[ELF] - Linkerscript: ignore AT

[grimar]

Summary

AT is tricky command and we even have a patch for it: D19272
Though looks we do not want to support it right now,
but probably it is worth to ignore then.
Patch implements that and just shows warning if it is used.

[emaste]

If we don't want to implement it fully just yet I'm happy with a warning for now. It will at least allow FreeBSD testing to continue.

[ruiu]

If a linker script contains an AT commands, the program should depend on it, otherwise it didn't contain it on the first place. So I think it's a hard error. I mean I ithin doing nothing and treat AT as an unknown token is the right approach.

[davide]

Hmm no. Please don't ignore this. I tried ignoring and the Xen loader gets very angry.

[emaste]

In that case we should just proceed with D19272

[grimar]

I`ll rebase and update the D19272 then.

ELF/LinkerScript.cpp

private:
   void readSections();
 
   void readLocationCounterValue();
   void readOutputSectionDescription(StringRef OutSec);
   std::vector<StringRef> readOutputSectionPhdrs();
   unsigned readPhdrType();
   void readSymbolAssignment(StringRef Name);
   std::vector<StringRef> readSectionsCommandExpr();
+  void readAt();
 
   const static StringMap<Handler> Cmd;
   ScriptConfiguration &Opt = *ScriptConfig;
   StringSaver Saver = {ScriptConfig->Alloc};
   bool IsUnderSysroot;
 };
 
 const StringMap<elf::ScriptParser::Handler> elf::ScriptParser::Cmd = {
void ScriptParser::readLocationCounterValue() {
   expect("=");
   std::vector<StringRef> Expr = readSectionsCommandExpr();
   if (Expr.empty())
     error("error in location counter expression");
   else
     Opt.Commands.push_back(llvm::make_unique<SymbolAssignment>(".", Expr));
 }
 
+void ScriptParser::readAt() {
+  warning("AT command is ignored");
+  expect("AT");
+  expect("(");
+  while (!Error) {
+    StringRef Tok = next();
+    if (Tok == ")")
+      break;
+  }
+}
+
 void ScriptParser::readOutputSectionDescription(StringRef OutSec) {
   OutputSectionCommand *Cmd = new OutputSectionCommand(OutSec);
   Opt.Commands.emplace_back(Cmd);
   expect(":");
+  if (peek() == "AT")
+    readAt();
   expect("{");
 
   while (!Error && !skip("}")) {
     StringRef Tok = next();
     if (Tok == "*") {
       auto *InCmd = new InputSectionDescription();
       Cmd->Commands.emplace_back(InCmd);
       expect("(");

test/ELF/linkerscript-at.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { \
+# RUN:  .aaa : AT(0x0) \
+# RUN:  { \
+# RUN:  } \
+# RUN: }" > %t.script
+# RUN: ld.lld -shared %t --script %t.script -o %t2 2>&1 | \
+# RUN:   FileCheck --check-prefix=IGNORED %s
+# IGNORED: AT command is ignored