ELFT>::X->addIgnored("__tls_get_addr");
 
   // If linker script do layout we do not need to create any standart symbols.
   if (ScriptConfig->HasSections)
     return;
 
-  // __ehdr_start is the location of program headers.
+  // __ehdr_start is the location of ELF file headers.
   ElfSym<ELFT>::EhdrStart =
-      addOptionalSynthetic<ELFT>("__ehdr_start", Out<ELFT>::ProgramHeaders, 0);
+      addOptionalSynthetic<ELFT>("__ehdr_start", Out<ELFT>::ElfHeader, 0);
 
   auto Define = [](StringRef S, DefinedSynthetic *&Sym1,
                    DefinedSynthetic *&Sym2) {
     Sym1 = addOptionalSynthetic<ELFT>(S, nullptr, 0, STV_DEFAULT);
     assert(S.startswith("_"));
     S = S.substr(1);
     Sym2 = addOptionalSynthetic<ELFT>(S, nullptr, 0, STV_DEFAULT);
   };
template <class ELFT> void Writer<ELFT>::finalizeSections() {
   addPredefinedSections();
   removeUnusedSyntheticSections<ELFT>(OutputSections);
 
   sortSections();
 
   // This is a bit of a hack. A value of 0 means undef, so we set it
   // to 1 t make __ehdr_start defined. The section number is not
   // particularly relevant.
-  Out<ELFT>::ProgramHeaders->SectionIndex = 1;
+  Out<ELFT>::ElfHeader->SectionIndex = 1;
 
   unsigned I = 1;
   for (OutputSectionBase *Sec : OutputSections) {
     Sec->SectionIndex = I++;
     Sec->ShName = In<ELFT>::ShStrTab->addString(Sec->getName());
   }
 
   // Binary and relocatable output does not have PHDRS.

lld/trunk/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: 0000000000200040 .text 00000000 .hidden __ehdr_start
+# RUN: llvm-readobj -symbols %t | FileCheck %s
+# CHECK:    Name: __ehdr_start (1)
+# CHECK-NEXT:    Value: 0x200000
+# CHECK-NEXT:    Size: 0
+# CHECK-NEXT:    Binding: Local (0x0)
+# CHECK-NEXT:    Type: None (0x0)
+# CHECK-NEXT:    Other [ (0x2)
+# CHECK-NEXT:      STV_HIDDEN (0x2)
+# CHECK-NEXT:    ]
+# CHECK-NEXT:    Section: .text (0x1)
 
 .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
+# RUN: llvm-readobj -symbols %t.r | FileCheck %s --check-prefix=RELOCATABLE
 
-# RELOCATABLE: 0000000000000000 *UND* 00000000 __ehdr_start
+# RELOCATABLE:    Name: __ehdr_start (1)
+# RELOCATABLE-NEXT:    Value: 0x0
+# RELOCATABLE-NEXT:    Size: 0
+# RELOCATABLE-NEXT:    Binding: Global (0x1)
+# RELOCATABLE-NEXT:    Type: None (0x0)
+# RELOCATABLE-NEXT:    Other: 0
+# RELOCATABLE-NEXT:    Section: Undefined (0x0)