[AsmPrinter] Print aliases in topological order

Print aliases in topological order, that is, for any 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.

GCC also prints aliases in topological order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265064 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1148,7 +1148,7 @@ bool AsmPrinter::doFinalization(Module &M) {
   }
 
   OutStreamer->AddBlankLine();
-  for (const auto &Alias : M.aliases()) {
+  const auto printAlias = [this, &M](const GlobalAlias &Alias) {
     MCSymbol *Name = getSymbol(&Alias);
 
     if (Alias.hasExternalLinkage() || !MAI->getWeakRefDirective())
@@ -1186,6 +1186,23 @@ bool AsmPrinter::doFinalization(Module &M) {
       OutStreamer->emitELFSize(cast<MCSymbolELF>(Name),
                                MCConstantExpr::create(Size, OutContext));
     }
+  };
+  // 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()) {
+    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 : reverse(AliasStack))
+      printAlias(*AncestorAlias);
+    AliasStack.clear();
   }
 
   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();

test/CodeGen/Generic/asm-printer-topological-order.ll

@@ -0,0 +1,15 @@
+; RUN: llc < %s | FileCheck %s
+
+@TestA = alias void (), void ()* @TestC
+@TestB = alias void (), void ()* @TestC
+@TestC = alias void (), void ()* @TestD
+
+define void @TestD() {
+entry:
+  ret void
+}
+
+; CHECK-LABEL: TestD:
+; CHECK: TestC = TestD
+; CHECK-DAG: TestB = TestC
+; CHECK-DAG: TestA = TestC