[ELF] - Stop mixing order of -defsym/-script commands.

[grimar]

Summary

Previously we always handled -defsym after other commands in command line.
That made impossible to overload values set by -defsym from linker script:

 test.script:            
  foo = 0x22;
-defsym=foo=0x11 -script t.script

would always set foo to 0x11.

That is inconstent with common logic which allows to override command line
options. it is inconsistent with bfd behavior and seems breaks assumption that
-defsym is the same as linker script assignment, as -defsyms always handled out of
command line order.

Patch fixes the handling order.

[grimar]

• Fixed comment.

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

void LinkerDriver::createFiles(opt::InputArgList &Args) {
   for (auto *Arg : Args) {
     switch (Arg->getOption().getUnaliasedOption().getID()) {
     case OPT_library:
       addLibrary(Arg->getValue());
       break;
     case OPT_INPUT:
       addFile(Arg->getValue(), /*WithLOption=*/false);
       break;
+    case OPT_defsym: {
+      StringRef From;
+      StringRef To;
+      std::tie(From, To) = StringRef(Arg->getValue()).split('=');
+      readDefsym(From, MemoryBufferRef(To, "-defsym"));
+      break;
+    }
     case OPT_script:
       if (Optional<std::string> Path = searchLinkerScript(Arg->getValue())) {
         if (Optional<MemoryBufferRef> MB = readFile(*Path))
           readLinkerScript(*MB);
         break;
       }
       error(Twine("cannot find linker script ") + Arg->getValue());
       break;
template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   for (auto *Arg : Args.filtered(OPT_trace_symbol))
     Symtab->trace(Arg->getValue());
 
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table.
   for (InputFile *F : Files)
     Symtab->addFile<ELFT>(F);
 
-  // Process -defsym option.
-  for (auto *Arg : Args.filtered(OPT_defsym)) {
-    StringRef From;
-    StringRef To;
-    std::tie(From, To) = StringRef(Arg->getValue()).split('=');
-    readDefsym(From, MemoryBufferRef(To, "-defsym"));
-  }
-
   // Now that we have every file, we can decide if we will need a
   // dynamic symbol table.
   // We need one if we were asked to export dynamic symbols or if we are
   // producing a shared library.
   // We also need one if any shared libraries are used and for pie executables
   // (probably because the dynamic linker needs it).
   Config->HasDynSymTab =
       !SharedFiles.empty() || Config->Pic || Config->ExportDynamic;

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

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "foo = 0x22;" > %t.script
+
+## This testcase checks that we apply -defsym and linker script
+## in the same order are they specified in a command line. 
+
+## Check that linker script can override -defsym assignments.
+# RUN: ld.lld %t.o -defsym=foo=0x11 -script %t.script -o %t
+# RUN: llvm-readobj -t %t | FileCheck %s
+# CHECK:      Name: foo
+# CHECK-NEXT:   Value: 0x22
+
+## Check that -defsym can override linker script. Check that multiple
+## -defsym commands for the same symbol are allowed.
+# RUN: ld.lld %t.o -script %t.script -defsym=foo=0x11 -defsym=foo=0x33 -o %t
+# RUN: llvm-readobj -t %t | FileCheck %s --check-prefix=REORDER
+# REORDER:      Name: foo
+# REORDER-NEXT:   Value: 0x33