[ELF] Allow overriding reserved symbols in linker script

[evgeny777]

Summary

This is a small bugfix for http://reviews.llvm.org/D22088

Resverved symbols are added in addReservedSymbols(), which is called before createSections()
Therefore if you define some reserved symbol in linker script then lld will report error because of name conflict

[ruiu]

test/ELF/linkerscript-symbols.s

4–7

Does this test check for the new code? I think you need two test cases, one is for symbol definition and the other is for symbol conflict.

[evgeny777]

Created separate test case. Review updated

[ruiu]

LGTM

ELF/LinkerScript.cpp

 int LinkerScript<ELFT>::compareSections(StringRef A, StringRef B) {
   int I = getSectionIndex(A);
   int J = getSectionIndex(B);
   if (I == INT_MAX && J == INT_MAX)
     return 0;
   return I < J ? -1 : 1;
 }
 
-template <class ELFT>
-void LinkerScript<ELFT>::addScriptedSymbols() {
+template <class ELFT> void LinkerScript<ELFT>::addScriptedSymbols() {
   for (SectionsCommand &Cmd : Opt.Commands)
-    if (Cmd.Kind == SymbolAssignmentKind)
+    if (Cmd.Kind == SymbolAssignmentKind &&
+        Symtab<ELFT>::X->find(Cmd.Name) == nullptr)
       Symtab<ELFT>::X->addAbsolute(Cmd.Name, STV_DEFAULT);
 }
 
 class elf::ScriptParser : public ScriptParserBase {
   typedef void (ScriptParser::*Handler)();
 
 public:
   ScriptParser(StringRef S, bool B) : ScriptParserBase(S), IsUnderSysroot(B) {}

test/ELF/linkerscript-symbols.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 
-# RUN: echo "SECTIONS {.text : {*(.text.*)} text_end = .;}" > %t.script
+# RUN: echo "SECTIONS {.text : {*(.text.*)} end = .;}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-objdump -t %t1 | FileCheck %s
-# CHECK: 0000000000000121         *ABS*    00000000 text_end
+# CHECK: 0000000000000121         *ABS*    00000000 end
 
 .global _start
 _start:
  nop