Teach ARM va_arg to ignore empty structs.

Empty structs are ignored for parameter passing purposes, but va_arg was
incrementing the pointer anyway which could lead to va_list getting out of
sync.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184605 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TargetInfo.cpp

@@ -3593,6 +3593,12 @@ llvm::Value *ARMABIInfo::EmitVAArg(llvm::Value *VAListAddr, QualType Ty,
   llvm::Value *VAListAddrAsBPP = Builder.CreateBitCast(VAListAddr, BPP, "ap");
   llvm::Value *Addr = Builder.CreateLoad(VAListAddrAsBPP, "ap.cur");
 
+  if (isEmptyRecord(getContext(), Ty, true)) {
+    // These are ignored for parameter passing purposes.
+    llvm::Type *PTy = llvm::PointerType::getUnqual(CGF.ConvertType(Ty));
+    return Builder.CreateBitCast(Addr, PTy);
+  }
+
   uint64_t Size = CGF.getContext().getTypeSize(Ty) / 8;
   uint64_t TyAlign = CGF.getContext().getTypeAlign(Ty) / 8;
   bool IsIndirect = false;

test/CodeGenCXX/arm-vaarg.cpp

@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -triple armv7-apple-ios -emit-llvm -o - %s | FileCheck %s
+struct Empty {};
+
+Empty emptyvar;
+
+int take_args(int a, ...) {
+  __builtin_va_list l;
+  __builtin_va_start(l, a);
+// CHECK: call void @llvm.va_start
+
+  emptyvar = __builtin_va_arg(l, Empty);
+// CHECK: load i8**
+// CHECK-NOT: getelementptr
+// CHECK: [[EMPTY_PTR:%[a-zA-Z0-9._]+]] = bitcast i8* {{%[a-zA-Z0-9._]+}} to %struct.Empty*
+
+  // It's conceivable that EMPTY_PTR may not actually be a valid pointer
+  // (e.g. it's at the very bottom of the stack and the next page is
+  // invalid). This doesn't matter provided it's never loaded (there's no
+  // well-defined way to tell), but it becomes a problem if we do try to use it.
+// CHECK-NOT: load %struct.Empty* [[EMPTY_PTR]]
+
+  int i = __builtin_va_arg(l, int);
+// CHECK: load i32*
+
+  __builtin_va_end(l);
+  return i;
+}