[ELF] Define __ehdr_start unconditionally even when using linker script

[phosek]

Summary

This behavior differs from the semantics implemented by GNU linkers which only define this symbol iff ELF headers are in the memory mapped segment.

[ruiu]

LGTM

lld/trunk/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
 
   // __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.
   if (!In<ELFT>::DynSymTab)
     Symtab<ELFT>::X->addIgnored("__tls_get_addr");
 
+  // __ehdr_start is the location of ELF file headers. Note that we define
+  // this symbol unconditionally even when using a linker script, which
+  // differs from the behavior implemented by GNU linker which only define
+  // this symbol if ELF headers are in the memory mapped segment.
+  addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
+
   // If linker script do layout we do not need to create any standart symbols.
   if (Script->Opt.HasSections)
     return;
 
-  // __ehdr_start is the location of ELF file headers.
-  addOptionalRegular<ELFT>("__ehdr_start", Out::ElfHeader, 0, STV_HIDDEN);
-
   auto Add = [](StringRef S) {
     return addOptionalRegular<ELFT>(S, Out::ElfHeader, 0, STV_DEFAULT);
   };
 
   ElfSym::Bss = Add("__bss_start");
   ElfSym::End1 = Add("end");
   ElfSym::End2 = Add("_end");
   ElfSym::Etext1 = Add("etext");

lld/trunk/test/ELF/linkerscript/ehdr_start.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: echo "SECTIONS { }" > %t.script
-# RUN: not ld.lld %t.o -script %t.script -o %t 2>&1 | FileCheck %s
-# CHECK: error: undefined symbol: __ehdr_start
-# CHECK: >>> referenced by {{.*}}:(.text+0x0)
+# RUN: ld.lld %t.o -script %t.script -o %t
+# RUN: llvm-readobj -symbols %t | FileCheck %s
+# CHECK:    Name: __ehdr_start (1)
+# CHECK-NEXT:    Value: 0x0
+# 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