[AIX] Avoid structor alias; die before bad alias codegen

Summary:
`AsmPrinter::emitGlobalIndirectSymbol` is dependent on
`MCStreamer::emitAssignment` to produce `.set` directives for alias
symbols; however, the `.set` pseudo-op on AIX is documented as not
usable with external relocatable terms or expressions, which limits its
applicability in generating alias symbols.

Disable generating aliases on AIX until a different implementation
strategy is available.

Reviewers: cebowleratibm, jasonliu, sfertile, daltenty, DiggerLin

Reviewed By: jasonliu

Differential Revision: https://reviews.llvm.org/D79044

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4675,8 +4675,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   // Enable -mconstructor-aliases except on darwin, where we have to work around
   // a linker bug (see <rdar://problem/7651567>), and CUDA device code, where
-  // aliases aren't supported.
-  if (!RawTriple.isOSDarwin() && !RawTriple.isNVPTX())
+  // aliases aren't supported. Similarly, aliases aren't yet supported for AIX.
+  if (!RawTriple.isOSDarwin() && !RawTriple.isNVPTX() && !RawTriple.isOSAIX())
     CmdArgs.push_back("-mconstructor-aliases");
 
   // Darwin's kernel doesn't support guard variables; just die if we

clang/test/Driver/aix-constructor-alias.c

@@ -0,0 +1,7 @@
+// Check that we don't pass -mconstructor-aliases when compiling for AIX.
+
+// RUN: %clang -### -target powerpc-ibm-aix7.1.0.0 %s -c -o %t.o 2>&1 \
+// RUN:   | FileCheck %s
+// RUN: %clang -### -target powerpc64-ibm-aix7.1.0.0 %s -c -o %t.o 2>&1 \
+// RUN:   | FileCheck %s
+// CHECK-NOT: "-mconstructor-aliases"

llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp

@@ -155,6 +155,14 @@ public:
 
   StringRef getPassName() const override { return "AIX PPC Assembly Printer"; }
 
+  bool doInitialization(Module &M) override {
+    if (M.alias_size() > 0u)
+      report_fatal_error(
+          "module has aliases, which LLVM does not yet support for AIX");
+
+    return PPCAsmPrinter::doInitialization(M);
+  }
+
   void SetupMachineFunction(MachineFunction &MF) override;
 
   void emitGlobalVariable(const GlobalVariable *GV) override;

llvm/test/CodeGen/PowerPC/aix-alias.ll

@@ -0,0 +1,10 @@
+; RUN: not --crash llc < %s -mtriple powerpc-ibm-aix-xcoff 2>&1 | FileCheck %s
+; RUN: not --crash llc < %s -mtriple powerpc64-ibm-aix-xcoff 2>&1 | FileCheck %s
+
+; Check that, while generation of aliases on AIX remains unimplemented, llc dies
+; with an appropriate message instead of generating incorrect output when an
+; alias is encountered.
+
+define i32 @a() { ret i32 0 }
+; CHECK: ERROR: module has aliases
+@b = internal alias i32 (), i32 ()* @a