[Clang] [Cygwin] va_list must be treated like normal Windows (llvm#143115)

Handling of va_list on Cygwin environment must be matched to normal
Windows environment.

The existing test `test/CodeGen/ms_abi.c` seems relevant, but it
contains `__attribute__((sysv_abi))`, which is not supported on Cygwin.
The new test is based on the `__attribute__((ms_abi))` portion of that
test.

---------

Co-authored-by: jeremyd2019 <[email protected]>

Author: 2 people

clang/lib/Basic/Targets/X86.h

@@ -997,6 +997,10 @@ class LLVM_LIBRARY_VISIBILITY CygwinX86_64TargetInfo : public X86_64TargetInfo {
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
   }
+
+  BuiltinVaListKind getBuiltinVaListKind() const override {
+    return TargetInfo::CharPtrBuiltinVaList;
+  }
 };
 
 class LLVM_LIBRARY_VISIBILITY DarwinX86_64TargetInfo

clang/test/CodeGen/X86/cygwin-varargs.c

@@ -0,0 +1,35 @@
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm < %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-pc-cygwin -emit-llvm < %s | FileCheck %s
+
+struct foo {
+  int x;
+  float y;
+  char z;
+};
+// CHECK: %[[STRUCT_FOO:.*]] = type { i32, float, i8 }
+
+void f(int a, ...) {
+  // CHECK-LABEL: define dso_local void @f
+  __builtin_va_list ap;
+  __builtin_va_start(ap, a);
+  // CHECK: %[[AP:.*]] = alloca ptr
+  // CHECK: call void @llvm.va_start
+  int b = __builtin_va_arg(ap, int);
+  // CHECK: %[[AP_CUR:.*]] = load ptr, ptr %[[AP]]
+  // CHECK-NEXT: %[[AP_NEXT:.*]] = getelementptr inbounds i8, ptr %[[AP_CUR]], i64 8
+  // CHECK-NEXT: store ptr %[[AP_NEXT]], ptr %[[AP]]
+  double _Complex c = __builtin_va_arg(ap, double _Complex);
+  // CHECK: %[[AP_CUR2:.*]] = load ptr, ptr %[[AP]]
+  // CHECK-NEXT: %[[AP_NEXT2:.*]] = getelementptr inbounds i8, ptr %[[AP_CUR2]], i64 8
+  // CHECK-NEXT: store ptr %[[AP_NEXT2]], ptr %[[AP]]
+  // CHECK-NEXT: load ptr, ptr %[[AP_CUR2]]
+  struct foo d = __builtin_va_arg(ap, struct foo);
+  // CHECK: %[[AP_CUR3:.*]] = load ptr, ptr %[[AP]]
+  // CHECK-NEXT: %[[AP_NEXT3:.*]] = getelementptr inbounds i8, ptr %[[AP_CUR3]], i64 8
+  // CHECK-NEXT: store ptr %[[AP_NEXT3]], ptr %[[AP]]
+  __builtin_va_list ap2;
+  __builtin_va_copy(ap2, ap);
+  // CHECK: call void @llvm.va_copy
+  __builtin_va_end(ap);
+  // CHECK: call void @llvm.va_end
+}