[CodeGen] Filter out available_externally aliases

[Hahnfeld]

Summary

The Language Reference says that aliases can have available_externally linkage if their aliasee is an available_externally global value. Using this kind of aliases resulted in crashes during code generation, filter them out (the same that the AsmPrinter also filters out GlobalVariables in emitSpecialLLVMGlobal(); Functions are discarded in the machine pass infrastructure).

[Hahnfeld]

Fix type of function alias.

[gchatelet]

I've only touched this file for refactoring purposes, I'm not qualified to do a proper review.

[lhames]

This looks right to me -- the verifier already requires available_externally aliases to point to other available_externally symbols, so if we're skipping an alias here then we know that we'll also skip all the things that it points to.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

bool AsmPrinter::doFinalization(Module &M) {
 
   // Print aliases in topological order, that is, for each alias a = b,
   // b must be printed before a.
   // This is because on some targets (e.g. PowerPC) linker expects aliases in
   // such an order to generate correct TOC information.
   SmallVector<const GlobalAlias *, 16> AliasStack;
   SmallPtrSet<const GlobalAlias *, 16> AliasVisited;
   for (const auto &Alias : M.aliases()) {
+    if (Alias.hasAvailableExternallyLinkage())
+      continue;
     for (const GlobalAlias *Cur = &Alias; Cur;
          Cur = dyn_cast<GlobalAlias>(Cur->getAliasee())) {
       if (!AliasVisited.insert(Cur).second)
         break;
       AliasStack.push_back(Cur);
     }
     for (const GlobalAlias *AncestorAlias : llvm::reverse(AliasStack))
       emitGlobalAlias(M, *AncestorAlias);

llvm/test/CodeGen/Generic/available_externally_alias.ll

+; RUN: llc < %s
+
+@v = available_externally global i32 42, align 4
+@va = available_externally alias i32, ptr @v
+
+define available_externally i32 @f() {
+entry:
+  ret i32 0
+}
+
+@fa = available_externally alias i32(), ptr @f