Fail early on unknown appending linkage variables

[• espindola]

Summary

In practice only a few well known appending linkage variables work.

Currently if codegen sees an unknown appending linkage variable it will just print it as a regular global. That is wrong as the symbol in the produced object file has different semantics as the one provided by the appending linkage.

This just errors early instead of producing a broken .o.

[dexonsmith]

The patch LGTM with an update to LangRef. Has this been discussed on
llvm-dev though? It seems like an IR change, since whether or not the
old behaviour was a good idea, it was certainly tested for...

[rengolin]

I'm ok with not silently assuming external linkage types from internal-only ones. This has to either be an explicit transformation during code emission or an error. Also, LangRef change. :)

[dexonsmith]

Marking as needing changes pending the LangRef change (and llvm-dev discussion).

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

if (MAI->hasWeakDefDirective()) {
       // .globl _foo
       OutStreamer->EmitSymbolAttribute(GVSym, MCSA_Global);
       //NOTE: linkonce is handled by the section the symbol was assigned to.
     } else {
       // .weak _foo
       OutStreamer->EmitSymbolAttribute(GVSym, MCSA_Weak);
     }
     return;
-  case GlobalValue::AppendingLinkage:
-    // FIXME: appending linkage variables should go into a section of
-    // their name or something.  For now, just emit them as external.
   case GlobalValue::ExternalLinkage:
-    // If external or appending, declare as a global symbol.
-    // .globl _foo
+    // If external, declare as a global symbol: .globl _foo
     OutStreamer->EmitSymbolAttribute(GVSym, MCSA_Global);
     return;
   case GlobalValue::PrivateLinkage:
   case GlobalValue::InternalLinkage:
     return;
+  case GlobalValue::AppendingLinkage:
   case GlobalValue::AvailableExternallyLinkage:
   case GlobalValue::ExternalWeakLinkage:
     llvm_unreachable("Should never emit this");
   }
   llvm_unreachable("Unknown linkage type!");
 }
 
 void AsmPrinter::getNameWithPrefix(SmallVectorImpl<char> &Name,
if (TM.getRelocationModel() == Reloc::Static &&
         MAI->hasStaticCtorDtorReferenceInStaticMode()) {
       StringRef Sym(".destructors_used");
       OutStreamer->EmitSymbolAttribute(OutContext.getOrCreateSymbol(Sym),
                                        MCSA_Reference);
     }
     return true;
   }
 
-  return false;
+  report_fatal_error("unknown special variable");
 }
 
 /// EmitLLVMUsedList - For targets that define a MAI::UsedDirective, mark each
 /// global in the specified llvm.used list for which emitUsedDirectiveFor
 /// is true, as being used with this directive.
 void AsmPrinter::EmitLLVMUsedList(const ConstantArray *InitList) {
   // Should be an array of 'i8*'.
   for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i) {

test/CodeGen/X86/2007-08-13-AppendingLinkage.ll

-; RUN: llc < %s -march=x86 | not grep drectve
-; PR1607
-
-%hlvm_programs_element = type { i8*, i32 (i32, i8**)* }
-@hlvm_programs = appending constant [1 x %hlvm_programs_element]
-zeroinitializer
-
-define %hlvm_programs_element* @hlvm_get_programs() {
-entry:
-  ret %hlvm_programs_element* getelementptr([1 x %hlvm_programs_element], [1 x %hlvm_programs_element]*  
-                                            @hlvm_programs, i32 0, i32 0)
-}

test/CodeGen/X86/AppendingLinkage.ll

+; RUN: not llc < %s -march=x86 2>&1 | FileCheck %s
+
+; CHECK: unknown special variable
+@foo = appending constant [1 x i32 ]zeroinitializer